| A selection of publications sorted by year |
| 2025 |
| Abstract: | Mathematics is an underexplored domain of human cognition. While many studies have focused on subsets of math concepts such as numbers, fractions, or geometric shapes, few have ventured beyond these elementary domains. Here, we attempted to map out the full space of math concepts and to answer two specific questions: can distributed semantic models, such a GloVe, provide a satisfactory fit to human semantic judgements in mathematics? And how does this fit vary with education? We first analyzed all of the French and English Wikipedia pages with math contents, and used a semi-automatic procedure to extract the 1000 most frequent math terms in both languages. In a second step, we collected extensive behavioral judgements of familiarity and semantic similarity between them. About half of the variance in human similarity judgements was explained by vector embeddings that attempt to capture latent semantic structures based on cooccurence statistics. Participants' self-reported level of education modulated familiarity and similarity, allowing us to create a partial hierarchy among high-level math concepts. Our results converge onto the proposal of a map of math space, organized as a database of math terms with information about their frequency, familiarity, grade of acquisition, and entanglement with other concepts. |
| 2024 |
| Abstract: | William James's use of "time in passing" and "stream of thoughts" may be two sides of the same coin that emerge from the brain segmenting the continuous flow of information into discrete events. Herein, we investigated how the density of events affects two temporal experiences: the felt duration and speed of time. Using a temporal bisection task, participants classified seconds-long videos of naturalistic scenes as short or long (duration), or slow or fast (passage of time). Videos contained a varying number and type of events. We found that a large number of events lengthened subjective duration and accelerated the felt passage of time. Surprisingly, participants were also faster at estimating their felt passage of time compared to duration. The perception of duration scaled with duration and event density, whereas the felt passage of time scaled with the rate of change. Altogether, our results suggest that distinct mechanisms underlie these two experiential times. |
| 2023 |
| Abstract: | Learning in a stochastic and changing environment is a difficult task. Models of learning typically postulate that observations that deviate from the learned predictions are surprising and used to update those predictions. Bayesian accounts further posit the existence of a confidence-weighting mechanism: learning should be modulated by the confidence level that accompanies those predictions. However, the neural bases of this confidence are much less known than the ones of surprise. Here, we used a dynamic probability learning task and high-field MRI to identify putative cortical regions involved in the representation of confidence about predictions during human learning. We devised a stringent test based on the conjunction of four criteria. We localized several regions in parietal and frontal cortices whose activity is sensitive to the confidence of an ideal observer, specifically so with respect to potential confounds (surprise and predictability), and in a way that is invariant to which item is predicted. We also tested for functionality in two ways. First, we localized regions whose activity patterns at the subject level showed an effect of both confidence and surprise in qualitative agreement with the confidence-weighting principle. Second, we found neural representations of ideal confidence that also accounted for subjective confidence. Taken together, those results identify a set of cortical regions potentially implicated in the confidence-weighting of learning. |
| Abstract: | Summary Access of sensory information to consciousness has been linked to the ignition of content-specific representations in association cortices. How does ignition interact with intrinsic cortical state fluctuations to give rise to conscious perception? We addressed this question in the prefrontal cortex (PFC) by combining multi-electrode recordings with a binocular rivalry (BR) paradigm inducing spontaneously driven changes in the content of consciousness, inferred from the reflexive optokinetic nystagmus (OKN) pattern. We find that fluctuations between low-frequency (LF, 1-9 Hz) and beta (~20-40 Hz) local field potentials (LFPs) reflect competition between spontaneous updates and stability of conscious contents, respectively. Both LF and beta events were locally modulated. The phase of the former locked differentially to the competing populations just before a spontaneous transition while the latter synchronized the neuronal ensemble coding the consciously perceived content. These results suggest that prefrontal state fluctuations gate conscious perception by mediating internal states that facilitate perceptual update and stability. |
| Abstract: | We often express our thoughts through words, but thinking goes well beyond language. Here we focus on an elementary but basic thinking process, disjunction elimination, elicited by elementary visual scenes deprived of linguistic content, describing its neural and oculomotor correlates. We track two main components of a nonverbal deductive process: the construction of a logical representation (A or B), and its simplification by deduction (not A, therefore B). We identify the network active in the two phases and show that in the latter, but not in the former, it overlaps with areas known to respond to verbal logical reasoning. Oculomotor markers consistently differentiate logical processing induced by the construction of a representation, its simplification by deductive inference, and its maintenance when inferences cannot be drawn. Our results reveal how integrative logical processes incorporate novel experience in the flow of thoughts induced by visual scenes. |
| Abstract: | Summary Detection of deviant stimuli is crucial to orient and adapt our behavior. Previous work shows that deviant stimuli elicit phasic activation of the locus coeruleus (LC), which releases noradrenaline and controls central arousal. However, it is unclear whether the detection of behaviorally relevant deviant stimuli selectively triggers LC responses or other neuromodulatory systems (dopamine, serotonin, and acetylcholine). We combine human functional MRI (fMRI) recordings optimized for brainstem imaging with pupillometry to perform a mapping of deviant-related responses in subcortical structures. Participants have to detect deviant items in a "local-global" paradigm that distinguishes between deviance based on the stimulus probability and the sequence structure. fMRI responses to deviant stimuli are distributed in many cortical areas. Both types of deviance elicit responses in the pupil, LC, and other neuromodulatory systems. Our results reveal that the detection of task-relevant deviant items recruits the same multiple subcortical systems across computationally different types of deviance. |
| 2022 |
| Abstract: | Neuroscientists are exploring whether shapes like squares and rectangles -- and our ability to recognize them -- are part of what makes our species special |
| 2021 |
| Abstract: | This thesis focuses on the application, to French students, of advances in the understanding of how children learn to read, what methods best train literacy and how we can better assess reading deficits-- so that these advances can fuel a virtuous circle between cognitive science and educational interventions. In the case of literacy, there is a global consensus that early explicit phonics instruction is the best means to learning how to read. In this thesis, we propose a tablet-based game to support phonics learning. We also address several unanswered questions of teaching methodology. For example, during what period of time should the teaching of the phonetic method be introduced? Can children learn to read using phonics software before formal literacy? We present the results from our randomized control study with French first graders (N=975). Data collected from pre- and post- tests shows that children only benefited from the phonics game when it was used early in the school year. In a second intervention, we improved the design of the application and research methodology, and then tested the game with French kindergarteners (N=1092). This time students improved in factors that support literacy, irrelevant of the period that the games were used. However, in a follow-up test, benefits disappeared once formal reading education began. In the process of seeking to develop a tablet-based approach to literacy, we also describe a program used by the game engine that automatically creates a phonics progression based on the most frequent and consistent grapheme-phoneme correspondences for any alphabetic language. Historically, some of the greatest insights into the processes involved in reading have come from the examination of the deficits caused by brain lesions. Based on the hypothesis that reading is a complex cognitive activity, and that a deficit in any part of the process could lead to dyslexia, we developed a screener that looks at the types of errors made by readers to assess for selective deficits. After normalizing the screener with French 6th and 7th graders, we examine the different types of errors made by students previously diagnosed with dyslexia by their school. We present the first French cases of two different types of selective deficits: letter position dyslexia and attention dyslexia. The body of this work was developed with the goal of practical application. To this end, the tablet-based game and phonics builder code were developed using open-source software and licenses. Our screener for dyslexia is also open for use in collaboration with our lab. In the general discussion, we discuss ideas for immediate improvements and future studies that could be done using these tools, to the benefit of improved literacy programs for all students. |
| Abstract: | Despite the widespread use of graphs, little is known about how fast and how accurately we can extract information from them. Through a series of four behavioral experiments, we characterized human performance in "mental regression", i.e. the perception of statistical trends from scatterplots. When presented with a noisy scatterplot, even as briefly as 100 ms, human adults could accurately judge if it was increasing or decreasing, fit a regression line, and extrapolate outside the original data range, for both linear and non-linear functions. Performance was highly consistent across those three tasks of trend judgment, line fitting and extrapolation. Participants' linear trend judgments took into account the slope, the noise, and the number of data points, and were tightly correlated with the t-test classically used to evaluate the significance of a linear regression. However, they overestimated the absolute value of the regression slope. This bias was inconsistent with ordinary least squares (OLS) regression, which minimizes the sum of square deviations, but consistent with the use of Deming regression, which treats the x and y axes symmetrically and minimizes the Euclidean distance to the fitting line. We speculate that this fast but biased perception of scatterplots may be based on a "neuronal recycling" of the human visual capacity to identify the medial axis of a shape. |
| Abstract: | The ability to detect the abstract pattern underlying a temporal sequence of events is crucial to many human activities, including language and mathematics, but its cortical correlates remain poorly understood. It is also unclear whether repeated exposure to the same sequence of sensory stimuli is sufficient to induce the encoding of an abstract amodal representation of the pattern. Using functional MRI, we probed the existence of such abstract codes for sequential patterns, their localization in the human brain, and their relation to existing language and math-responsive networks. We used a passive sequence violation paradigm, in which a given sequence is repeatedly presented before rare deviant sequences are introduced. We presented two binary patterns, AABB and ABAB, in four presentation formats, either visual or auditory, and either cued by the identity of the stimuli or by their spatial location. Regardless of the presentation format, a habituation to the repeated pattern and a response to pattern violations were seen in a set of inferior frontal, intraparietal and temporal areas. Within language areas, such pattern-violation responses were only found in the inferior frontal gyrus (IFG), whereas all math-responsive regions responded to pattern changes. Most of these regions also responded whenever the modality or the cue changed, suggesting a general sensitivity to violation detection. Thus, the representation of sequence patterns appears to be distributed, yet to include a core set of abstract amodal regions, particularly the IFG. |
| Abstract: | Anesthesia induces a reconfiguration of the repertoire of functional brain states leading to a high function-structure similarity. However, it is unclear how these functional changes lead to loss of consciousness. Here we suggest that the mechanism of conscious access is related to a general dynamical rearrangement of the intrinsic hierarchical organization of the cortex. To measure cortical hierarchy, we applied the Intrinsic Ignition analysis to resting-state fMRI data acquired in awake and anesthetized macaques. Our results reveal the existence of spatial and temporal hierarchical differences of neural activity within the macaque cortex, with a strong modulation by the depth of anesthesia and the employed anesthetic agent. Higher values of Intrinsic Ignition correspond to rich and flexible brain dynamics whereas lower values correspond to poor and rigid, structurally driven brain dynamics. Moreover, spatial and temporal hierarchical dimensions are disrupted in a different manner, involving different hierarchical brain networks. All together suggest that disruption of brain hierarchy is a new signature of consciousness loss. |
| 2020 |
| Abstract: | Detecting and learning temporal regularities is essential to accurately predict the future. Past research indicates that humans are sensitive to two types of sequential regularities: deterministic rules, which afford sure predictions, and statistical biases, which govern the probabilities of individual items and their transitions. How does the human brain arbitrate between those two types? We used finger tracking to continuously monitor the online build-up of evidence, confidence, false alarms and changes-of-mind during sequence learning. All these aspects of behaviour conformed tightly to a hierarchical Bayesian inference model with distinct hypothesis spaces for statistics and rules, yet linked by a single probabilistic currency. Alternative models based either on a single statistical mechanism or on two non-commensurable systems were rejected. Our results indicate that a hierarchical Bayesian inference mechanism, capable of operating over several distinct hypothesis spaces, underlies the human capability to learn both statistics and rules. |
| Abstract: | Nonhuman primate neuroimaging is on the cusp of a transformation, much in the same way its human counterpart was in 2010, when the Human Connectome Project was launched to accelerate progress. Inspired by an open data-sharing initiative, the global community recently met and, in this article, breaks through obstacles to define its ambitions. |
| Abstract: | We present an extension of the Individual Brain Charting dataset -a high spatial-resolution, multi-task, functional Magnetic Resonance Imaging dataset, intended to support the investigation on the functional principles governing cognition in the human brain. The concomitant data acquisition from the same 12 participants, in the same environment, allows to obtain in the long run finer cognitive topographies, free from inter-subject and inter-site variability. This second release provides more data from psychological domains present in the first release, and also yields data featuring new ones. It includes tasks on e.g. mental time travel, reward, theory-of-mind, pain, numerosity, self-reference effect and speech recognition. In total, 13 tasks with 86 contrasts were added to the dataset and 63 new components were included in the cognitive description of the ensuing contrasts. As the dataset becomes larger, the collection of the corresponding topographies becomes more comprehensive, leading to better brain-atlasing frameworks. This dataset is an open-access facility; raw data and derivatives are publicly available in neuroimaging repositories. Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.12958181 |
| Abstract: | Humans have much more sophisticated communication skills than other species. They are not limited to emotional cries, alarm calls, and soothing demands; they also interpret the inner and outer world in a symbolic way, resulting in a collective intelligence and an accumulation of knowledge called culture. This culture permeates the child and fosters efficient learning, based on the knowledge accumulated through generations. To develop this collective intelligence, it requires (a) a social brain predisposed to learn from conspecifics, (b) awareness of one's mental state and knowledge and those of others, (c) a shared common language of thought, and (d) a communication system for exchanging this information. We insist on the value of symbolic representations as a compressed, necessary format for representing information to ourselves and exchanging information with others. We propose that human cognition has been boosted beyond the cognition of other primates by the multiplicative advantage of codevelopment of social cognition, language but also symbolic thinking that can be observed from the first months of life on |
| 2019 |
| Abstract: | As human beings, we can understand spoken language, recognize the opening bars of Beethoven's 5th Symphony, notice the tide-induced fluctuations of the level of the ocean, predict the color of traffic lights, and identify many more of the ubiquitous temporal regularities that characterize our daily environment. How does the human brain detect, identify, process and leverage those regularities in spite of their striking diversity? In this dissertation, I studied the mechanisms through which the human brain acquires knowledge of sequences and of regularities they may entail. To do so, I recorded behavioural and neural responses to auditory binary sequences characterized by various types of regularities. In parallel, I derived mathematical models of sequence processing that rest upon normative principles of probabilistic inference, but that are yet characterized by different computational architectures, and used human data to arbitrate among them. Using this same general approach, I investigated three different facets of the human sensitivity to sequences. Firstly, I demonstrated that a simple machinery for inferring transition structures between sequence items supports various aspects of the human perception of sequences encountered in seemingly disparate studies. Secondly, I then found that this learning algorithm was implemented in distinct brain systems which extracted statistical trends over different timescales, thereby providing a mechanistic explanation for the human sensitivity to both global statistical biases and to the recent history of observations. In addition to statistical learning, humans also possess the ability to quickly detect and identify deterministic rules. Thirdly, I showed that statistics and rules correspond to two distinct hypothesis spaces, instead of a continuum; and that human subjects could rationally arbitrate among them given the observed sequence. Altogether, my investigations of the cognitive foundations, computational principles and neural architectures supporting sequence processing suggest that the human brain is equipped with several systems that conform to normative principles of probabilistic inference but that are specialized in different aspects of sequences, thereby providing a putative explanation for the human perception of a vast repertoire of temporal regularities. |
| Abstract: | A central goal in cognitive science is to parse the series of processing stages underlying a cognitive task. A powerful yet simple behavioral method that can resolve this problem is finger trajectory tracking: by continuously tracking the finger position and speed as a participant chooses a response, and by analyzing which stimulus features affect the trajectory at each time point during the trial, we can estimate the absolute timing and order of each processing stage, and detect transient effects, changes of mind, serial versus parallel processing, and real-time fluctuations in subjective confidence. We suggest that trajectory tracking, which provides considerably more information than mere response times, may provide a comprehensive understanding of the fast temporal dynamics of cognitive operations. |
| Abstract: | Recent work has shown that LSTMs trained on a generic language modeling objective capture syntax-sensitive generalizations such as long-distance number agreement. We have however no mechanistic understanding of how they accomplish this remarkable feat. Some have conjectured it depends on heuristics that do not truly take hierarchical structure into account. We present here a detailed study of the inner mechanics of number tracking in LSTMs at the single neuron level. We discover that long-distance number information is largely managed by two {``}number units{''}. Importantly, the behaviour of these units is partially controlled by other units independently shown to track syntactic structure. We conclude that LSTMs are, to some extent, implementing genuinely syntactic processing mechanisms, paving the way to a more general understanding of grammatical encoding in LSTMs. |
| 2018 |
| Abstract: | The left hemisphere specialization for language is a well-established asymmetry in the human brain. Structural and functional asymmetries are observed as early as the prenatal period suggesting genetically determined differences between both hemispheres. The corpus callosum is a large tract connecting mostly homologous areas; some have proposed that it might participate in an enhancement of the left-hemispheric advantage to process speech. To investigate its role in early development, we compared 13 3--4-month-old infants with an agenesis of the corpus callosum (``AgCC'') with 18 typical infants using high-density electroencephalography in an auditory task. We recorded event-related potentials for speech stimuli (syllables and babbling noise), presented binaurally (same syllable in both ears), monaurally (babbling noise in one ear) and dichotically (syllable in one ear and babbling noise in the other ear). In response to these stimuli, both groups developed an anterior positivity synchronous with a posterior negativity, yet the topography significantly differed between groups likely due to the atypical gyration of the medial surface in AgCC. In particular, the anterior positivity was lateral in AgCC infants while it covered the midline in typical infants. We then measured the latencies of the main auditory response (P2 at this age) for the different conditions on the symmetrical left and right clusters. The main difference between groups was a {\textasciitilde}{\thinspace}60ms delay in typical infants relative to AgCC, for the ipsilateral response (i.e. left hemisphere) to babbling noise presented in the left ear, whereas no difference was observed in the case of right-ear stimulation. We suggest that our results highlight an asymmetrical callosal connectivity favoring the right-to-left hemisphere direction in typical infants. This asymmetry, similar to recent descriptions in adults, might contribute to an enhancement of left lateralization for language processing beyond the initial cortical left-hemisphere advantage. |
| Abstract: | Self-initiated movements are reliably preceded by a gradual buildup of neuronal activity known as the readiness potential (RP). Recent evidence suggests that the RP may reflect sub-threshold stochastic fluctuations in neural activity that can be modeled as a process of accumulation to bound. One element of accumulator models that has been largely overlooked in the literature is the stochastic term, which is traditionally modeled as Gaussian white noise. While there may be practical reasons for this choice, we have long known that noise in neural systems is not white as it is long-term correlated with spectral density of the form 1/f (with roughly 1 < < 3) across a broad range of spatial scales. I explored the behavior of a leaky stochastic accumulator when the noise over which it accumulates is temporally autocorrelated. I also allowed for the possibility that the RP, as measured at the scalp, might reflect the input to the accumulator (i.e. its stochastic noise component) rather than its output. These two premises led to two novel predictions that I empirically confirmed on behavioral and electroencephalography data from human subjects performing a self-initiated movement task. In addition to generating these two predictions, the model also suggested biologically plausible levels of autocorrelation, consistent with the degree of autocorrelation in our empirical data and in prior reports. These results expose new perspectives for accumulator models by suggesting that the spectral properties of the stochastic input should be allowed to vary, consistent with the nature of biological neural noise.Significance Statement The cortical â??readiness potentialâ?? (RP) is a gradual buildup of scalp electrical potential, and underlying neural activity in motor areas, that reliably precedes the onset of voluntary self-initiated movements by up to one second or more. More than fifty years after its discovery, the functional nature of the RP remains unclear. Here I argue, based on empirical evidence, that the RP reflects the stochastic input to an accumulation-to-bound decision process, and that this stochastic input is temporally autocorrelated, and not Gaussian white noise as it is traditionally modeled. The argument is supported by testing and confirming two novel predictions that emerge from an accumulator model when the stochastic input noise is autocorrelated rather than white. |
| 2017 |
| Abstract: | In physics â??entrainmentâ?? refers to the synchronization of two coupled oscillators with similar fundamental frequencies. In behavioral science, entrainment refers to the tendency of humans to synchronize their movements with rhythmic stimuli. Here, we asked whether human subjects performing a tapping task would entrain their tapping to an undetected auditory rhythm surreptitiously introduced in the guise of ambient background noise in the room. Subjects performed two different tasks, one in which they tapped their finger at a steady rate of their own choosing and one in which they performed a single abrupt finger tap on each trial after a delay of their own choosing. In both cases we found that subjects tended to tap in phase with the inducing modulation, with some variability in the preferred phase across subjects, consistent with prior research. In the repetitive tapping task, if the frequency of the inducing stimulus was far from the subjectâ??s own self-paced frequency, then entrainment was abolished, consistent with the properties of entrainment in physics. Thus, undetected ambient noise can influence self-generated movements. This suggests that uncued decisions to act are never completely endogenous, but are subject to subtle unnoticed influences from the sensory environment. |
| 2016 |
| Abstract: | The origins of human abilities for mathematics are debated: Some theories suggest that they are founded upon evolutionarily ancient brain circuits for number and space and others that they are grounded in language competence. To evaluate what brain systems underlie higher mathematics, we scanned professional mathematicians and mathematically naive subjects of equal academic standing as they evaluated the truth of advanced mathematical and nonmathematical statements. In professional mathematicians only, mathematical statements, whether in algebra, analysis, topology or geometry, activated a reproducible set of bilateral frontal, Intraparietal, and ventrolateral temporal regions. Crucially, these activations spared areas related to language and to general-knowledge semantics. Rather, mathematical judgments were related to an amplification of brain activity at sites that are activated by numbers and formulas in nonmathematicians, with a corresponding reduction in nearby face responses. The evidence suggests that high-level mathematical expertise and basic number sense share common roots in a nonlinguistic brain circuit. |
| Abstract: | The meaning of words referring to concrete items is thought of as a multidimensional representation that includes both perceptual (e.g., average size, prototypical color) and conceptual (e.g., taxonomic class) dimensions. Are these different dimensions coded in different brain regions? In healthy human subjects, we tested the presence of a mapping between the implied real object size (a perceptual dimension) and the taxonomic categories at different levels of specificity (conceptual dimensions) of a series of words, and the patterns of brain activity recorded with functional magnetic resonance imaging in six areas along the ventral occipito-temporal cortical path. Combining multivariate pattern classification and representational similarity analysis, we found that the real object size implied by a word appears to be primarily encoded in early visual regions, while the taxonomic category and sub-categorical cluster in more anterior temporal regions. This anteroposterior gradient of information content indicates that different areas along the ventral stream encode complementary dimensions of the semantic space. |
| Abstract: | Syntax allows human beings to build an infinite number of sentences from a finite number of words. How this unique, productive power of human language unfolds over the course of language development is still hotly debated. When they listen to sentences comprising newly-learned words, do children generalize from their knowledge of the legal combinations of word categories or do they instead rely on strings of words stored in memory to detect syntactic errors? Using novel words taught in the lab, we recorded Evoked Response Potentials (ERPs) in two-year-olds and adults listening to grammatical and ungrammatical sentences containing syntactic contexts that had not been used during training. In toddlers, the ungrammatical use of words, even when they have been just learned, induced an early left anterior negativity (surfacing 100-400ms after target word onset) followed by a late posterior positivity (surfacing 700-900ms after target word onset) that was not observed in grammatical sentences. This late effect was remarkably similar to the P600 displayed by adults, suggesting that toddlers and adults perform similar syntactic computations. Our results thus show that toddlers build on-line expectations regarding the syntactic category of upcoming words in a sentence. |
| Abstract: | To comprehend language, listeners need to encode the relationship between words within sentences. This entails categorizing words into their appropriate word classes. Function words, consistently preceding words from specific categories (e.g., the ballNOUN, I speakVERB), provide invaluable information for this task, and children's sensitivity to such adjacent relationships develops early on in life. However, neighboring words are not the sole source of information regarding an item's word class. Here we examine whether young children also take into account preceding sentence context online during syntactic categorization. To address this question, we use the ambiguous French function word la which, depending on sentence context, can either be used as determiner (the, preceding nouns) or as object clitic (it, preceding verbs). French-learning 18-month-olds' evoked potentials (ERPs) were recorded while they listened to sentences featuring this ambiguous function word followed by either a noun or a verb (thus yielding a locally felicitous co-occurrence of la + noun or la + verb). Crucially, preceding sentence context rendered the sentence either grammatical or ungrammatical. Ungrammatical sentences elicited a late positivity (resembling a P600) that was not observed for grammatical sentences. Toddlers' analysis of the unfolding sentence was thus not limited to local co-occurrences, but rather took into account non-adjacent sentence context. These findings suggest that by 18 months of age, online word categorization is already surprisingly robust. This could be greatly beneficial for the acquisition of novel words. |
| Abstract: | The ability to detect our own errors is an essential component of action monitoring. Using a masking paradigm in normal adults, we recently discovered that some error-detection processes can proceed without awareness, while other markers of performance monitoring such as the Error-Related Negativity (ERN) are tightly linked to conscious perception. Interestingly, research on cognitive deficit in schizophrenia has shown that the ERN is altered in these patients. In the present study, we therefore tested if the error detection impairment in schizophrenia is specific to conscious perception or is also found under non-conscious conditions, probing whether these performance monitoring processes are truly distinct. Thirteen patients with schizophrenia and thirteen age-matched healthy control subjects performed a speeded number comparison task on masked stimuli while EEG and MEG signals were recorded. Conscious perception and error-detection were assessed on a trial-by-trial basis using subjective reports of visibility and confidence. We found that patients with schizophrenia presented altered cingulate error-detection responses in conscious trials, as reflected by a decreased ERN. By contrast, on unconscious trials, both controls and schizophrenia patients performed above chance in evaluating the likelihood of having made an error. This dissociation confirms the existence of two distinct performance monitoring systems, and suggests that conscious metacognition in schizophrenia is specifically altered while non-conscious performance monitoring remains preserved. |
| Abstract: | Pure alexia is an acquired reading disorder, typically due to a left occipito-temporal lesion affecting the Visual Word Form Area (VWFA). It is unclear whether the VWFA acts as a unique bottleneck for reading, or whether alternative routes are available for recovery. Here, we address this issue through the single-case longitudinal study of a neuroscientist who experienced pure alexia and participated in 17 behavioral, 9 anatomical, and 9 fMRI assessment sessions over a period of two years. The origin of the impairment was assigned to a small left fusiform lesion, accompanied by a loss of VWFA responsivity and by the degeneracy of the associated white matter pathways. fMRI experiments allowed us to image longitudinally the visual perception of words, as compared to other classes of stimuli, as well as the mechanisms of letter-by-letter reading. The progressive improvement of reading was not associated with the re-emergence of a new area selective to words, but with increasing responses in spared occipital cortex posterior to the lesion and in contralateral right occipital cortex. Those regions showed a non-specific increase of activations over time and an increase in functional correlation with distant language areas. Those results confirm the existence of an alternative occipital route for reading, bypassing the VWFA, but they also point to its key limitation: the patient remained a slow letter-by-letter reader, thus supporting the critical importance of the VWFA for the efficient parallel recognition of written words. |
| Abstract: | Can babies think? A fundamental challenge for cognitive neuroscience is to answer when brain functions begin and in what form they first emerge. This is challenging with behavioral tasks, as it is difficult to communicate to an infant what a task requires, and motor function is impoverished, making execution of the appropriate response difficult. To circumvent these requirements, neuroimaging provides a complementary route for assessing the emergence of cognition. Starting from the prerequisites of cognitive function and building stepwise, we review when the cortex forms and when it becomes gyrated and regionally differentiated. We then discuss when white matter tracts mature and when functional brain networks arise. Finally, we assess the responsiveness of these brain systems to external events. We find that many cognitive systems are observed surprisingly early. Some emerge before birth, with activations in the frontal lobe even in the first months of gestation. These discoveries are changing our understanding of the nature of cognitive networks and their early function, transforming cognitive neuroscience, and opening new windows for education and investigation. Infant neuroimaging also has tremendous clinical potential, for both detecting atypical development and facilitating earlier intervention. Finally, we discuss the key technical developments that are enabling this nascent field. |
| Abstract: | Linguistic processing is based on a close collaboration between temporal and frontal regions connected by two pathways: the "dorsal" and "ventral pathways" (assumed to support phonological and semantic processing, respectively, in adults). We investigated here the development of these pathways at the onset of language acquisition, during the first post-natal weeks, using cross-sectional diffusion imaging in 21 healthy infants (6-22 weeks of age) and 17 young adults. We compared the bundle organization and microstructure at these two ages using tractography and original clustering analyses of diffusion tensor imaging parameters. We observed structural similarities between both groups, especially concerning the dorsal/ventral pathway segregation and the arcuate fasciculus asymmetry. We further highlighted the developmental tempos of the linguistic bundles: The ventral pathway maturation was more advanced than the dorsal pathway maturation, but the latter catches up during the first post-natal months. Its fast development during this period might relate to the learning of speech cross-modal representations and to the first combinatorial analyses of the speech input. |
| Abstract: | The human species has developed complex mathematical skills which likely emerge from a combination of multiple foundational abilities. One of them seems to be a preverbal capacity to extract and manipulate the numerosity of sets of objects which is shared with other species and in humans is thought to be integrated with symbolic knowledge to result in a more abstract representation of numerical concepts. For what concerns the functional neuroanatomy of this capacity, neuropsychology and functional imaging have localized key substrates of numerical processing in parietal and frontal cortex. However, traditional fMRI mapping relying on a simple subtraction approach to compare numerical and nonnumerical conditions is limited to tackle with sufficient precision and detail the issue of the underlying code for number, a question which more easily lends itself to investigation by methods with higher spatial resolution, such as neurophysiology. In recent years, progress has been made through the introduction of approaches sensitive to within-category discrimination in combination with fMRI (adaptation and multivariate pattern recognition), and the present review summarizes what these have revealed so far about the neural coding of individual numbers in the human brain, the format of these representations and parallels between human and monkey neurophysiology findings. |
| Abstract: | The ability to imagine ourselves in the past, in the future or in different spatial locations suggests that the brain can generate cognitive maps that are independent of the experiential self in the here and now. Using three experiments, we asked to which extent Mental Time Travel (MTT; imagining the self in time) and Mental Space Navigation (MSN; imagining the self in space) shared similar cognitive operations. For this, participants judged the ordinality of real historical events in time and in space with respect to different mental perspectives: for instance, participants mentally projected themselves in Paris in nine years, and judged whether an event occurred before or after, or, east or west, of where they mentally stood. In all three experiments, symbolic distance effects in time and space dimensions were quantified using Reaction Times (RT) and Error Rates (ER). When self-projected, participants were slower and were less accurate (absolute distance effects); participants were also faster and more accurate when the spatial and temporal distances were further away from their mental viewpoint (relative distance effects). These effects show that MTT and MSN require egocentric mapping and that self-projection requires map transformations. Additionally, participants' performance was affected when self-projection was made in one dimension but judgements in another, revealing a competition between temporal and spatial mapping (Experiment 2 & 3). Altogether, our findings suggest that MTT and MSN are separately mapped although they require comparable allo- to ego-centric map conversion. |
| Abstract: | During speech listening, the brain parses a continuous acoustic stream of information into computational units (e.g. syllables or words) necessary for speech comprehension. Recent neuroscientific hypotheses propose that neural oscillations contribute to speech parsing, but whether they do so on the basis of acoustic cues (bottom-up acoustic parsing) or as a function of available linguistic representations (top-down linguistic parsing) is unknown. In this magnetoencephalography study, we contrasted acoustic and linguistic parsing using bistable speech sequences. While listening to the speech sequences, participants were asked to maintain one of the two possible speech percepts through volitional control. We predicted that the tracking of speech dynamics by neural oscillations would not only follow the acoustic properties but also shift in time according to the participant's conscious speech percept. Our results show that the latency of high-frequency activity (specifically, beta and gamma bands) varied as a function of the perceptual report. In contrast, the phase of low-frequency oscillations was not strongly affected by top-down control. While changes in low-frequency neural oscillations were compatible with the encoding of pre-lexical segmentation cues, high-frequency activity specifically informed on an individual's conscious speech percept. |
| Abstract: | During the last trimester of human gestation, neurons reach their final destination and establish long- and short-distance connections. Due to the difficulties obtaining functional data at this age, the characteristics of the functional architecture at the onset of sensory thalamocortical connectivity in humans remain largely unknown. In particular, it is unknown to what extent responses evoked by an external stimulus are general or already sensitive to certain stimuli. In the present study, we recorded high-density event-related potentials (ERPs) in 19 neonates, tested ten weeks before term (28-32 weeks gestational age (wGA), that is, at an average age of 30 wGA) by means of a syllable discrimination task (i.e., a phonetic change: ba vs. ga; and a voice change: male vs. female voice). We first observed that the syllables elicited 4 peaks with distinct topographies implying a progression of the sensory input along a processing hierarchy; second, repetition induced a decrease in the amplitude (repetition suppression) of these peaks, but their latencies and topographies remained stable; and third, a change of stimulus generated mismatch responses, which were more precisely time-locked to event onset in the case of a phonetic change than in the case of a voice change. A hierarchical and parallel functional architecture is therefore able to process environmental sounds in a timely precise fashion, well before term birth. This elaborate functional architecture at the onset of extrinsic neural activity suggests that specialized areas weakly dependent on the environment are present in the perisylvian region as part of the genetic endowment of the human species. |
| Abstract: | Number is a complex category, as with the word "number" we may refer to different entities. First, it is a perceptual property that characterizes any set of individual items, namely its cardinality. The ability to extract the (approximate) cardinality of sets is almost universal in the animal domain and present in humans since birth. In primates, posterior parietal cortex seems to be a crucial site for this ability, even if the degree of selectivity of numerical representations in parietal cortex reported to date appears much lower compared to that of other semantic categories in the ventral stream. Number can also be intended as a mathematical object, which we humans use to count, measure, and order: a (verbal or visual) symbol that stands for the cardinality of a set, the intensity of a continuous quantity or the position of an item on a list. Evidence points to a convergence towards parietal cortex for the semantic coding of numerical symbols and to the bilateral occipitotemporal cortex for the shape coding of Arabic digits and other number symbols. |
| Abstract: | It is generally accepted in neuroscience that anatomy and function go hand in hand. Accordingly, a local morphological variability could lead to a corresponding functional variability. In this study, we tested this hypothesis by linking the variability of the cortical folding pattern of 252 right-handed subjects to the localization or the pattern of functional activations induced by hand motion or silent reading. Three regions are selected: the central sulcus, the precentral sulcus and the superior temporal sulcus (STS). "Essential morphological variability traits" are identified using a method building upon multidimensional scaling. The link between variability in anatomy and function is confirmed by the perfect match between the central sulcus morphological "hand knob" and the corresponding motor activation: as the location of the hand knob moves more or less dorsally along the central sulcus, the motor hand activation moves accordingly. Furthermore, the size of the left hand activation in the right hemisphere is correlated with the knob location in the central sulcus. A new link between functional and morphological variability is discovered relative to the location of a premotor activation induced by silent reading. While this reading activation is located next to the wall of the central sulcus when the hand knob has a ventral positioning, it is pushed into a deep gyrus interrupting the precentral sulcus when the knob is more dorsal. Finally, it is shown that the size of the reading activation along the STS is larger when the posterior branches are less developed. |
| Abstract: | Primate brains can detect a variety of unexpected deviations in auditory sequences. The local-global paradigm dissociates two hierarchical levels of auditory predictive coding by examining the brain responses to first-order (local) and second-order (global) sequence violations. Using the macaque model, we previously demonstrated that, in the awake state, local violations cause focal auditory responses while global violations activate a brain circuit comprising prefrontal, parietal and cingulate cortices. Here we used the same local-global auditory paradigm to clarify the encoding of the hierarchical auditory regularities in anesthetized monkeys and compared their brain responses to those obtained in the awake state as measured with fMRI. Both, propofol, a GABAA-agonist, and ketamine, an NMDA-antagonist, left intact or even enhanced the cortical response to auditory inputs. The local effect vanished during propofol anesthesia and shifted spatially during ketamine anesthesia compared with wakefulness. Under increasing levels of propofol, we observed a progressive disorganization of the global effect in prefrontal, parietal and cingulate cortices and its complete suppression under ketamine anesthesia. Anesthesia also suppressed thalamic activations to the global effect. These results suggest that anesthesia preserves initial auditory processing, but disturbs both short-term and long-term auditory predictive coding mechanisms. The disorganization of auditory novelty processing under anesthesia relates to a loss of thalamic responses to novelty and to a disruption of higher-order functional cortical networks in parietal, prefrontal and cingular cortices. |
| Abstract: | This study examines the structural integrity and the hemispheric lateralization patterns of four major association fiber pathways in a group of French dyslexic children and age-matched controls (from 9 to 14 years), using high angular diffusion imaging combined with spherical deconvolution tractography. Compared with age-matched controls, dyslexic children show increased hindrance-modulated oriented anisotropy (HMOA) in the right superior longitudinal fasciculus (SLF). They also show a reduced leftward asymmetry of the inferior fronto-occipital fasciculus (IFOF) and an increased rightward asymmetry of the second branch of the SLF (SLF II). The lateralization pattern of IFOF and SLF II also accounts for individual differences in dyslexic children's reading abilities. These data provide evidence for an abnormal lateralization of occipito-frontal and parieto-frontal pathways in developmental dyslexia. |
| 2015 |
| Abstract: | Distinct preference for visual number symbols was recently discovered in the human right inferior temporal gyrus (rITG). It remains unclear how this preference emerges, what is the contribution of shape biases to its formation and whether visual processing underlies it. Here we use congenital blindness as a model for brain development without visual experience. During fMRI, we present blind subjects with shapes encoded using a novel visual-to-music sensory-substitution device (The EyeMusic). Greater activation is observed in the rITG when subjects process symbols as numbers compared with control tasks on the same symbols. Using resting-state fMRI in the blind and sighted, we further show that the areas with preference for numerals and letters exhibit distinct patterns of functional connectivity with quantity and language-processing areas, respectively. Our findings suggest that specificity in the ventral 'visual' stream can emerge independently of sensory modality and visual experience, under the influence of distinct connectivity patterns. |
| Abstract: | At rest, the brain is traversed by spontaneous functional connectivity patterns. Two hypotheses have been proposed for their origins: they may reflect a continuous stream of ongoing cognitive processes as well as random fluctuations shaped by a fixed anatomical connectivity matrix. Here we show that both sources contribute to the shaping of resting-state networks, yet with distinct contributions during consciousness and anesthesia. We measured dynamical functional connectivity with functional MRI during the resting state in awake and anesthetized monkeys. Under anesthesia, the more frequent functional connectivity patterns inherit the structure of anatomical connectivity, exhibit fewer small-world properties, and lack negative correlations. Conversely, wakefulness is characterized by the sequential exploration of a richer repertoire of functional configurations, often dissimilar to anatomical structure, and comprising positive and negative correlations among brain regions. These results reconcile theories of consciousness with observations of long-range correlation in the anesthetized brain and show that a rich functional dynamics might constitute a signature of consciousness, with potential clinical implications for the detection of awareness in anesthesia and brain-lesioned patients. |
| Abstract: | Isolated corpus callosum dysgenesis (CCD) is a congenital malformation which occurs during early development of the brain. In this study, we aimed to identify and describe its consequences beyond the lack of callosal fibres, on the morphology, microstructure and asymmetries of the main white matter bundles with diffusion imaging and fibre tractography. Seven children aged between 9 and 13 years old and seven age- and gender-matched control children were studied. First, we focused on bundles within the mesial region of the cerebral hemispheres: the corpus callosum, Probst bundles and cingulum which were selected using a conventional region-based approach. We demonstrated that the Probst bundles have a wider connectivity than the previously described rostrocaudal direction, and a microstructure rather distinct from the cingulum but relatively close to callosal remnant fibres. A sigmoid bundle was found in two partial ageneses. Second, the corticospinal tract, thalamic radiations and association bundles were extracted automatically via an atlas of adult white matter bundles to overcome bias resulting from a priori knowledge of the bundles' anatomical morphology and trajectory. Despite the lack of callosal fibres and the colpocephaly observed in CCD, all major white matter bundles were identified with a relatively normal morphology, and preserved microstructure (i.e. fractional anisotropy, mean diffusivity) and asymmetries. Consequently the bundles' organisation seems well conserved in brains with CCD. These results await further investigations with functional imaging before apprehending the cognition variability in children with isolated dysgenesis. |
| Abstract: | By adulthood, literate humans have been exposed to millions of visual scenes and pages of text. Does the human visual system become attuned to the statistics of its inputs? Using functional magnetic resonance imaging, we examined whether the brain responses to line configurations are proportional to their natural-scene frequency. To further distinguish prior cortical competence from adaptation induced by learning to read, we manipulated whether the selected configurations formed letters and whether they were presented on the horizontal meridian, the familiar location where words usually appear, or on the vertical meridian. While no natural-scene frequency effect was observed, we observed letter-status and letter frequency effects on bilateral occipital activation, mainly for horizontal stimuli. The findings suggest a reorganization of the visual pathway resulting from reading acquisition under genetic and connectional constraints. Even early retinotopic areas showed a stronger response to letters than to rotated versions of the same shapes, suggesting an early visual tuning to large visual features such as letters. |
| Abstract: | The acquisition of literacy transforms the human brain. By reviewing studies of illiterate subjects, we propose specific hypotheses on how the functions of core brain systems are partially reoriented or 'recycled' when learning to read. Literacy acquisition improves early visual processing and reorganizes the ventral occipito-temporal pathway: responses to written characters are increased in the left occipito-temporal sulcus, whereas responses to faces shift towards the right hemisphere. Literacy also modifies phonological coding and strengthens the functional and anatomical link between phonemic and graphemic representations. Literacy acquisition therefore provides a remarkable example of how the brain reorganizes to accommodate a novel cultural skill. |
| Abstract: | A sequence of images, sounds, or words can be stored at several levels of detail, from specific items and their timing to abstract structure. We propose a taxonomy of five distinct cerebral mechanisms for sequence coding: transitions and timing knowledge, chunking, ordinal knowledge, algebraic patterns, and nested tree structures. In each case, we review the available experimental paradigms and list the behavioral and neural signatures of the systems involved. Tree structures require a specific recursive neural code, as yet unidentified by electrophysiology, possibly unique to humans, and which may explain the singularity of human language and cognition. |
| Abstract: | Macaque electrophysiology has revealed neurons responsive to number in lateral (LIP) and ventral (VIP) intraparietal areas. Recently, fMRI pattern recognition revealed information discriminative of individual numbers in human parietal cortex but without precisely localizing the relevant sites or testing for subregions with different response profiles. Here, we defined the human functional equivalents of LIP (feLIP) and VIP (feVIP) using neurophysiologically motivated localizers. We applied multivariate pattern recognition to investigate whether both regions represent numerical information and whether number codes are position specific or invariant. In a delayed number comparison paradigm with laterally presented numerosities, parietal cortex discriminated between numerosities better than early visual cortex, and discrimination generalized across hemifields in parietal, but not early visual cortex. Activation patterns in the 2 parietal regions of interest did not differ in the coding of position-specific or position-independent number information, but in the expression of a numerical distance effect which was more pronounced in feLIP.Thus, the representation of number in parietal cortex is at least partially position invariant. Both feLIP and feVIP contain information about individual numerosities in humans, but feLIP hosts a coarser representation of numerosity than feVIP, compatible with either broader tuning or a summation code. |
| Abstract: | Magnetoencephalography and electroencephalography (M/EEG) measure non-invasively the weak electromagnetic fields induced by post-synaptic neural currents. The estimation of the spatial covariance of the signals recorded on M/EEG sensors is a building block of modern data analysis pipelines. Such covariance estimates are used in brain-computer interfaces (BCI) systems, in nearly all source localization methods for spatial whitening as well as for data covariance estimation in beamformers. The rationale for such models is that the signals can be modeled by a zero mean Gaussian distribution. While maximizing the Gaussian likelihood seems natural, it leads to a covariance estimate known as empirical covariance (EC). It turns out that the EC is a poor estimate of the true covariance when the number of samples is small. To address this issue the estimation needs to be regularized. The most common approach downweights off-diagonal coefficients, while more advanced regularization methods are based on shrinkage techniques or generative models with low rank assumptions: probabilistic PCA (PPCA) and factor analysis (FA). Using cross-validation all of these models can be tuned and compared based on Gaussian likelihood computed on unseen data. We investigated these models on simulations, one electroencephalography (EEG) dataset as well as magnetoencephalography (MEG) datasets from the most common MEG systems. First, our results demonstrate that different models can be the best, depending on the number of samples, heterogeneity of sensor types and noise properties. Second, we show that the models tuned by cross-validation are superior to models with hand-selected regularization. Hence, we propose an automated solution to the often overlooked problem of covariance estimation of M/EEG signals. The relevance of the procedure is demonstrated here for spatial whitening and source localization of MEG signals. |
| Abstract: | At school, children with Developmental Coordination Disorder (DCD) struggle with mathematics. However, little attention has been paid to their numerical cognition abilities. The goal of this study was to better understand the cognitive basis for mathematical difficulties in children with DCD. Twenty 7-to-10 years-old children with DCD were compared to twenty age-matched typically developing children using dot and digit comparison tasks to assess symbolic and nonsymbolic number processing and in a task of single digits additions. Results showed that children with DCD had lower performance in nonsymbolic and symbolic number comparison tasks than typically developing children. They were also slower to solve simple addition problems. Moreover, correlational analyses showed that children with DCD who experienced greater impairments in the nonsymbolic task also performed more poorly in the symbolic tasks. These findings suggest that DCD impairs both nonsymbolic and symbolic number processing. A systematic assessment of numerical cognition in children with DCD could provide a more comprehensive picture of their deficits and help in proposing specific remediation. |
| Abstract: | Deep in the occipitotemporal cortex lie two functional regions, the visual word form area (VWFA) and the number form area (NFA), which are thought to play a special role in letter and number recognition, respectively. We review recent progress made in characterizing the origins of these symbol form areas in children or adults, sighted or blind subjects, and humans or monkeys. We propose two non-mutually-exclusive hypotheses on the origins of the VWFA and NFA: the presence of a connectivity bias, and a sensitivity to shape features. We assess the explanatory power of these hypotheses, describe their consequences, and offer several experimental tests. |
| Abstract: | Complex number words (e.g., "twenty two") are formed by merging together several simple number words (e.g., "twenty" and "two"). In the present study, we explored the neural correlates of this operation and investigated to what extent it engages brain areas involved processing numerical quantity and linguistic syntactic structure. Participants speaking two typologically distinct languages, French and Chinese, were required to read aloud sequences of simple number words while their cerebral activity was recorded by functional magnetic resonance imaging. Each number word could either be merged with the previous ones (e.g., 'twenty three') or not (e.g., 'three twenty'), thus forming four levels ranging from lists of number words to complex numerals. When a number word could be merged with the preceding ones, it was named faster than when it could not. Neuroimaging results showed that the number of merges correlated with activation in the left inferior frontal gyrus and in the left inferior parietal lobule. Consistent findings across Chinese and French participants suggest that these regions serve as the neural bases for forming complex number words in different languages. |
| Abstract: | This research tests the hypothesis that comprehension of human events will engage an extended semantic representation system, independent of the input modality (sentence vs. picture). To investigate this, we examined brain activation and connectivity in 19 subjects who read sentences and viewed pictures depicting everyday events, in a combined fMRI and DTI study. Conjunction of activity in understanding sentences and pictures revealed a common fronto-temporo-parietal network that included the middle and inferior frontal gyri, the parahippocampal-retrosplenial complex, the anterior and middle temporal gyri, the inferior parietal lobe in particular the temporo-parietal cortex. DTI tractography seeded from this temporo-parietal cortex hub revealed a multi-component network reaching into the temporal pole, the ventral frontal pole and premotor cortex. A significant correlation was found between the relative pathway density issued from the temporo-parietal cortex and the imageability of sentences for individual subjects, suggesting a potential functional link between comprehension and the temporo-parietal connectivity strength. These data help to define a "meaning" network that includes components of recently characterized systems for semantic memory, embodied simulation, and visuo-spatial scene representation. The network substantially overlaps with the "default mode" network implicated as part of a core network of semantic representation, along with brain systems related to the formation of mental models, and reasoning. These data are consistent with a model of real-world situational understanding that is highly embodied. Crucially, the neural basis of this embodied understanding is not limited to sensorimotor systems, but extends to the highest levels of cognition, including autobiographical memory, scene analysis, mental model formation, reasoning and theory of mind. |
| Abstract: | Using electroencephalography, we examined 8-month-old infants' ability to discover a systematic dependency between the first and third syllables of successive words, concatenated into a monotonous speech stream, and to subsequently generalize this regularity to new items presented in isolation. Full-term and preterm infants, while exposed to the stream, displayed a significant entrainment (phase-locking) to the syllabic and word frequencies, demonstrating that they were sensitive to the word unit. The acquisition of the systematic dependency defining words was confirmed by the significantly different neural responses to rule-words and part-words subsequently presented during the test phase. Finally, we observed a correlation between syllabic entrainment during learning and the difference in phase coherence between the test conditions (rule-words vs part-words) suggesting that temporal processing of the syllable unit might be crucial in linguistic learning. No group difference was observed suggesting that non-adjacent statistical computations are already robust at 8months, even in preterm infants, and thus develop during the first year of life, earlier than expected from behavioral studies. |
| Abstract: | Identifying potentially unique features of the human cerebral cortex is a first step to understanding how evolution has shaped the brain in our species. By analyzing MR images obtained from 177 humans and 73 chimpanzees, we observed a human-specific asymmetry in the superior temporal sulcus at the heart of the communication regions and which we have named the "superior temporal asymmetrical pit" (STAP). This 45-mm-long segment ventral to Heschl's gyrus is deeper in the right hemisphere than in the left in 95\% of typical human subjects, from infanthood till adulthood, and is present, irrespective of handedness, language lateralization, and sex although it is greater in males than in females. The STAP also is seen in several groups of atypical subjects including persons with situs inversus, autistic spectrum disorder, Turner syndrome, and corpus callosum agenesis. It is explained in part by the larger number of sulcal interruptions in the left than in the right hemisphere. Its early presence in the infants of this study as well as in fetuses and premature infants suggests a strong genetic influence. Because this asymmetry is barely visible in chimpanzees, we recommend the STAP region during midgestation as an important phenotype to investigate asymmetrical variations of gene expression among the primate lineage. This genetic target may provide important insights regarding the evolution of the crucial cognitive abilities sustained by this sulcus in our species, namely communication and social cognition. |
| Abstract: | Humans readily introspect upon their thoughts and their behavior, but how reliable are these subjective reports? In the present study, we explored the consistencies of and differences between the observer's subjective report and actual behavior within a single trial. On each trial of a serial search task, we recorded eye movements and the participants' beliefs of where their eyes moved. The comparison of reported versus real eye movements revealed that subjects successfully reported a subset of their eye movements. Limits in subjective reports stemmed from both the number and the type of eye movements. Furthermore, subjects sometimes reported eye movements they actually never made. A detailed examination of these reports suggests that they could reflect covert shifts of attention during overt serial search. Our data provide quantitative and qualitative measures of observers' subjective reports and reveal experimental effects of visual search that would otherwise be inaccessible. |
| Abstract: | We report the case of a 14-year-old girl suffering from severe developmental visual impairment along with delayed language and cognitive development, and featuring a clear-cut dissociation between spared dorsal and impaired ventral visual pathways. Visual recognition of objects, including faces and printed words, was affected. In contrast, movement perception and visually guided motor control were preserved. Structural MRI was normal on inspection, but Voxel Based Morphometry (VBM) revealed reduced grey matter density in the mesial occipital and ventral occipito-temporal cortex. Functional MRI during the perception of line drawings uncovered impaired differentiation which is normally observed at even younger ages: no local category preferences could be identified within the occipito-temporal cortex for faces, houses, words or tools. In contrast, movement-related activations appeared to be normal. Finally, those abnormalities evolved on the background of chronic bilateral occipital epileptic activity, including continuous spike-wave discharges during sleep, which may be considered as the primary cause of non-specific intellectual disability and visual impairment. |
| Abstract: | The auditory neural representations of infants can easily be studied with electroencephalography using mismatch experimental designs. We recorded high-density event-related potentials while 3-month-old infants were listening to trials consisting of CV syllables produced with different vowels (/bX/ or /gX/). The consonant remained the same for the first three syllables, followed (or not) by a change in the fourth position. A consonant change evoked a significant difference around the second auditory peak (400-600 ms) relative to control trials. This mismatch response demonstrates that the infants robustly categorized the consonant despite coarticulation that blurs the phonetic cues, and at an age at which they do not produce these consonants themselves. This response was obtained even when infants had no visual articulatory information to help them to track the consonant repetition. In combination with previous studies establishing categorical perception and normalization across speakers, this result demonstrates that preverbal infants already have abstract phonetic representation integrating over acoustical features in the first months of life. |
| Abstract: | Learning in a stochastic environment consists of estimating a model from a limited amount of noisy data, and is therefore inherently uncertain. However, many classical models reduce the learning process to the updating of parameter estimates and neglect the fact that learning is also frequently accompanied by a variable "feeling of knowing" or confidence. The characteristics and the origin of these subjective confidence estimates thus remain largely unknown. Here we investigate whether, during learning, humans not only infer a model of their environment, but also derive an accurate sense of confidence from their inferences. In our experiment, humans estimated the transition probabilities between two visual or auditory stimuli in a changing environment, and reported their mean estimate and their confidence in this report. To formalize the link between both kinds of estimate and assess their accuracy in comparison to a normative reference, we derive the optimal inference strategy for our task. Our results indicate that subjects accurately track the likelihood that their inferences are correct. Learning and estimating confidence in what has been learned appear to be two intimately related abilities, suggesting that they arise from a single inference process. We show that human performance matches several properties of the optimal probabilistic inference. In particular, subjective confidence is impacted by environmental uncertainty, both at the first level (uncertainty in stimulus occurrence given the inferred stochastic characteristics) and at the second level (uncertainty due to unexpected changes in these stochastic characteristics). Confidence also increases appropriately with the number of observations within stable periods. Our results support the idea that humans possess a quantitative sense of confidence in their inferences about abstract non-sensory parameters of the environment. This ability cannot be reduced to simple heuristics, it seems instead a core property of the learning process. |
| Abstract: | Research on confidence spreads across several sub-fields of psychology and neuroscience. Here, we explore how a definition of confidence as Bayesian probability can unify these viewpoints. This computational view entails that there are distinct forms in which confidence is represented and used in the brain, including distributional confidence, pertaining to neural representations of probability distributions, and summary confidence, pertaining to scalar summaries of those distributions. Summary confidence is, normatively, derived or "read out" from distributional confidence. Neural implementations of readout will trade off optimality versus flexibility of routing across brain systems, allowing confidence to serve diverse cognitive functions. |
| Abstract: | Recent human neurophysiological recordings have uncovered two fundamental modes of cerebral cortex activity with distinct dynamics: an active mode characterized by a rapid and sustained activity ("ignition") and a spontaneous (resting-state) mode, manifesting ultra-slow fluctuations of low amplitude. We propose that both dynamics reflect two faces of the same recurrent loop mechanism: an integration device that accumulates ongoing stochastic activity and, either spontaneously or in a task-driven manner, crosses a dynamic threshold and ignites, leading to content-specific awareness. The hypothesis can explain a rich set of behavioral and neuronal phenomena, such as perceptual threshold, the high non-linearity of visual responses, the subliminal nature of spontaneous activity fluctuations, and the slow activity buildup anticipating spontaneous behavior (e.g., readiness potential). Further elaborations of this unified scheme, such as a cascade of integrators with different ignition thresholds or multi-stable states, can account for additional complexities in the repertoire of human cortical dynamics. |
| Abstract: | The neural correlates of consciousness are typically sought by comparing the overall brain responses to perceived and unperceived stimuli. However, this comparison may be contaminated by non-specific attention, alerting, performance, and reporting confounds. Here, we pursue a novel approach, tracking the neuronal coding of consciously and unconsciously perceived contents while keeping behavior identical (blindsight). EEG and MEG were recorded while participants reported the spatial location and visibility of a briefly presented target. Multivariate pattern analysis demonstrated that considerable information about spatial location traverses the cortex on blindsight trials, but that starting ?270 ms post-onset, information unique to consciously perceived stimuli, emerges in superior parietal and superior frontal regions. Conscious access appears characterized by the entry of the perceived stimulus into a series of additional brain processes, each restricted in time, while the failure of conscious access results in the breaking of this chain and a subsequent slow decay of the lingering unconscious activity. |
| Abstract: | In humans and some other species perceptual decision-making is complemented by the ability to make confidence judgements about the certainty of sensory evidence. While both forms of decision process have been studied empirically, the precise relationship between them remains poorly understood. We performed an experiment that combined a perceptual decision-making task (identifying the category of a faint visual stimulus) with a confidence-judgement task (wagering on the accuracy of each perceptual decision). The visual stimulation paradigm required steady fixation, so we used eye-tracking to control for stray eye movements. Our data analyses revealed an unexpected and counterintuitive interaction between the steadiness of fixation (prior to and during stimulation), perceptual decision making, and post-decision wagering: greater variability in gaze direction during fixation was associated with significantly increased visual-perceptual sensitivity, but significantly decreased reliability of confidence judgements. The latter effect could not be explained by a simple change in overall confidence (i.e. a criterion artifact), but rather was tied to a change in the degree to which high wagers predicted correct decisions (i.e. the sensitivity of the confidence judgement). We found no evidence of a differential change in pupil diameter that could account for the effect and thus our results are consistent with fixational eye movements being the relevant covariate. However, we note that small changes in pupil diameter can sometimes cause artefactual fluctuations in measured gaze direction and this possibility could not be fully ruled out. In either case, our results suggest that perceptual decisions and confidence judgements can be processed independently and point toward a new avenue of research into the relationship between them. |
| Abstract: | According to recent evidence, stimulus-tuned neurons in the cerebral cortex exhibit reduced variability in firing rate across trials, after the onset of a stimulus. However, in order for a reduction in variability to be directly relevant to perception and behavior, it must be realized within trial--the pattern of activity must be relatively stable. Stability is characteristic of decision states in recurrent attractor networks, and its possible relevance to conscious perception has been suggested by theorists. However, it is difficult to measure on the within-trial time scales and broadly distributed spatial scales relevant to perception. We recorded simultaneous magneto- and electroencephalography (MEG and EEG) data while subjects observed threshold-level visual stimuli. Pattern-similarity analyses applied to the data from MEG gradiometers uncovered a pronounced decrease in variability across trials after stimulus onset, consistent with previous single-unit data. This was followed by a significant divergence in variability depending upon subjective report (seen/unseen), with seen trials exhibiting less variability. Applying the same analysis across time, within trial, we found that the latter effect coincided in time with a difference in the stability of the pattern of activity. Stability alone could be used to classify data from individual trials as "seen" or "unseen." The same metric applied to EEG data from patients with disorders of consciousness exposed to auditory stimuli diverged parametrically according to clinically diagnosed level of consciousness. Differences in signal strength could not account for these results. Conscious perception may involve the transient stabilization of distributed cortical networks, corresponding to a global brain-scale decision. |
| Abstract: | When presented with an auditory sequence, the brain acts as a predictive-coding device that extracts regularities in the transition probabilities between sounds and detects unexpected deviations from these regularities. Does such prediction require conscious vigilance, or does it continue to unfold automatically in the sleeping brain? The mismatch negativity and P300 components of the auditory event-related potential, reflecting two steps of auditory novelty detection, have been inconsistently observed in the various sleep stages. To clarify whether these steps remain during sleep, we recorded simultaneous electroencephalographic and magnetoencephalographic signals during wakefulness and during sleep in normal subjects listening to a hierarchical auditory paradigm including short-term (local) and long-term (global) regularities. The global response, reflected in the P300, vanished during sleep, in line with the hypothesis that it is a correlate of high-level conscious error detection. The local mismatch response remained across all sleep stages (N1, N2, and REM sleep), but with an incomplete structure; compared with wakefulness, a specific peak reflecting prediction error vanished during sleep. Those results indicate that sleep leaves initial auditory processing and passive sensory response adaptation intact, but specifically disrupts both short-term and long-term auditory predictive coding. |
| 2014 |
| Abstract: | The last two decades have seen an unprecedented development of human brain mapping approaches at various spatial and temporal scales. Together, these have provided a large fundus of information on many different aspects of the human brain including micro- and macrostructural segregation, regional specialization of function, connectivity, and temporal dynamics. Atlases are central in order to integrate such diverse information in a topographically meaningful way. It is noteworthy, that the brain mapping field has been developed along several major lines such as structure vs. function, postmortem vs. in vivo, individual features of the brain vs. population-based aspects, or slow vs. fast dynamics. In order to understand human brain organization, however, it seems inevitable that these different lines are integrated and combined into a multimodal human brain model. To this aim, we held a workshop to determine the constraints of a multi-modal human brain model that are needed to enable (i) an integration of different spatial and temporal scales and data modalities into a common reference system, and (ii) efficient data exchange and analysis. As detailed in this report, to arrive at fully interoperable atlases of the human brain will still require much work at the frontiers of data acquisition, analysis, and representation. Among them, the latter may provide the most challenging task, in particular when it comes to representing features of vastly different scales of space, time and abstraction. The potential benefits of such endeavor, however, clearly outweigh the problems, as only such kind of multi-modal human brain atlas may provide a starting point from which the complex relationships between structure, function, and connectivity may be explored. |
| Abstract: | The adult human brain quickly adapts to regular temporal sequences, and emits a sequence of novelty responses when these regularities are violated. These novelty responses have been interpreted as error signals that reflect the difference between the incoming signal and predictions generated at multiple cortical levels. Do infants already possess such a hierarchy of violation-detection mechanisms? Using high-density recordings of event-related potentials during an auditory local-global violation paradigm, we show that three-month-old infants process novelty in temporal sequences at two distinct levels. Violations of local expectancies, such as perceiving a deviant vowel "a" after repeated presentation of another vowel i-i-i, elicited an early auditory mismatch response. Conversely, violations of global expectancies, such as hearing the rare sequence a-a-a-a instead of the frequent sequence a-a-a-i, modulated this early mismatch response and led to a late frontal negative slow wave, whose cortical sources included the left inferior frontal region. These results suggest that the infant brain already possesses two dissociable systems for temporal sequence learning. |
| Abstract: | Recent experimental studies have shown that early brain activity is crucial for neuronal survival and the development of brain networks; however, it has been challenging to assess its role in the developing human brain. We employed serial quantitative magnetic resonance imaging to measure the rate of growth in circumscribed brain tissues from preterm to term age, and compared it with measures of electroencephalographic (EEG) activity during the first postnatal days by 2 different methods. EEG metrics of functional activity were computed: EEG signal peak-to-peak amplitude and the occurrence of developmentally important spontaneous activity transients (SATs). We found that an increased brain activity in the first postnatal days correlates with a faster growth of brain structures during subsequent months until term age. Total brain volume, and in particular subcortical gray matter volume, grew faster in babies with less cortical electrical quiescence and with more SAT events. The present findings are compatible with the idea that (1) early cortical network activity is important for brain growth, and that (2) objective measures may be devised to follow early human brain activity in a biologically reasoned way in future research as well as during intensive care treatment. |
| Abstract: | The visual word form area (VWFA), a region systematically involved in the identification of written words, occupies a reproducible location in the left occipitotemporal sulcus in expert readers of all cultures. Such a reproducible localization is paradoxical, given that reading is a recent invention that could not have influenced the genetic evolution of the cortex. Here, we test the hypothesis that the VWFA recycles a region of the ventral visual cortex that shows a high degree of anatomical connectivity to perisylvian language areas, thus providing an efficient circuit for both grapheme-phoneme conversion and lexical access. In two distinct experiments, using high-resolution diffusion-weighted data from 75 human subjects, we show that (1) the VWFA, compared with the fusiform face area, shows higher connectivity to left-hemispheric perisylvian superior temporal, anterior temporal and inferior frontal areas; (2) on a posterior-to-anterior axis, its localization within the left occipitotemporal sulcus maps onto a peak of connectivity with language areas, with slightly distinct subregions showing preferential projections to areas respectively involved in grapheme-phoneme conversion and lexical access. In agreement with functional data on the VWFA in blind subjects, the results suggest that connectivity to language areas, over and above visual factors, may be the primary determinant of VWFA localization. |
| Abstract: | Studying how the healthy human brain develops is important to understand early pathological mechanisms and to assess the influence of fetal or perinatal events on later life. Brain development relies on complex and intermingled mechanisms especially during gestation and first post-natal months, with intense interactions between genetic, epigenetic and environmental factors. Although the baby's brain is organized early on, it is not a miniature adult brain: regional brain changes are asynchronous and protracted, i.e. sensory-motor regions develop early and quickly, whereas associative regions develop later and slowly over decades. Concurrently, the infant/child gradually achieves new performances, but how brain maturation relates to changes in behavior is poorly understood, requiring non-invasive in vivo imaging studies such as magnetic resonance imaging (MRI). Two main processes of early white matter development are reviewed: (1) establishment of connections between brain regions within functional networks, leading to adult-like organization during the last trimester of gestation, (2) maturation (myelination) of these connections during infancy to provide efficient transfers of information. Current knowledge from post-mortem descriptions and in vivo MRI studies is summed up, focusing on T1- and T2-weighted imaging, diffusion tensor imaging, and quantitative mapping of T1/T2 relaxation times, myelin water fraction and magnetization transfer ratio. |
| Abstract: | Diffusion imaging techniques such as DTI and HARDI are difficult to implement in infants because of their sensitivity to subject motion. A short acquisition time is generally preferred, at the expense of spatial resolution and signal-to-noise ratio. Before estimating the local diffusion model, most pre-processing techniques only register diffusion-weighted volumes, without correcting for intra-slice artifacts due to motion or technical problems. Here, we propose a fully automated strategy, which takes advantage of a high orientation number and is based on spherical-harmonics decomposition of the diffusion signal.The correction strategy is based on two successive steps: 1) automated detection and resampling of corrupted slices; 2) correction for eddy current distortions and realignment of misregistered volumes. It was tested on DTI data from adults and non-sedated healthy infants.The methodology was validated through simulated motions applied to an uncorrupted dataset and through comparisons with an unmoved reference. Second, we showed that the correction applied to an infant group enabled to improve DTI maps and to increase the reliability of DTI quantification in the immature cortico-spinal tract.This automated strategy performed reliably on DTI datasets and can be applied to spherical single- and multiple-shell diffusion imaging. |
| Abstract: | Auditory novelty detection has been associated with different cognitive processes. Bekinschtein et al. (2009) developed an experimental paradigm to dissociate these processes, using local and global novelty, which were associated, respectively, with automatic versus strategic perceptual processing. They have mostly been studied using event-related potentials (ERPs), but local spiking activity as indexed by gamma (60-120 Hz) power and interactions between brain regions as indexed by modulations in beta-band (13-25 Hz) power and functional connectivity have not been explored. We thus recorded 9 epileptic patients with intracranial electrodes to compare the precise dynamics of the responses to local and global novelty. Local novelty triggered an early response observed as an intracranial mismatch negativity (MMN) contemporary with a strong power increase in the gamma band and an increase in connectivity in the beta band. Importantly, all these responses were strictly confined to the temporal auditory cortex. In contrast, global novelty gave rise to a late ERP response distributed across brain areas, contemporary with a sustained power decrease in the beta band (13-25 Hz) and an increase in connectivity in the alpha band (8-13 Hz) within the frontal lobe. We discuss these multi-facet signatures in terms of conscious access to perceptual information. |
| Abstract: | Developmental research, as well as paediatric clinical activity crucially depends on non-invasive and painless brain recording techniques, such as electroencephalography (EEG), and near infrared spectroscopy (NIRS). However, both of these techniques measure cortical activity from the scalp without precise knowledge of the recorded cerebral structures. An accurate and reliable mapping between external anatomical landmarks and internal cerebral structures is therefore fundamental to localise brain sources in a non-invasive way. Here, using MRI, we examined the relations between the 10-20 sensor placement system and cerebral structures in 16 infants (3-17 weeks post-term). We provided an infant template parcelled in 94 regions on which we reported the variability of sensors locations, concurrently with the anatomical variability of six main cortical sulci (superior and inferior frontal sulcus, central sulcus, sylvian fissure, superior temporal sulcus, and intraparietal sulcus) and of the distances between the sensors and important cortical landmarks across these infants. The main difference between infants and adults was observed for the channels O1-O2, T5-T6, which projected over lower structures than in adults. We did not find any asymmetry in the distances between the scalp and the brain envelope. However, because of the Yakovlean torque pushing dorsally and frontally the right sylvian fissure, P3-P4 were not at the same distance from the posterior end of this structure. This study should help to refine hypotheses on functional cognitive development by providing an accurate description of the localization of standardised channels relative to infants' brain structures. Template and atlas are publicly available on our Web site (http://www.unicog.org/pm/pmwiki.php/Site/InfantTemplate). |
| Abstract: | Parsing a cognitive task into a sequence of operations is a central problem in cognitive neuroscience. We argue that a major advance is now possible owing to the application of pattern classifiers to time-resolved recordings of brain activity [electroencephalography (EEG), magnetoencephalography (MEG), or intracranial recordings]. By testing at which moment a specific mental content becomes decodable in brain activity, we can characterize the time course of cognitive codes. Most importantly, the manner in which the trained classifiers generalize across time, and from one experimental condition to another, sheds light on the temporal organization of information-processing stages. A repertoire of canonical dynamical patterns is observed across various experiments and brain regions. This method thus provides a novel way to understand how mental representations are manipulated and transformed. |
| Abstract: | Subliminal perception studies have shown that one can objectively discriminate a stimulus without subjectively perceiving it. We show how a minimalist framework based on Signal Detection Theory and Bayesian inference can account for this dissociation, by describing subjective and objective tasks with similar decision-theoretic mechanisms. Each of these tasks relies on distinct response classes, and therefore distinct priors and decision boundaries. As a result, they may reach different conclusions. By formalizing, within the same framework, forced-choice discrimination responses, subjective visibility reports and confidence ratings, we show that this decision model suffices to account for several classical characteristics of conscious and unconscious perception. Furthermore, the model provides a set of original predictions on the nonlinear profiles of discrimination performance obtained at various levels of visibility. We successfully test one such prediction in a novel experiment: when varying continuously the degree of perceptual ambiguity between two visual symbols presented at perceptual threshold, identification performance varies quasi-linearly when the stimulus is unseen and in an 'all-or-none' manner when it is seen. The present model highlights how conscious and non-conscious decisions may correspond to distinct categorizations of the same stimulus encoded by a high-dimensional neuronal population vector. |
| Abstract: | THE BRAIN RESPONSE TO AUDITORY NOVELTY COMPRISES TWO MAIN EEG COMPONENTS: an early mismatch negativity and a late P300. Whereas the former has been proposed to reflect a prediction error, the latter is often associated with working memory updating. Interestingly, these two proposals predict fundamentally different dynamics: prediction errors are thought to propagate serially through several distinct brain areas, while working memory supposes that activity is sustained over time within a stable set of brain areas. Here we test this temporal dissociation by showing how the generalization of brain activity patterns across time can characterize the dynamics of the underlying neural processes. This method is applied to magnetoencephalography (MEG) recordings acquired from healthy participants who were presented with two types of auditory novelty. Following our predictions, the results show that the mismatch evoked by a local novelty leads to the sequential recruitment of distinct and short-lived patterns of brain activity. In sharp contrast, the global novelty evoked by an unexpected sequence of five sounds elicits a sustained state of brain activity that lasts for several hundreds of milliseconds. The present results highlight how MEG combined with multivariate pattern analyses can characterize the dynamics of human cortical processes. |
| Abstract: | The operational momentum (OM) effect describes a cognitive bias whereby we overestimate the results of mental addition problems while underestimating for subtraction. To test whether the OM emerges from psychophysical characteristics of the mental magnitude representation we measured two basic parameters (Weber fraction and numerical estimation accuracy) characterizing the mental magnitude representation and participants' performance in cross-notational addition and subtraction problems. Although participants were able to solve the cross-notational problems, they consistently chose relatively larger results in addition problems than in subtraction problems, thus replicating and extending previous results. Combining the above measures in a psychophysical model allowed us to partially predict the chosen results. Most crucially, however, we were not able to fully model the OM bias on the basis of these psychophysical parameters. Our results speak against the idea that the OM is due to basic characteristics of the mental magnitude representation. In turn, this might be interpreted as evidence for the assumption that the OM effect is better explained by attentional shifts along the mental magnitude representation during mental calculation. |
| Abstract: | Human cognition is characterized by severe capacity limits: we can accurately track, enumerate, or hold in mind only a small number of items at a time. It remains debated whether capacity limitations across tasks are determined by a common system. Here we measure brain activation of adult subjects performing either a visual short-term memory (vSTM) task consisting of holding in mind precise information about the orientation and position of a variable number of items, or an enumeration task consisting of assessing the number of items in those sets. We show that task-specific capacity limits (three to four items in enumeration and two to three in vSTM) are neurally reflected in the activity of the posterior parietal cortex (PPC): an identical set of voxels in this region, commonly activated during the two tasks, changed its overall response profile reflecting task-specific capacity limitations. These results, replicated in a second experiment, were further supported by multivariate pattern analysis in which we could decode the number of items presented over a larger range during enumeration than during vSTM. Finally, we simulated our results with a computational model of PPC using a saliency map architecture in which the level of mutual inhibition between nodes gives rise to capacity limitations and reflects the task-dependent precision with which objects need to be encoded (high precision for vSTM, lower precision for enumeration). Together, our work supports the existence of a common, flexible system underlying capacity limits across tasks in PPC that may take the form of a saliency map. |
| Abstract: | In vivo evaluation of the brain white matter maturation is still a challenging task with no existing gold standards. In this article we propose an original approach to evaluate the early maturation of the white matter bundles, which is based on comparison of infant and adult groups using the Mahalanobis distance computed from four complementary MRI parameters: quantitative qT1 and qT2 relaxation times, longitudinal ?? and transverse ?? diffusivities from diffusion tensor imaging. Such multi-parametric approach is expected to better describe maturational asynchrony than conventional univariate approaches because it takes into account complementary dependencies of the parameters on different maturational processes, notably the decrease in water content and the myelination. Our approach was tested on 17 healthy infants (aged 3- to 21-week old) for 18 different bundles. It finely confirmed maturational asynchrony across the bundles: the spino-thalamic tract, the optic radiations, the cortico-spinal tract and the fornix have the most advanced maturation, while the superior longitudinal and arcuate fasciculi, the anterior limb of the internal capsule and the external capsule have the most delayed maturation. Furthermore, this approach was more reliable than univariate approaches as it revealed more maturational relationships between the bundles and did not violate a priori assumptions on the temporal order of the bundle maturation. Mahalanobis distances decreased exponentially with age in all bundles, with the only difference between them explained by different onsets of maturation. Estimation of these relative delays confirmed that the most dramatic changes occur during the first post-natal year. |
| Abstract: | Time perception is a critical component of conscious experience. To be in synchrony with the environment, the brain must deal not only with differences in the speed of light and sound but also with its computational and neural transmission delays. Here, we asked whether the brain could actively compensate for temporal delays by changing its processing time. Specifically, can changes in neural timing or in the phase of neural oscillation index perceived timing? For this, a lag-adaptation paradigm was used to manipulate participants' perceived audiovisual (AV) simultaneity of events while they were recorded with magnetoencephalography (MEG). Desynchronized AV stimuli were presented rhythmically to elicit a robust 1Hz frequency-tagging of auditory and visual cortical responses. As participants' perception of AV simultaneity shifted, systematic changes in the phase of entrained neural oscillations were observed. This suggests that neural entrainment is not a passive response and that the entrained neural oscillation shifts in time. Crucially, our results indicate that shifts in neural timing in auditory cortices linearly map participants' perceived AV simultaneity. To our knowledge, these results provide the first mechanistic evidence for active neural compensation in the encoding of sensory event timing in support of the emergence of time awareness. |
| Abstract: | Magnetic resonance imaging has proved to be suitable and efficient for in vivo investigation of the early process of brain gyrification in fetuses and preterm newborns but the question remains as to whether cortical-related measurements derived from both cases are comparable or not. Indeed, the developmental folding trajectories drawn up from both populations have not been compared so far, neither from cross-sectional nor from longitudinal datasets. The present study aimed to compare features of cortical folding between healthy fetuses and early imaged preterm newborns on a cross-sectional basis, over a developmental period critical for the folding process (21-36 weeks of gestational age [GA]). A particular attention was carried out to reduce the methodological biases between the 2 populations. To provide an accurate group comparison, several global parameters characterizing the cortical morphometry were derived. In both groups, those metrics provided good proxies for the dramatic brain growth and cortical folding over this developmental period. Except for the cortical volume and the rate of sulci appearance, they depicted different trajectories in both groups suggesting that the transition from into ex utero has a visible impact on cortical morphology that is at least dependent on the GA at birth in preterm newborns. |
| Abstract: | How does the human brain extract regularities from its environment? There is evidence that short range or 'local' regularities (within seconds) are automatically detected by the brain while long range or 'global' regularities (over tens of seconds or more) require conscious awareness. In the present experiment, we asked whether participants' attention was needed to acquire such auditory regularities, to detect their violation or both. We designed a paradigm in which participants listened to predictable sounds. Subjects could be distracted by a visual task at two moments: when they were first exposed to a regularity or when they detected violations of this regularity. MEG recordings revealed that early brain responses (100-130 ms) to violations of short range regularities were unaffected by visual distraction and driven essentially by local transitional probabilities. Based on global workspace theory and prior results, we expected that visual distraction would eliminate the long range global effect, but unexpectedly, we found the contrary, i.e. late brain responses (300-600 ms) to violations of long range regularities on audio-visual trials but not on auditory only trials. Further analyses showed that, in fact, visual distraction was incomplete and that auditory and visual stimuli interfered in both directions. Our results show that conscious, attentive subjects can learn the long range dependencies present in auditory stimuli even while performing a visual task on synchronous visual stimuli. Furthermore, they acquire a complex regularity and end up making different predictions for the very same stimulus depending on the context (i.e. absence or presence of visual stimuli). These results suggest that while short-range regularity detection is driven by local transitional probabilities between stimuli, the human brain detects and stores long-range regularities in a highly flexible, context dependent manner. |
| Abstract: | Learning to read requires the acquisition of an efficient visual procedure for quickly recognizing fine print. Thus, reading practice could induce a perceptual learning effect in early vision. Using functional magnetic resonance imaging (fMRI) in literate and illiterate adults, we previously demonstrated an impact of reading acquisition on both high- and low-level occipitotemporal visual areas, but could not resolve the time course of these effects. To clarify whether literacy affects early vs. late stages of visual processing, we measured event-related potentials to various categories of visual stimuli in healthy adults with variable levels of literacy, including completely illiterate subjects, early-schooled literate subjects, and subjects who learned to read in adulthood (ex-illiterates). The stimuli included written letter strings forming pseudowords, on which literacy is expected to have a major impact, as well as faces, houses, tools, checkerboards, and false fonts. To evaluate the precision with which these stimuli were encoded, we studied repetition effects by presenting the stimuli in pairs composed of repeated, mirrored, or unrelated pictures from the same category. The results indicate that reading ability is correlated with a broad enhancement of early visual processing, including increased repetition suppression, suggesting better exemplar discrimination, and increased mirror discrimination, as early as ? 100-150 ms in the left occipitotemporal region. These effects were found with letter strings and false fonts, but also were partially generalized to other visual categories. Thus, learning to read affects the magnitude, precision, and invariance of early visual processing. |
| Abstract: | The ability to recognize 2 mirror images as the same picture across left-right inversions exists early on in humans and other primates. In order to learn to read, however, one must discriminate the left-right orientation of letters and distinguish, for instance, b from d. We therefore reasoned that literacy may entail a loss of mirror invariance. To evaluate this hypothesis, we asked adult literates, illiterates, and ex-illiterates to perform a speeded same-different task with letter strings, false fonts, and pictures regardless of their orientation (i.e., they had to respond "same" to mirror pairs such as "iblo oldi"). Literates presented clear difficulties with mirror invariance. This "mirror cost" effect was strongest with letter strings, but crucially, it was also observed with false fonts and even with pictures. In contrast, illiterates did not present any cost for mirror pairs. Interestingly, subjects who learned to read as adults also exhibited a mirror cost, suggesting that modest reading practice, late in life, can suffice to break mirror invariance. (PsycINFO Database Record (c) 2014 APA, all rights reserved). |
| Abstract: | Gaze following is an essential human communication cue that orients the attention of two interacting people to the same external object. This capability is robustly observed after 7 months of age in full-term infants. Do healthy preterm infants benefit from their early exposure to face-to-face interactions with other humans to acquire this capacity sooner than full-term infants of the same chronological age, despite their immature brains? In two different experiments, we demonstrated that 7-month-old preterm infants performed like 7-month-old full-term infants (with whom they shared the same chronological age) and not like 4-month-old full-term infants (with whom they shared the same postmenstrual age). The duration of exposure to visual experience thus appears to have a greater impact on the development of early gaze following than does postmenstrual age. |
| Abstract: | Two areas of the occipitotemporal cortex show a remarkable hemispheric lateralization: written words activate the visual word form area (VWFA) in the left fusiform gyrus and faces activate a symmetrical site in the right hemisphere, the fusiform face area (FFA). While the lateralization of the VWFA fits with the leftward asymmetry of the speech processing network, origin of the rightward asymmetry for faces is still unclear. Using fMRI data from 64 subjects (including 16 monozygotic (MZ) and 13 dizygotic (DZ) twin pairs), we investigated how activations evoked by written words, faces, and spoken language are co-lateralized in the temporal lobe, and whether this organization reflects genetic factors or individual reading expertise. We found that the lateralization of the left superior temporal activation for spoken language correlates with the lateralization of occipitotemporal activations for both written words and faces. Behavioral reading scores also modulate the responses to words and faces. Estimation of genetic and environmental contributions shows that activations of the VWFA, the occipital face area, and the temporal speech areas are partially under genetic control whereas activation of the FFA is primarily influenced by individual experience. Our results stress the importance of both genetic factors and acquired expertise in the occipitotemporal organization. |
| Abstract: | In recent years, numerous electrophysiological signatures of consciousness have been proposed. Here, we perform a systematic analysis of these electroencephalography markers by quantifying their efficiency in differentiating patients in a vegetative state from those in a minimally conscious or conscious state. Capitalizing on a review of previous experiments and current theories, we identify a series of measures that can be organized into four dimensions: (i) event-related potentials versus ongoing electroencephalography activity; (ii) local dynamics versus inter-electrode information exchange; (iii) spectral patterns versus information complexity; and (iv) average versus fluctuations over the recording session. We analysed a large set of 181 high-density electroencephalography recordings acquired in a 30 minutes protocol. We show that low-frequency power, electroencephalography complexity, and information exchange constitute the most reliable signatures of the conscious state. When combined, these measures synergize to allow an automatic classification of patients' state of consciousness. |
| Abstract: | How does reading expertise change the visual system? Here, we explored whether the visual system could develop dedicated perceptual mechanisms in early and intermediate visual cortex under the pressure for fast processing that is particularly strong in reading. We compared fMRI activations in Chinese participants with limited knowledge of French and in French participants with no knowledge of Chinese, exploiting these doubly dissociated reading skills as a tool to study the neural correlates of visual expertise. All participants viewed the same stimuli: words in both languages and matched visual controls, presented at a fast rate comparable with fluent reading. In the Visual Word Form Area, all participants showed enhanced responses to their known scripts. However, group differences were found in occipital cortex. In French readers reading French, activations were enhanced in left-hemisphere visual area V1, with the strongest differences between French words and their controls found at the central and horizontal meridian representations. Chinese participants , who were not expert French readers, did not show these early visual activations. In contrast, Chinese readers reading Chinese showed enhanced activations in intermediate visual areas V3v/hV4, absent in French participants. Together with our previous findings [Szwed, M., Dehaene, S., Kleinschmidt, A., Eger, E., Valabregue, R., Amadon, A., et al. Specialization for written words over objects in the visual cortex. Neuroimage, 56, 330-344, 2011], our results suggest that the effects of extensive practice can be found at the lowest levels of the visual system. They also reveal their cross-script variability: Alphabetic reading involves enhanced engagement of central and right meridian V1 representations that are particularly used in left-to-right reading, whereas Chinese characters put greater emphasis on intermediate visual areas. |
| Abstract: | How does general anesthesia (GA) work? Anesthetics are pharmacological agents that target specific central nervous system receptors. Once they bind to their brain receptors, anesthetics modulate remote brain areas and end up interfering with global neuronal networks, leading to a controlled and reversible loss of consciousness. This remarkable manipulation of consciousness allows millions of people every year to undergo surgery safely most of the time. However, despite all the progress that has been made, we still lack a clear and comprehensive insight into the specific neurophysiological mechanisms of GA, from the molecular level to the global brain propagation. During the last decade, the exponential progress in neuroscience and neuro-imaging led to a significant step in the understanding of the neural correlates of consciousness, with direct consequences for clinical anesthesia. Far from shutting down all brain activity, anesthetics lead to a shift in the brain state to a distinct, highly specific and complex state, which is being increasingly characterized by modern neuro-imaging techniques. There are several clinical consequences and challenges that are arising from the current efforts to dissect GA mechanisms: the improvement of anesthetic depth monitoring, the characterization and avoidance of intra-operative awareness and post-anesthesia cognitive disorders, and the development of future generations of anesthetics. |
| Abstract: | What are the limits of unconscious language processing? Can language circuits process simple grammatical constructions unconsciously and integrate the meaning of several unseen words? Using behavioural priming and electroencephalography (EEG), we studied a specific rule-based linguistic operation traditionally thought to require conscious cognitive control: the negation of valence. In a masked priming paradigm, two masked words were successively (Experiment 1) or simultaneously presented (Experiment 2), a modifier ('not'/'very') and an adjective (e.g. 'good'/'bad'), followed by a visible target noun (e.g. 'peace'/'murder'). Subjects indicated whether the target noun had a positive or negative valence. The combination of these three words could either be contextually consistent (e.g. 'very bad - murder') or inconsistent (e.g. 'not bad - murder'). EEG recordings revealed that grammatical negations could unfold partly unconsciously, as reflected in similar occipito-parietal N400 effects for conscious and unconscious three-word sequences forming inconsistent combinations. However, only conscious word sequences elicited P600 effects, later in time. Overall, these results suggest that multiple unconscious words can be rapidly integrated and that an unconscious negation can automatically 'flip the sign' of an unconscious adjective. These findings not only extend the limits of subliminal combinatorial language processes, but also highlight how consciousness modulates the grammatical integration of multiple words. |
| Abstract: | The estimation of duration can be affected by context and surprise. Using MagnetoEncephaloGraphy (MEG), we tested whether increased neural activity during surprise and following neural suppression in two different contexts supported subjective time dilation (Eagleman & Pariyadath, 2009; Pariyadath & Eagleman, 2012). Sequences of three 300ms frequency-modulated (FM, control) or pure tones (test) were presented and followed by a fourth FM varying in duration. In test, the last FM was perceived as significantly longer than veridical duration (Tse, Intriligator, Rivest, & Cavanagh, 2004) but did not differ from the perceived duration in control. Several novel and distinct neural signatures were observed in duration estimation: first, neural suppression of standard stimuli was observed for the onset but not for the offset auditory evoked responses. Second, ramping activity increased with veridical duration in control whereas at the same latency in test, the amplitude of the midlatency response increased with the distance of deviant durations. Third, in both conditions, the amplitude of the offset auditory evoked responses accounted well for participants' performance: the longer the perceived duration, the larger the offset response. Fourth, neural duration demarcated by the peak latencies of the onset and ramping evoked activities indexed a systematic time compression that reliably predicted subjective time perception. Our findings suggest that interval timing undergoes time compression by capitalizing on the predicted offset of an auditory event. |
| Abstract: | Abstract The SNARC effect refers to faster reaction times for larger numbers with right-sided responses, and for smaller numbers with left-sided responses (Dehaene et al., 1993), even when numerical magnitude is irrelevant. Although the SNARC is generally thought to reflect a mapping between numbers and space, the question of which spatial reference frame(s) are critical for the effect has not been systematically explored. We propose a dynamic hierarchical organization of the reference frames (from a global left-right frame to body- and object-related frames), where the influence of each frame can be modulated by experimental context. We conducted two experiments based on predictions derived from this organizational system. Experiment 1 compared instructions that differed only in focusing participants' attention on either the response buttons or the hands. Instructions focusing on a hand-based reference frame eliminated the SNARC. Experiment 2 provided the opportunity for an object-centered reference frame to manifest itself in the SNARC. Although we did not observe an effect of an object-centered reference frame, we observed the influence of other reference frames in a context where an object-centered reference frame was emphasized. Altogether, these results support the proposed organization of the reference frames. |
| 2013 |
| Abstract: | Developmental dyslexia is a learning disability that specifically affects reading acquisition. Cortical anomalies and gray matter volume differences in various temporal regions have been reported in dyslexic subjects compared with controls. However, consistency between studies is lacking. In the present experiments, we focused our structural analyses on the ventral occipitotemporal regions, defined by their functional response to visual categories. We applied a subject-by-subject functionally guided approach on a total of 76 participants (31 dyslexic children). Cortical thickness was estimated for each participant around his/her peak of specific functional activation to visual words, faces, or places. Results from two independent datasets showed a reduction in thickness in dyslexic children compared with controls in the region responsive to words, in the left hemisphere. Additionally, a gender-by-diagnosis interaction was observed at the same location, due to differences in girls only. To avoid the potential confound of reading level, we also contrasted dyslexic and control children matched for reading performance, and we observed a similar difference, although in a smaller extent of cortex. The present study thus provides the first account of a focal cortical thickness reduction in dyslexia in the subregion of ventral occipitotemporal cortex specifically responsive to visual words, when age, gender, and reading performance are taken into account. |
| Abstract: | Neuroimaging studies of audiovisual speech processing have exclusively addressed listeners' native language (L1). Yet, several behavioural studies now show that AV processing plays an important role in non-native (L2) speech perception. The current fMRI study measured brain activity during auditory, visual, audiovisual congruent and audiovisual incongruent utterances in L1 and L2. BOLD responses to congruent AV speech in the pSTS were stronger than in either unimodal condition in both L1 and L2. Yet no differences in AV processing were expressed according to the language background in this area. Instead, the regions in the bilateral occipital lobe had a stronger congruency effect on the BOLD response (congruent higher than incongruent) in L2 as compared to L1. According to these results, language background differences are predominantly expressed in these unimodal regions, whereas the pSTS is similarly involved in AV integration regardless of language dominance. |
| Abstract: | Metacognition, the ability to monitor one's own cognitive processes, is frequently assumed to be univocally associated with conscious processing. However, some monitoring processes, such as those associated with the evaluation of one's own performance, may conceivably be sufficiently automatized to be deployed non-consciously. Here, we used simultaneous electro- and magneto-encephalography (EEG/MEG) to investigate how error detection is modulated by perceptual awareness of a masked target digit. The Error-Related Negativity (ERN), an EEG component occurring ~100 ms after an erroneous response, was exclusively observed on conscious trials: regardless of masking strength, the amplitude of the ERN showed a step-like increase when the stimulus became visible. Nevertheless, even in the absence of an ERN, participants still managed to detect their errors at above-chance levels under subliminal conditions. Error detection on conscious trials originated from the posterior cingulate cortex, while a small response to non-conscious errors was seen in dorsal anterior cingulate. We propose the existence of two distinct brain mechanisms for metacognitive judgements: a conscious all-or-none process of single-trial response evaluation, and a non-conscious statistical assessment of confidence. |
| Abstract: | In the primate brain, sensory information is processed along two partially segregated cortical streams: the ventral stream, mainly coding for objects' shape and identity, and the dorsal stream, mainly coding for objects' quantitative information (including size, number, and spatial position). Neurophysiological measures indicate that such functional segregation is present early on in infancy, and that the two streams follow independent maturational trajectories during childhood. Here we collected, in a large sample of young children and adults, behavioural measures on an extensive set of functions typically associated with either the dorsal or the ventral stream. We then used a correlational approach to investigate the presence of inter-individual variability resulting in clustering of functions. Results show that dorsal- and ventral-related functions follow two uncorrelated developmental trajectories. Moreover, within each stream, some functions show age-independent correlations: finger gnosis, non-symbolic numerical abilities and spatial abilities within the dorsal stream, and object and face recognition abilities within the ventral stream. This pattern of clear within-stream cross-task correlation seems to be lost in adults, with two notable exceptions: performance in face and object recognition on one side, and in symbolic and non-symbolic comparison on the other, remain correlated, pointing to distinct shape recognition and quantity comparison systems. |
| Abstract: | Humans as social beings are profoundly affected by exclusion. Short experiences with people differing in their degree of prosocial behaviour can induce reliable preferences for including partners, but the neural mechanisms of this learning remain unclear. Here, we asked participants to play a short social interaction game based on "cyber-ball" where one fictive partner included and another excluded the subject, thus defining social roles (includer - "good", excluder - "bad"). We then used multivariate pattern recognition on high-resolution functional magnetic resonance imaging (fMRI) data acquired before and after this game to test whether neural responses to the partners' and neutral control faces during a perceptual task reflect their learned social valence. Support vector classification scores revealed a learning-related increase in neural discrimination of social status in anterior insula and anterior cingulate regions, which was mainly driven by includer faces becoming distinguishable from excluder and control faces. Thus, face-evoked responses in anterior insula and anterior cingulate cortex contain fine-grained information shaped by prior social interactions that allow for categorisation of faces according to their learned social status. These lasting traces of social experience in cortical areas important for emotional and social processing could provide a substrate of how social inclusion shapes future behaviour and promotes cooperative interactions between individuals. |
| Abstract: | Anomalous activations of the prefrontal cortex (PFC) and posterior cerebral areas have been reported in previous studies of working memory in schizophrenia. Several interpretations have been reported: e.g., neural inefficiency, the use of different strategies and differences in the functional organization of the cerebral cortex. To better understand these abnormal activations, we investigated the cerebral bases of a working memory component process, namely refreshing (i.e., thinking briefly of a just-activated representation). Fifteen patients with schizophrenia and 15 control subjects participated in this functional magnetic resonance imaging (fMRI) study. Participants were told that whenever they saw a word on the screen, they had to read it silently to themselves (read and repeat conditions), and when they saw a dot, they had to think of the just-previous word (refresh condition). The refresh condition (in comparison with the read condition) was associated with significantly increased activation in the left inferior frontal gyrus and significantly decreased connectivity within the prefrontal cortex and between the prefrontal and parietal cortices in patients with schizophrenia in comparison with control subjects. These results suggest that prefrontal dysfunctions in schizophrenia might be related to a defective ability to initiate (rather than to execute) specific cognitive processes. |
| Abstract: | Magnetoencephalographic (MEG) recordings are a rich source of information about the neural dynamics underlying cognitive processes in the brain, with excellent temporal and good spatial resolution. In recent years there have been considerable advances in MEG hardware developments and methods. Sophisticated analysis techniques are now routinely applied and continuously improved, leading to fascinating insights into the intricate dynamics of neural processes. However, the rapidly increasing level of complexity of the different steps in a MEG study make it difficult for novices, and sometimes even for experts, to stay aware of possible limitations and caveats. Furthermore, the complexity of MEG data acquisition and data analysis requires special attention when describing MEG studies in publications, in order to facilitate interpretation and reproduction of the results. This manuscript aims at making recommendations for a number of important data acquisition and data analysis steps and suggests details that should be specified in manuscripts reporting MEG studies. These recommendations will hopefully serve as guidelines that help to strengthen the position of the MEG research community within the field of neuroscience, and may foster discussion in order to further enhance the quality and impact of MEG research. |
| Abstract: | To assess whether there was an adverse effect on brain growth after hydrocortisone (HC) treatment for bronchopulmonary dysplasia (BPD) in a large cohort of infants without dexamethasone exposure.Infants who received HC for BPD between 2005 and 2011 and underwent magnetic resonance imaging at term-equivalent age were included. Control infants born in Geneva (2005-2006) and Utrecht (2007-2011) were matched to the infants treated with HC according to segmentation method, sex, and gestational age. Infants with overt parenchymal pathology were excluded. Multivariable analysis was used to determine if there was a difference in brain volumes between the 2 groups.Seventy-three infants treated with HC and 73 matched controls were included. Mean gestational age was 26.7 weeks, and mean birth weight was 906 g. After correction for gestational age, postmenstrual age at time of scanning, the presence of intraventricular hemorrhage, and birth weight z-score, no differences were found between infants treated with HC and controls in total brain tissue or cerebellar volumes.In the absence of associated parenchymal brain injury, no reduction in brain tissue or cerebellar volumes could be found at term-equivalent age between infants with or without treatment with HC for BPD. |
| Abstract: | Detecting residual consciousness in unresponsive patients is a major clinical concern and a challenge for theoretical neuroscience. To tackle this issue, we recently designed a paradigm that dissociates two electro-encephalographic (EEG) responses to auditory novelty. Whereas a local change in pitch automatically elicits a mismatch negativity (MMN), a change in global sound sequence leads to a late P300b response. The latter component is thought to be present only when subjects consciously perceive the global novelty. Unfortunately, it can be difficult to detect because individual variability is high, especially in clinical recordings. Here, we show that multivariate pattern classifiers can extract subject-specific EEG patterns and predict single-trial local or global novelty responses. We first validate our method with 38 high-density EEG, MEG and intracranial EEG recordings. We empirically demonstrate that our approach circumvents the issues associated with multiple comparisons and individual variability while improving the statistics. Moreover, we confirm in control subjects that local responses are robust to distraction whereas global responses depend on attention. We then investigate 104 vegetative state (VS), minimally conscious state (MCS) and conscious state (CS) patients recorded with high-density EEG. For the local response, the proportion of significant decoding scores (M=60\%) does not vary with the state of consciousness. By contrast, for the global response, only 14\% of the VS patients' EEG recordings presented a significant effect, compared to 31\% in MCS patients' and 52\% in CS patients'. In conclusion, single-trial multivariate decoding of novelty responses provides valuable information in non-communicating patients and paves the way towards real-time monitoring of the state of consciousness. |
| Abstract: | Neuronal theories of conscious access tentatively relate conscious perception to the integration and global broadcasting of information across distant cortical and thalamic areas. Experiments contrasting visible and invisible stimuli support this view and suggest that global neuronal communication may be detectable using scalp electroencephalography (EEG). However, whether global information sharing across brain areas also provides a specific signature of conscious state in awake but noncommunicating patients remains an active topic of research. We designed a novel measure termed "weighted symbolic mutual information" (wSMI) and applied it to 181 high-density EEG recordings of awake patients recovering from coma and diagnosed in various states of consciousness. The results demonstrate that this measure of information sharing systematically increases with consciousness state, particularly across distant sites. This effect sharply distinguishes patients in vegetative state (VS), minimally conscious state (MCS), and conscious state (CS) and is observed regardless of etiology and delay since insult. The present findings support distributed theories of conscious processing and open up the possibility of an automatic detection of conscious states, which may be particularly important for the diagnosis of awake but noncommunicating patients. |
| Abstract: | Infants have a sophisticated behavioral and cognitive repertoire suggestive of a capacity for conscious reflection. Yet, demonstrating conscious access in infants remains challenging, mainly because they cannot report their thoughts. Here, to circumvent this problem, we studied whether an electrophysiological signature of consciousness found in adults, corresponding to a late nonlinear cortical response [~300 milliseconds (ms)] to brief pictures, already exists in infants. We recorded event-related potentials while 5-, 12-, and 15-month-old infants (N = 80) viewed masked faces at various levels of visibility. In all age groups, we found a late slow wave showing a nonlinear profile at the expected perceptual thresholds. However, this late component shifted from a weak and delayed response in 5-month-olds (starting around 900 ms) to a more sustained and faster response in older infants (around 750 ms). These results reveal that the brain mechanisms underlying the threshold for conscious perception are already present in infancy but undergo a slow acceleration during development. |
| Abstract: | In this article we propose to investigate the analogy between early cortical folding process and cortical smoothing by mean curvature flow. First, we introduce a one-parameter model that is able to fit a developmental trajectory as represented in a Volume-Area plot and we propose an efficient optimization strategy for parameter estimation. Second, we validate the model on forty cortical surfaces of preterm newborns by comparing global geometrical indices and trajectories of central sulcus along developmental and simulation time. |
| Abstract: | The ontogeny of linguistic functions in the human brain remains elusive. Although some auditory capacities are described before term, whether and how such immature cortical circuits might process speech are unknown. Here we used functional optical imaging to evaluate the cerebral responses to syllables at the earliest age at which cortical responses to external stimuli can be recorded in humans (28- to 32-wk gestational age). At this age, the cortical organization in layers is not completed. Many neurons are still located in the subplate and in the process of migrating to their final location. Nevertheless, we observed several points of similarity with the adult linguistic network. First, whereas syllables elicited larger right than left responses, the posterior temporal region escaped this general pattern, showing faster and more sustained responses over the left than over the right hemisphere. Second, discrimination responses to a change of phoneme (ba vs. ga) and a change of human voice (male vs. female) were already present and involved inferior frontal areas, even in the youngest infants (29-wk gestational age). Third, whereas both types of changes elicited responses in the right frontal region, the left frontal region only reacted to a change of phoneme. These results demonstrate a sophisticated organization of perisylvian areas at the very onset of cortical circuitry, 3 mo before term. They emphasize the influence of innate factors on regions involved in linguistic processing and social communication in humans. |
| Abstract: | To examine the influence of age and reading proficiency on the development of the spoken language network, we tested 6- and 9-years-old children listening to native and foreign sentences in a slow event-related fMRI paradigm. We observed a stable organization of the peri-sylvian areas during this time period with a left dominance in the superior temporal sulcus and inferior frontal region. A year of reading instruction was nevertheless sufficient to increase activation in regions involved in phonological representations (posterior superior temporal region) and sentence integration (temporal pole and pars orbitalis). A top-down activation of the left inferior temporal cortex surrounding the visual word form area, was also observed but only in 9year-olds (3years of reading practice) listening to their native language. These results emphasize how a successful cultural practice, reading, slots in the biological constraints of the innate spoken language network. |
| Abstract: | All humans share a universal, evolutionarily ancient approximate number system (ANS) that estimates and combines the numbers of objects in sets with ratio-limited precision. Interindividual variability in the acuity of the ANS correlates with mathematical achievement, but the causes of this correlation have never been established. We acquired psychophysical measures of ANS acuity in child and adult members of an indigene group in the Amazon, the Mundurucu, who have a very restricted numerical lexicon and highly variable access to mathematics education. By comparing Mundurucu subjects with and without access to schooling, we found that education significantly enhances the acuity with which sets of concrete objects are estimated. These results indicate that culture and education have an important effect on basic number perception. We hypothesize that symbolic and nonsymbolic numerical thinking mutually enhance one another over the course of mathematics instruction. |
| Abstract: | Twin studies have long suggested a genetic influence on inter-individual variations in mathematical abilities, and candidate genes have been identified by genome-wide association studies. However, the localization of the brain regions under genetic influence during number manipulation is still unexplored. Here we investigated fMRI data from a group of 19 MZ (monozygotic) and 13 DZ (dizygotic) adult twin pairs, scanned during a mental calculation task. We examined both the activation and the degree of functional lateralization in regions of interest (ROIs) centered on the main activated peaks. Heritability was first investigated by comparing the respective MZ and DZ correlations. Then, genetic and environmental contributions were jointly estimated by fitting a ACE model classically used in twin studies. We found that a subset of the activated network was under genetic influence, encompassing the bilateral posterior superior parietal lobules (PSPL), the right intraparietal sulcus (IPS) and a left superior frontal region. An additional region of the left inferior parietal cortex (IPC), whose deactivation correlated with a behavioral calculation score, also presented higher similarity between MZ than between DZ twins, thus offering a plausible physiological basis for the observable inheritance of math scores. Finally, the main impact of the shared environment was found in the lateralization of activation within the intraparietal sulcus. These maps of genetic and environmental contributions provide precise candidate phenotypes for further genetic association analyses, and illuminate how genetics and education shape the development of number processing networks. |
| Abstract: | Multi-sensor technologies such as EEG, MEG, and ECoG result in high-dimensional data sets. Given the high temporal resolution of such techniques, scientific questions very often focus on the time-course of an experimental effect. In many studies, researchers focus on a single sensor or the average over a subset of sensors covering a "region of interest" (ROI). However, single-sensor or ROI analyses ignore the fact that the spatial focus of activity is constantly changing, and fail to make full use of the information distributed over the sensor array.We describe a technique that exploits the optimality and simplicity of matched spatial filters in order to reduce experimental effects in multivariate time series data to a single time course. Each (multi-sensor) time sample of each trial is replaced with its projection onto a spatial filter that is matched to an observed experimental effect, estimated from the remaining trials (Effect-Matched Spatial filtering, or EMS filtering). The resulting set of time courses (one per trial) can be used to reveal the temporal evolution of an experimental effect, which distinguishes this approach from techniques that reveal the temporal evolution of an anatomical source or region of interest.We illustrate the technique with data from a dual-task experiment and use it to track the temporal evolution of brain activity during the psychological refractory period. We demonstrate its effectiveness in separating the means of two experimental conditions, and in significantly improving the signal-to-noise ratio at the single-trial level. It is fast to compute and results in readily-interpretable time courses and topographies. The technique can be applied to any data-analysis question that can be posed independently at each sensor, and we provide one example, using linear regression, that highlights the versatility of the technique.The approach described here combines established techniques in a way that strikes a balance between power, simplicity, speed of processing, and interpretability. We have used it to provide a direct view of parallel and serial processes in the human brain that previously could only be measured indirectly. An implementation of the technique in MatLab is freely available via the internet. |
| Abstract: | Casali et al. recently showed that the complexity of the electrophysiological brain response to a transcranial magnetic stimulation pulse distinguishes conscious from unconscious humans in a variety of conditions. In addition to its theoretical implications, this novel method paves the way to a quantitative assessment of the states of consciousness. |
| Abstract: | Writing was invented too recently to have influenced the human genome. Consequently, reading acquisition must rely on partial recycling of pre-existing brain systems. Prior fMRI evidence showed that in literates a left-hemispheric visual region increases its activation to written strings relative to illiterates and reduces its response to faces. Increasing literacy also leads to a stronger right-hemispheric lateralization for faces. Here, we evaluated whether this reorganization of the brain's face system has behavioral consequences for the processing of non-linguistic visual stimuli. Three groups of adult illiterates, ex-illiterates and literates were tested with the sequential composite face paradigm that evaluates the automaticity with which faces are processed as wholes. Illiterates were consistently more holistic than participants with reading experience in dealing with faces. A second experiment replicated this effect with both faces and houses. Brain reorganization induced by literacy seems to reduce the influence of automatic holistic processing of faces and houses by enabling the use of a more analytic and flexible processing strategy, at least when holistic processing is detrimental to the task. |
| Abstract: | Whether unconscious stimuli can modulate the preparation of a cognitive task is still controversial. Using a backward masking paradigm, we investigated whether the modulation could be observed even if the prime was made unconscious in 100\% of the trials. In two behavioral experiments, subjects were instructed to initiate a phonological or semantic task on an upcoming word, following an explicit instruction and an unconscious prime. When the SOA between prime and instruction was sufficiently long (84ms), primes congruent with the task set instruction led to speedier responses than incongruent primes. In the other condition (36ms), no task set priming was observed. Repetition priming had the opposite tendency, suggesting the observed task set facilitation cannot be ascribed solely to perceptual repetition priming. Our results therefore confirm that unconscious information can modulate cognitive control for currently active task sets, providing sufficient time is available before the conscious decision. |
| 2012 |
| Abstract: | The acquisition of literacy results from an effortful learning process that leads to functional changes in several cortical regions. We explored whether learning to read also leads to anatomical changes within the left intrahemispheric white matter pathways that interconnect these regions. Using diffusion tensor imaging tractography, we compared illiterates with ex-illiterates who learned to read during adulthood and literates who learned to read during their childhood. Literacy related to an increase in fractional anisotropy and a decrease in perpendicular diffusivity in the temporo-parietal portion of the left arcuate fasciculus. The microstructure within this pathway correlated with the reading performance and the degree of functional activation within 2 dominant brain regions involved in reading: The Visual Word Form Area in response to letter strings, and the posterior superior temporal cortex in response to spoken language. Thus, the acquisition of literacy is associated with a reinforcement of left temporo-parietal connections whose microstructure predicts overall reading performance and the functional specialization of the Visual Word Form Area. This anatomical magnetic resonance imaging marker may be useful to predict developmental reading disorders. |
| Abstract: | Humans can perform sequential and recursive computations, as when calculating 23x74. However, this comes at a cost: flexible computations are slow and effortful. We argue that this competence involves serial chains of successive decisions, each based on the accumulation of evidence up to a threshold and forwarding the result to the subsequent step. Such serial 'programs' require a specific neurobiological architecture, approximating the operation of a slow serial Turing machine. We review recent progress in understanding how the brain implements such multi-step decisions and briefly examine how they might be realized in models of primate cortex. |
| Abstract: | Improving our ability to detect conscious processing in non communicating patients remains a major goal of clinical cognitive neurosciences. In this perspective, several functional brain imaging tools are currently under development. Bedside cognitive event-related potentials (ERPs) derived from the EEG signal are a good candidate to explore consciousness in these patients because: (1) they have an optimal time resolution within the millisecond range able to monitor the stream of consciousness, (2) they are fully non-invasive and relatively cheap, (3) they can be recorded continuously on dedicated individual systems to monitor consciousness and to communicate with patients, (4) and they can be used to enrich patients' autonomy through brain-computer interfaces. We recently designed an original auditory rule extraction ERP test that evaluates cerebral responses to violations of temporal regularities that are either local in time or global across several seconds. Local violations led to an early response in auditory cortex, independent of attention or the presence of a concurrent visual task, while global violations led to a late and spatially distributed response that was only present when subjects were attentive and aware of the violations. In the present work, we report the results of this test in 65 successive recordings obtained at bedside from 49 non-communicating patients affected with various acute or chronic neurological disorders. At the individual level, we confirm the high specificity of the 'global effect': only conscious patients presented this proposed neural signature of conscious processing. Here, we also describe in details the respective neural responses elicited by violations of local and global auditory regularities, and we report two additional ERP effects related to stimuli expectancy and to task learning, and we discuss their relations to consciousness. |
| Abstract: | Both our environment and our behavior contain many spatiotemporal regularities. Preferential and differential tuning of neural populations to these regularities can be demonstrated by assessing rate dependence of neural responses evoked during continuous periodic stimulation. Here, we used functional magnetic resonance imaging to measure regional variations of temporal sensitivity along the human ventral visual stream. By alternating one face and one house stimulus, we combined sufficient low-level signal modulation with changes in semantic meaning and could therefore drive all tiers of visual cortex strongly enough to assess rate dependence. We found several dissociations between early visual cortex and middle- and higher-tier regions. First, there was a progressive slowing down of stimulation rates yielding peak responses along the ventral visual stream. This finding shows the width of temporal integration windows to increase at higher hierarchical levels. Next, for fixed rates, early but not higher visual cortex responses additionally depended on the length of stimulus exposure, which may indicate increased persistence of responses to short stimuli at higher hierarchical levels. Finally, attention, which was recruited by an incidental task, interacted with stimulation rate and shifted tuning peaks toward lower frequencies. Together, these findings quantify neural response properties that are likely to be operational during natural vision and that provide putative neurofunctional substrates of mechanisms that are relevant in several psychophysical phenomena as masking and the attentional blink. Moreover, they illustrate temporal constraints for translating the deployment of attention into enhanced neural responses and thereby account for lower limits of attentional dwell time. |
| Abstract: | The description of cortical folding pattern (CFP) is challenging because of geometric complexity and inter-subject variability. On a cortical surface mesh, curvature estimation provides a good scalar proxy of CFP. The oscillations of this function can be studied using a Fourier-like analysis to produce a power spectrum representative of the spatial frequency composition of CFP. First, we introduce an original method for the SPectral ANalysis of GYrication (Spangy), which performs a spectral decomposition of the mean curvature of the grey/white interface mesh based on the Laplace-Beltrami operator eigenfunctions. Spangy produces an ordered 7 bands power spectrum of curvature (B0-B6) and provides an anatomically relevant segmentation of CFP based on local spectral composition. A spatial frequency being associated with each eigenfunction, the bandwidth design assumes frequency doubling between consecutive spectral bands. Next, we observed that the last 3 spectral bands (B4, 5 and 6) accounted for 93\% of the analyzed spectral power and were associated with fold-related variations of curvature, whereas the lower frequency bands were related to global brain shape. The spectral segmentation of CFP revealed 1st, 2nd and 3rd order elements associated with B4, B5 and B6 respectively. These elements could be related to developmentally-defined primary, secondary and tertiary folds. Finally, we used allometric scaling of frequency bands power and segmentation to analyze the relationship between the spectral composition of CFP and brain size in a large adult dataset. Total folding power followed a positive allometric scaling which did not divide up proportionally between the bands: B4 contribution was constant, B5 increased like total folding power and B6 much faster. Besides, apparition of new elements of pattern with increasing size only concerned the 3rd order. Hence, we demonstrate that large brains are twistier than smaller ones because of an increased number of high spatial frequency folds, ramifications and kinks that accommodate the allometric increase of cortical surface. |
| Abstract: | Humans and many other animals use acoustical signals to mediate social interactions with conspecifics. The evolution of sound-based communication is still poorly understood and its neural correlates have only recently begun to be investigated. In the present study, we applied functional MRI to humans and macaque monkeys listening to identical stimuli in order to compare the cortical networks involved in the processing of vocalizations. At the first stages of auditory processing, both species showed similar fMRI activity maps within and around the lateral sulcus (the Sylvian fissure in humans). Monkeys showed remarkably similar responses to monkey calls and to human vocal sounds (speech or otherwise), mainly in the lateral sulcus and the adjacent superior temporal gyrus (STG). In contrast, a preference for human vocalizations and especially for speech was observed in the human STG and superior temporal sulcus (STS). The STS and Broca's region were especially responsive to intelligible utterances. The evolution of the language faculty in humans appears to have recruited most of the STS. It may be that in monkeys, a much simpler repertoire of vocalizations requires less involvement of this temporal territory. |
| Abstract: | Few phenomena are as suitable as perceptual multistability to demonstrate that the brain constructively interprets sensory input. Several studies have outlined the neural circuitry involved in generating perceptual inference but only more recently has the individual variability of this inferential process been appreciated. Studies of the interaction of evoked and ongoing neural activity show that inference itself is not merely a stimulus-triggered process but is related to the context of the current brain state into which the processing of external stimulation is embedded. As brain states fluctuate, so does perception of a given sensory input. In multistability, perceptual fluctuation rates are consistent for a given individual but vary considerably between individuals. There has been some evidence for a genetic basis for these individual differences and recent morphometric studies of parietal lobe regions have identified neuroanatomical substrates for individual variability in spontaneous switching behaviour. Moreover, disrupting the function of these latter regions by transcranial magnetic stimulation yields systematic interference effects on switching behaviour, further arguing for a causal role of these regions in perceptual inference. Together, these studies have advanced our understanding of the biological mechanisms by which the brain constructs the contents of consciousness from sensory input. |
| Abstract: | In natural environments, sensory information is embedded in temporally contiguous streams of events. This is typically the case when seeing and listening to a speaker or when engaged in scene analysis. In such contexts, two mechanisms are needed to single out and build a reliable representation of an event (or object): the temporal parsing of information and the selection of relevant information in the stream. It has previously been shown that rhythmic events naturally build temporal expectations that improve sensory processing at predictable points in time. Here, we asked to which extent temporal regularities can improve the detection and identification of events across sensory modalities. To do so, we used a dynamic visual conjunction search task accompanied by auditory cues synchronized or not with the color change of the target (horizontal or vertical bar). Sounds synchronized with the visual target improved search efficiency for temporal rates below 1.4 Hz but did not affect efficiency above that stimulation rate. Desynchronized auditory cues consistently impaired visual search below 3.3 Hz. Our results are interpreted in the context of the Dynamic Attending Theory: specifically, we suggest that a cognitive operation structures events in time irrespective of the sensory modality of input. Our results further support and specify recent neurophysiological findings by showing strong temporal selectivity for audiovisual integration in the auditory-driven improvement of visual search efficiency. |
| Abstract: | Doing two things at once is difficult. When two tasks have to be performed within a short interval, the second is sharply delayed, an effect called the Psychological Refractory Period (PRP). Similarly, when two successive visual targets are briefly flashed, people may fail to detect the second target (Attentional Blink or AB). Although AB and PRP are typically studied in very different paradigms, a recent detailed neuromimetic model suggests that both might arise from the same serial stage during which stimuli gain access to consciousness and, as a result, can be arbitrarily routed to any other appropriate processor. Here, in agreement with this model, we demonstrate that AB and PRP can be obtained on alternate trials of the same cross-modal paradigm and result from limitations in the same brain mechanisms. We asked participants to respond as fast as possible to an auditory target T1 and then to a visual target T2 embedded in a series of distractors, while brain activity was recorded with magneto-encephalography (MEG). For identical stimuli, we observed a mixture of blinked trials, where T2 was entirely missed, and PRP trials, where T2 processing was delayed. MEG recordings showed that PRP and blinked trials underwent identical sensory processing in visual occipito-temporal cortices, even including the non-conscious separation of targets from distractors. However, late activations in frontal cortex (>350ms), strongly influenced by the speed of task-1 execution, were delayed in PRP trials and absent in blinked trials. Our findings suggest that PRP and AB arise from similar cortical stages, can occur with the same exact stimuli, and are merely distinguished by trial-by-trial fluctuations in task processing. |
| Abstract: | Written mathematical notation conveys, in a compact visual form, the nested functional relations among abstract concepts such as operators, numbers or sets. Is the comprehension of mathematical expressions derived from the human capacity for processing the recursive structure of language? Or does algebraic processing rely only on a language-independent network, jointly involving the visual system for parsing the string of mathematical symbols and the intraparietal system for representing numbers and operators? We tested these competing hypotheses by scanning mathematically trained adults while they viewed simple strings ranging from randomly arranged characters to mathematical expressions with up to three levels of nested parentheses. Syntactic effects were observed in behavior and in brain activation measured with functional magnetic resonance imaging (fMRI) and magneto-encephalography (MEG). Bilateral occipito-temporal cortices and right parietal and precentral cortices appeared as the primary nodes for mathematical syntax. MEG estimated that a mathematical expression could be parsed by posterior visual regions in less than 180ms. Nevertheless, a small increase in activation with increasing expression complexity was observed in linguistic regions of interest, including the left inferior frontal gyrus and the posterior superior temporal sulcus. We suggest that mathematical syntax, although arising historically from language competence, becomes "compiled" into visuo-spatial areas in well-trained mathematics students. |
| Abstract: | In dyslexia, anomalous activations have been described in both left temporo-parietal language cortices and in left ventral visual occipito-temporal cortex. However, the reproducibility, task-dependency, and presence of these brain anomalies in childhood rather than adulthood remain debated. We probed the large-scale organization of ventral visual and spoken language areas in dyslexic children using minimal target-detection tasks that were performed equally well by all groups. In 23 normal and 23 dyslexic 10-year-old children from two different socio-economic status (SES) backgrounds, we compared fMRI activity to visually presented houses, faces, and written strings, and to spoken sentences in the native or in a foreign language. Our results confirm a disorganization of both ventral visual and spoken language areas in dyslexic children. Visually, dyslexic children showed a normal lateral-to-medial mosaic of preferences, as well as normal responses to houses and checkerboards, but a reduced activation to words in the visual word form area (VWFA) and to faces in the right fusiform face area (FFA). Auditorily, dyslexic children exhibited reduced responses to speech in posterior temporal cortex, left insula and supplementary motor area, as well as reduced responses to maternal language in subparts of the planum temporale, left basal language area and VWFA. By correlating these two findings, we identify spoken-language predictors of VWFA activation to written words, which differ for dyslexic and normal readers. Similarities in fMRI deficits in both SES groups emphasize the existence of a core set of brain activation anomalies in dyslexia, regardless of culture, language and SES, without however resolving whether these anomalies are a cause or a consequence of impaired reading. |
| Abstract: | Do the neural circuits for reading vary across culture? Reading of visually complex writing systems such as Chinese has been proposed to rely on areas outside the classical left-hemisphere network for alphabetic reading. Here, however, we show that, once potential confounds in cross-cultural comparisons are controlled for by presenting handwritten stimuli to both Chinese and French readers, the underlying network for visual word recognition may be more universal than previously suspected. Using functional magnetic resonance imaging in a semantic task with words written in cursive font, we demonstrate that two universal circuits, a shape recognition system (reading by eye) and a gesture recognition system (reading by hand), are similarly activated and show identical patterns of activation and repetition priming in the two language groups. These activations cover most of the brain regions previously associated with culture-specific tuning. Our results point to an extended reading network that invariably comprises the occipitotemporal visual word-form system, which is sensitive to well-formed static letter strings, and a distinct left premotor region, Exner's area, which is sensitive to the forward or backward direction with which cursive letters are dynamically presented. These findings suggest that cultural effects in reading merely modulate a fixed set of invariant macroscopic brain circuits, depending on surface features of orthographies. |
| Abstract: | Recent advances have been made in the genetics of two human communication skills: speaking and reading. Mutations of the FOXP2 gene cause a severe form of language impairment and orofacial dyspraxia, while single-nucleotide polymorphisms (SNPs) located within a KIAA0319/TTRAP/THEM2 gene cluster and affecting the KIAA0319 gene expression are associated with reading disability. Neuroimaging studies of clinical populations point to partially distinct cerebral bases for language and reading impairments. However, alteration of FOXP2 and KIAA0319/TTRAP/THEM2 polymorphisms on typically developed language networks has never been explored. Here, we genotyped and scanned 94 healthy subjects using fMRI during a reading task. We studied the correlation of genetic polymorphisms with interindividual variability in brain activation and functional asymmetry in frontal and temporal cortices. In FOXP2, SNPs rs6980093 and rs7799109 were associated with variations of activation in the left frontal cortex. In the KIAA0319/TTRAP/THEM2 locus, rs17243157 was associated with asymmetry in functional activation of the superior temporal sulcus (STS). Interestingly, healthy subjects bearing the KIAA0319/TTRAP/THEM2 variants previously identified as enhancing the risk of dyslexia showed a reduced left-hemispheric asymmetry of the STS. Our results confirm that both FOXP2 and KIAA0319/TTRAP/THEM2 genes play an important role in human language development, but probably through different cerebral pathways. The observed cortical effects mirror previous fMRI results in developmental language and reading disorders, and suggest that a continuum may exist between these pathologies and normal interindividual variability. |
| Abstract: | Mathematics shares with language an essential reliance on the human capacity for recursion, permitting the generation of an infinite range of embedded expressions from a finite set of symbols. We studied the role of syntax in arithmetic thinking, a neglected component of numerical cognition, by examining eye movement sequences during the calculation of arithmetic expressions. Specifically, we investigated whether, similar to language, an expression has to be scanned sequentially while the nested syntactic structure is being computed or, alternatively, whether this structure can be extracted quickly and in parallel. Our data provide evidence for the latter: fixations sequences were stereotypically organized in clusters that reflected a fast identification of syntactic embeddings. A syntactically relevant pattern of eye movement was observed even when syntax was defined by implicit procedural rules (precedence of multiplication over addition) rather than explicit parentheses. While the total number of fixations was determined by syntax, the duration of each fixation varied with the complexity of the arithmetic operation at each step. These findings provide strong evidence for a syntactic organization for arithmetic thinking, paving the way for further comparative analysis of differences and coincidences in the instantiation of recursion in language and mathematics. |
| Abstract: | Typical cohorts in brain imaging studies are not large enough for systematic testing of all the information contained in the images. To build testable working hypotheses, investigators thus rely on analysis of previous work, sometimes formalized in a so-called meta-analysis. In brain imaging, this approach underlies the specification of regions of interest (ROIs) that are usually selected on the basis of the coordinates of previously detected effects. In this paper, we propose to use a database of images, rather than coordinates, and frame the problem as transfer learning: learning a discriminant model on a reference task to apply it to a different but related new task. To facilitate statistical analysis of small cohorts, we use a sparse discriminant model that selects predictive voxels on the reference task and thus provides a principled procedure to define ROIs. The benefits of our approach are twofold. First it uses the reference database for prediction, i.e., to provide potential biomarkers in a clinical setting. Second it increases statistical power on the new task. We demonstrate on a set of 18 pairs of functional MRI experimental conditions that our approach gives good prediction. In addition, on a specific transfer situation involving different scanners at different locations, we show that voxel selection based on transfer learning leads to higher detection power on small cohorts. |
| Abstract: | Using a visual-to-auditory sensory-substitution algorithm, congenitally fully blind adults were taught to read and recognize complex images using "soundscapes"--sounds topographically representing images. fMRI was used to examine key questions regarding the visual word form area (VWFA): its selectivity for letters over other visual categories without visual experience, its feature tolerance for reading in a novel sensory modality, and its plasticity for scripts learned in adulthood. The blind activated the VWFA specifically and selectively during the processing of letter soundscapes relative to both textures and visually complex object categories and relative to mental imagery and semantic-content controls. Further, VWFA recruitment for reading soundscapes emerged after 2 hr of training in a blind adult on a novel script. Therefore, the VWFA shows category selectivity regardless of input sensory modality, visual experience, and long-term familiarity or expertise with the script. The VWFA may perform a flexible task-specific rather than sensory-specific computation, possibly linking letter shapes to phonology. |
| Abstract: | The acquisition of reading has an extensive impact on the developing brain and leads to enhanced abilities in phonological processing and visual letter perception. Could this expertise also extend to early visual abilities outside the reading domain? Here we studied the performance of illiterate, ex-illiterate and literate adults closely matched in age, socioeconomic and cultural characteristics, on a contour integration task known to depend on early visual processing. Stimuli consisted of a closed egg-shaped contour made of disconnected Gabor patches, within a background of randomly oriented Gabor stimuli. Subjects had to decide whether the egg was pointing left or right. Difficulty was varied by jittering the orientation of the Gabor patches forming the contour. Contour integration performance was lower in illiterates than in both ex-illiterate and literate controls. We argue that this difference in contour perception must reflect a genuine difference in visual function. According to this view, the intensive perceptual training that accompanies reading acquisition also improves early visual abilities, suggesting that the impact of literacy on the visual system is more widespread than originally proposed. |
| Abstract: | Humans can understand spoken or written sentences presented at extremely fast rates of ?400 wpm, far exceeding the normal speech rate (?150 wpm). How does the brain cope with speeded language? And what processing bottlenecks eventually make language incomprehensible above a certain presentation rate? We used time-resolved fMRI to probe the brain responses to spoken and written sentences presented at five compression rates, ranging from intelligible (60-100\% of the natural duration) to challenging (40\%) and unintelligible (20\%). The results show that cortical areas differ sharply in their activation speed and amplitude. In modality-specific sensory areas, activation varies linearly with stimulus duration. However, a large modality-independent left-hemispheric language network, including the inferior frontal gyrus (pars orbitalis and triangularis) and the superior temporal sulcus, shows a remarkably time-invariant response, followed by a sudden collapse for unintelligible stimuli. Finally, linear and nonlinear responses, reflecting a greater effort as compression increases, are seen at various prefrontal and parietal sites. We show that these profiles fit with a simple model according to which the higher stages of language processing operate at a fixed speed and thus impose a temporal bottleneck on sentence comprehension. At presentation rates faster than this internal processing speed, incoming words must be buffered, and intelligibility vanishes when buffer storage and retrieval operations are saturated. Based on their temporal and amplitude profiles, buffer regions can be identified with the left inferior frontal/anterior insula, precentral cortex, and mesial frontal cortex. |
| Abstract: | The mismatch negativity (MMN) is thought to index the activation of specialized neural networks for active prediction and deviance detection. However, a detailed neuronal model of the neurobiological mechanisms underlying the MMN is still lacking, and its computational foundations remain debated. We propose here a detailed neuronal model of auditory cortex, based on predictive coding, that accounts for the critical features of MMN. The model is entirely composed of spiking excitatory and inhibitory neurons interconnected in a layered cortical architecture with distinct input, predictive, and prediction error units. A spike-timing dependent learning rule, relying upon NMDA receptor synaptic transmission, allows the network to adjust its internal predictions and use a memory of the recent past inputs to anticipate on future stimuli based on transition statistics. We demonstrate that this simple architecture can account for the major empirical properties of the MMN. These include a frequency-dependent response to rare deviants, a response to unexpected repeats in alternating sequences (ABABAA?), a lack of consideration of the global sequence context, a response to sound omission, and a sensitivity of the MMN to NMDA receptor antagonists. Novel predictions are presented, and a new magnetoencephalography experiment in healthy human subjects is presented that validates our key hypothesis: the MMN results from active cortical prediction rather than passive synaptic habituation. |
| 2011 |
| Abstract: | Regions of human ventral extrastriate visual cortex develop specializations for natural categories (e.g., faces) and cultural artifacts (e.g., words). In adults, category-based specializations manifest as greater neural responses in visual regions of the brain (e.g., fusiform gyrus) to some categories over others. However, few studies have examined how these specializations originate in the brains of children. Moreover, it is as yet unknown whether the development of visual specializations hinges on "increases" in the response to the preferred categories, "decreases" in the responses to nonpreferred categories, or "both." This question is relevant to a long-standing debate concerning whether neural development is driven by building up or pruning back representations. To explore these questions, we measured patterns of visual activity in 4-year-old children for 4 categories (faces, letters, numbers, and shoes) using functional magnetic resonance imaging. We report 2 key findings regarding the development of visual categories in the brain: 1) the categories "faces" and "symbols" doubly dissociate in the fusiform gyrus before children can read and 2) the development of category-specific responses in young children depends on cortical responses to nonpreferred categories that decrease as preferred category knowledge is acquired. |
| Abstract: | Recent studies have established a relation between ongoing brain activity fluctuations and intertrial variability in evoked neural responses, perception, and motor performance. Here, we extended these investigations into the domain of cognitive control. Using functional neuroimaging and a sparse event-related design (with long and unpredictable intervals), we measured ongoing activity fluctuations and evoked responses in volunteers performing a Stroop task with color-word interference. Across trials, prestimulus activity of several regions predicted subsequent response speed and across subjects this effect scaled with the Stroop effect size, being significant only in subjects manifesting behavioral interference. These effects occurred only in task relevant as the dorsal anterior cingulate and dorsolateral prefrontal cortex as well as ventral visual areas sensitive to color and visual words. Crucially, in subjects showing a Stroop effect, reaction times were faster when prestimulus activity was higher in task-relevant (color) regions and slower when activity was higher in irrelevant (word form) regions. These findings suggest that intrinsic brain activity fluctuations modulate neural mechanisms underpinning selective voluntary attention and cognitive control. Rephrased in terms of predictive coding models, ongoing activity can hence be considered a proxy of the precision (gain) with which prediction error signals are transmitted upon sensory stimulation. |
| Abstract: | Human decisions are based on accumulating evidence over time for different options. Here we ask a simple question: How is the accumulation of evidence affected by the level of awareness of the information? We examined the influence of awareness on decision-making using combined behavioral methods and magneto-encephalography (MEG). Participants were required to make decisions by accumulating evidence over a series of visually presented arrow stimuli whose visibility was modulated by masking. Behavioral results showed that participants could accumulate evidence under both high and low visibility. However, a top-down strategic modulation of the flow of incoming evidence was only present for stimuli with high visibility: once enough evidence had been accrued, participants strategically reduced the impact of new incoming stimuli. Also, decision-making speed and confidence were strongly modulated by the strength of the evidence for high-visible but not low-visible evidence, even though direct priming effects were identical for both types of stimuli. Neural recordings revealed that, while initial perceptual processing was independent of visibility, there was stronger top-down amplification for stimuli with high visibility than low visibility. Furthermore, neural markers of evidence accumulation over occipito-parietal cortex showed a strategic bias only for highly visible sensory information, speeding up processing and reducing neural computations related to the decision process. Our results indicate that the level of awareness of information changes decision-making: while accumulation of evidence already exists under low visibility conditions, high visibility allows evidence to be accumulated up to a higher level, leading to important strategical top-down changes in decision-making. Our results therefore suggest a potential role of awareness in deploying flexible strategies for biasing information acquisition in line with one's expectations and goals. |
| Abstract: | Recent experimental studies and theoretical models have begun to address the challenge of establishing a causal link between subjective conscious experience and measurable neuronal activity. The present review focuses on the well-delimited issue of how an external or internal piece of information goes beyond nonconscious processing and gains access to conscious processing, a transition characterized by the existence of a reportable subjective experience. Converging neuroimaging and neurophysiological data, acquired during minimal experimental contrasts between conscious and nonconscious processing, point to objective neural measures of conscious access: late amplification of relevant sensory activity, long-distance cortico-cortical synchronization at beta and gamma frequencies, and "ignition" of a large-scale prefronto-parietal network. We compare these findings to current theoretical models of conscious processing, including the Global Neuronal Workspace (GNW) model according to which conscious access occurs when incoming information is made globally available to multiple brain systems through a network of neurons with long-range axons densely distributed in prefrontal, parieto-temporal, and cingulate cortices. The clinical implications of these results for general anesthesia, coma, vegetative state, and schizophrenia are discussed. |
| Abstract: | Reading systematically activates the left lateral occipitotemporal sulcus, at a site known as the visual word form area (VWFA). This site is reproducible across individuals/scripts, attuned to reading-specific processes, and partially selective for written strings relative to other categories such as line drawings. Lesions affecting the VWFA cause pure alexia, a selective deficit in word recognition. These findings must be reconciled with the fact that human genome evolution cannot have been influenced by such a recent and culturally variable activity as reading. Capitalizing on recent functional magnetic resonance imaging experiments, we provide strong corroborating evidence for the hypothesis that reading acquisition partially recycles a cortical territory evolved for object and face recognition, the prior properties of which influenced the form of writing systems. |
| Abstract: | As Turing (1936, Proceedings of the London Mathematical Society) noted, a fundamental process in human cognition is to effect chained sequential operations in which the second operation requires an input from the preceding one. Although a great deal is known about the costs associated with 'independent' (unrelated) operations, e.g., from the classic psychological refractory period paradigm, far less is known about those operations to which Turing referred. We present the results of two behavioural experiments, where participants were required to perform two speeded sequential tasks that were either chained or independent. Both experiments reveal the reaction time cost of chaining, over and above classical dual-task serial costs. Moreover, the chaining operation significantly altered the distribution of reaction times relative to the Independent condition in terms of an increased mean and variance. These results are discussed in terms of the cognitive architecture underlying the serial chaining of cognitive operations. |
| Abstract: | We report performance measures for lexical decision (LD), word naming (NMG), and progressive demasking (PDM) for a large sample of monosyllabic monomorphemic French words (N?=?1,482). We compare the tasks and also examine the impact of word length, word frequency, initial phoneme, orthographic and phonological distance to neighbors, age-of-acquisition, and subjective frequency. Our results show that objective word frequency is by far the most important variable to predict reaction times in LD. For word naming, it is the first phoneme. PDM was more influenced by a semantic variable (word imageability) than LD, but was also affected to a much greater extent by perceptual variables (word length, first phoneme/letters). This may reduce its usefulness as a psycholinguistic word recognition task. |
| Abstract: | In order to understand how genetic mutations might have favored language development in our species, we need a better description of the human brain at the beginning of life. As the linguistic network mainly involves the left perisylvian regions in adults, we used anatomical MRI to study the structural asymmetries of these regions in 14 preverbal infants. Our results show four significant asymmetries. First and foremost, they stress an important but little-known asymmetry: the larger depth of the right superior temporal sulcus (STS) at the base of Heschl's gyrus. Then, we characterized the early forward and upward shift of the posterior end of the right Sylvian fissure, the elongation of the left planum temporale as well as the thickening of the left Heschl's gyrus. The rightward bias of the STS is robust and large, and is not correlated with the leftward asymmetries of the planum and Heschl's gyrus, suggesting that different morphogenetic factors drive these asymmetries. As this sulcus is engaged in multiple high-level functions (e.g. language and theory of mind), and has been spotted as abnormal in several developmental disorders (e.g. schizophrenia, autism), this early rightward asymmetry should be further explored as a target for a genetic evolutionary pressure. |
| Abstract: | Human performance exhibits strong multi-tasking limitations in simple response time tasks. In the psychological refractory period (PRP) paradigm, where two tasks have to be performed in brief succession, central processing of the second task is delayed when the two tasks are performed at short time intervals. Here, we aimed to probe the cortical network underlying this postponement of central processing by simultaneously recording electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) data while 12 subjects performed two simple number-comparison tasks. Behavioral data showed a significant slowing of response times to the second target stimulus at short stimulus-onset asynchronies, together with significant correlations between response times to the first and second target stimulus, i.e., the hallmarks of the PRP effect. The analysis of EEG data showed a significant delay of the post-perceptual P3 component evoked by the second target, which was of similar magnitude as the effect on response times. fMRI data revealed an involvement of parietal and prefrontal regions in dual-task processing. The combined analysis of fMRI and EEG data-based on the trial-by-trial variability of the P3-revealed that BOLD signals in two bilateral regions in the inferior parietal lobe and precentral gyrus significantly covaried with P3 related activity. Our results show that combining neuroimaging methods of high spatial and temporal resolutions can help to identify cortical regions underlying the central bottleneck of information processing, and strengthen the conclusion that fronto-parietal cortical regions participate in a distributed "global neuronal workspace" system that underlies the generation of the P3 component and may be one of the key cerebral underpinnings of the PRP bottleneck. |
| Abstract: | Kant argued that Euclidean geometry is synthesized on the basis of an a priori intuition of space. This proposal inspired much behavioral research probing whether spatial navigation in humans and animals conforms to the predictions of Euclidean geometry. However, Euclidean geometry also includes concepts that transcend the perceptible, such as objects that are infinitely small or infinitely large, or statements of necessity and impossibility. We tested the hypothesis that certain aspects of nonperceptible Euclidian geometry map onto intuitions of space that are present in all humans, even in the absence of formal mathematical education. Our tests probed intuitions of points, lines, and surfaces in participants from an indigene group in the Amazon, the Mundurucu, as well as adults and age-matched children controls from the United States and France and younger US children without education in geometry. The responses of Mundurucu adults and children converged with that of mathematically educated adults and children and revealed an intuitive understanding of essential properties of Euclidean geometry. For instance, on a surface described to them as perfectly planar, the Mundurucu's estimations of the internal angles of triangles added up to ?180 degrees, and when asked explicitly, they stated that there exists one single parallel line to any given line through a given point. These intuitions were also partially in place in the group of younger US participants. We conclude that, during childhood, humans develop geometrical intuitions that spontaneously accord with the principles of Euclidean geometry, even in the absence of training in mathematics. |
| Abstract: | Does extensive practice reduce or eliminate central interference in dual-task processing? We explored the reorganization of task architecture with practice by combining interference analysis (delays in dual-task experiment) and random-walk models of decision making (measuring the decision and non-decision contributions to RT). The main delay observed in the Psychologically Refractory Period at short stimulus onset asynchronies (SOA) values was largely unaffected by training. However, the range of SOAs over which this interference regime held diminished with learning. This was consistent with an overall shift observed in single-task performance from a highly variable decision time to a reliable (non-decision time) contribution to response time. Executive components involved in coordinating dual-task performance decreased (and became more stable) after extensive practice. The results suggest that extensive practice reduces the duration of central decision stages, but that the qualitative property of central seriality remains a structural invariant. |
| Abstract: | Boly et al. (Reports, 13 May 2011, p. 858) investigated cortical connectivity patterns in patients suffering from a disorder of consciousness, using electroencephalography in an auditory oddball paradigm. We point to several inconsistencies in their data, including a failure to replicate the classical mismatch negativity. Data quality, source reconstruction, and statistics would need to be improved to support their conclusions. |
| Abstract: | A classical approach in the neurosciences is to study neural activity modulations induced by a stimulus, a task, etc. This approach is anchored in a behaviourist culture and has proven informative within certain limits. The present paper shows that this approach nonetheless neglects aspects of neural activity that can also contribute important information about brain function. Over the last years, the contributions with the strongest impact on progress in cognitive neuroscience have used other approaches that exploit a spatial or temporal variability of neural activity that standard analyses consider as noise and hence do not take into account. By applying multi-variate analyses, spatial variability of evoked responses has permitted decoding sensory and cognitive representations in the brain. Temporal variability of ongoing neural activity influences how stimuli are perceived trial by trial as well as the associated evoked responses which points out the importance of spontaneous brain activity for cognition. We describe these two kind of approaches based on experiments using functional neuroimaging but the conclusions generalize to other techniques applied in the neurosciences. |
| Abstract: | That disconnection causes clinical symptoms is a very influential concept in behavioral neurology. Criteria for subcortical disconnection usually are symptoms that are distinct from those following cortical lesions and damage to a single, long-range fiber tract. Yet, a recent study combining functional magnetic resonance imaging and fiber tracking concluded that a focal lesion in left parietal white matter provides the only tenable explanation for pure Gerstmann's syndrome, an enigmatic tetrad of acalculia, agraphia, finger agnosia, and left-right disorientation. Such a lesion would affect not only a single fiber tract but crossing or "kissing" of different fiber tracts and hence disconnect separate cortical networks. As fiber crossing is prominent in the cerebral white matter, the authors propose an extension to the subcortical disconnection framework that opens the door to ascribing a more diversified clinical phenomenology to white matter damage and ensuing disconnection than has been the case so far. |
| Abstract: | Human infants, unlike even closely related primates, exhibit a remarkable capacity for language learning. Yet how the underlying anatomical network matures remains largely unknown. The classical view is that of a largely immature brain comprising only a few islands of maturity in primary cortices. This view has favored a description of learning based on bottom-up algorithms and has tended to discard the role of frontal regions, which were assumed to be barely functional early on. Here, using an index based on the normalized T2-weighted magnetic resonance signal, we have quantified maturation within the linguistic network in fourteen 1- to 4-month-old infants. Our results show first that the ventral superior temporal sulcus (STS), and not the inferior frontal area, is the less mature perisylvian region. A significant difference of maturation in the STS favoring the right side is an early testimony of the distinctive left-right development of this structure observed during the whole life. Second, asymmetries of maturation in Broca's area were correlated with asymmetries in the posterior STS and in the parietal segment of the arcuate fasciculus, suggesting that an efficient frontotemporal dorsal pathway might provide infants with a phonological loop circuitry much earlier than expected. |
| Abstract: | Patients with schizophrenia have pronounced deficits in face recognition memory that severely hamper their social skills. The functional mechanisms of these impairments remain unknown. According to the dual-process theory, recognition memory comprises two distinct components: recollection and familiarity. Studies using the Remember/Know procedure in patients with schizophrenia showed impairments in conscious recollection as measured by remember responses, but not in familiarity as measured by know responses. Unfortunately, none of these studies used face material. We investigated both recognition memory components using words and faces and the 'Remember/Know' procedure in 25 patients with schizophrenia and 24 control participants. In the same task, size congruency of stimuli was manipulated between the study and test phases to have a selective impact on know responses for faces. Patients reported fewer remember responses than controls. Size changes between the study and the test affected know responses in controls but not in patients. These results reveal that patients with schizophrenia are impaired in terms of their ability to recollect details about previously seen faces as they are for words. |
| Abstract: | Many common tasks require us to individuate in parallel two or more objects out of a complex scene. Although the mechanisms underlying our abilities to count the number of items, remember the visual properties of objects and to make saccadic eye movements towards targets have been studied separately, each of these tasks require selection of individual objects and shows a capacity limit. Here we show that a common factor-salience-determines the capacity limit in the various tasks. We manipulated bottom-up salience (visual contrast) and top-down salience (task relevance) in enumeration and visual memory tasks. As one item became increasingly salient, the subitizing range was reduced and memory performance for all other less-salient items was decreased. Overall, the pattern of results suggests that our abilities to enumerate and remember small groups of stimuli are grounded in an attentional priority or salience map which represents the location of important items. |
| Abstract: | Inverse inference has recently become a popular approach for analyzing neuroimaging data, by quantifying the amount of information contained in brain images on perceptual, cognitive, and behavioral parameters. As it outlines brain regions that convey information for an accurate prediction of the parameter of interest, it allows to understand how the corresponding information is encoded in the brain. However, it relies on a prediction function that is plagued by the curse of dimensionality, as there are far more features (voxels) than samples (images), and dimension reduction is thus a mandatory step. We introduce in this paper a new model, called Multiclass Sparse Bayesian Regression (MCBR), that, unlike classical alternatives, automatically adapts the amount of regularization to the available data. MCBR consists in grouping features into several classes and then regularizing each class differently in order to apply an adaptive and efficient regularization. We detail these framework and validate our algorithm on simulated and real neuroimaging data sets, showing that it performs better than reference methods while yielding interpretable clusters of features. |
| Abstract: | While medical imaging typically provides massive amounts of data, the extraction of relevant information for predictive diagnosis remains a difficult challenge. Functional magnetic resonance imaging (fMRI) data, that provide an indirect measure of task-related or spontaneous neuronal activity, are classically analyzed in a mass-univariate procedure yielding statistical parametric maps. This analysis framework disregards some important principles of brain organization: population coding, distributed and overlapping representations. Multivariate pattern analysis, i.e., the prediction of behavioral variables from brain activation patterns better captures this structure. To cope with the high dimensionality of the data, the learning method has to be regularized. However, the spatial structure of the image is not taken into account in standard regularization methods, so that the extracted features are often hard to interpret. More informative and interpretable results can be obtained with the l(1) norm of the image gradient, also known as its total variation (TV), as regularization. We apply for the first time this method to fMRI data, and show that TV regularization is well suited to the purpose of brain mapping while being a powerful tool for brain decoding. Moreover, this article presents the first use of TV regularization for classification. |
| Abstract: | Linguistic analyses suggest that sentences are not mere strings of words but possess a hierarchical structure with constituents nested inside each other. We used functional magnetic resonance imaging (fMRI) to search for the cerebral mechanisms of this theoretical construct. We hypothesized that the neural assembly that encodes a constituent grows with its size, which can be approximately indexed by the number of words it encompasses. We therefore searched for brain regions where activation increased parametrically with the size of linguistic constituents, in response to a visual stream always comprising 12 written words or pseudowords. The results isolated a network of left-hemispheric regions that could be dissociated into two major subsets. Inferior frontal and posterior temporal regions showed constituent size effects regardless of whether actual content words were present or were replaced by pseudowords (jabberwocky stimuli). This observation suggests that these areas operate autonomously of other language areas and can extract abstract syntactic frames based on function words and morphological information alone. On the other hand, regions in the temporal pole, anterior superior temporal sulcus and temporo-parietal junction showed constituent size effect only in the presence of lexico-semantic information, suggesting that they may encode semantic constituents. In several inferior frontal and superior temporal regions, activation was delayed in response to the largest constituent structures, suggesting that nested linguistic structures take increasingly longer time to be computed and that these delays can be measured with fMRI. |
| Abstract: | The visual word form area (VWFA) is a ventral stream visual area that develops expertise for visual reading [1-3]. It is activated across writing systems and scripts [4, 5] and encodes letter strings irrespective of case, font, or location in the visual field [1] with striking anatomical reproducibility across individuals [6]. In the blind, comparable reading expertise can be achieved using Braille. This study investigated which area plays the role of the VWFA in the blind. One would expect this area to be at either parietal or bilateral occipital cortex, reflecting the tactile nature of the task and crossmodal plasticity, respectively [7, 8]. However, according to the metamodal theory [9], which suggests that brain areas are responsive to a specific representation or computation regardless of their input sensory modality, we predicted recruitment of the left-hemispheric VWFA, identically to the sighted. Using functional magnetic resonance imaging, we show that activation during Braille reading in blind individuals peaks in the VWFA, with striking anatomical consistency within and between blind and sighted. Furthermore, the VWFA is reading selective when contrasted to high-level language and low-level sensory controls. Thus, we propose that the VWFA is a metamodal reading area that develops specialization for reading regardless of visual experience. |
| Abstract: | Using simultaneous electroencephalography as a measure of ongoing activity and functional magnetic resonance imaging (fMRI) as a measure of the stimulus-driven neural response, we examined whether the amplitude and phase of occipital alpha oscillations at the onset of a brief visual stimulus affects the amplitude of the visually evoked fMRI response. When accounting for intrinsic coupling of alpha amplitude and occipital fMRI signal by modeling and subtracting pseudo-trials, no significant effect of prestimulus alpha amplitude on the evoked fMRI response could be demonstrated. Regarding the effect of alpha phase, we found that stimuli arriving at the peak of the alpha cycle yielded a lower blood oxygenation level-dependent (BOLD) fMRI response in early visual cortex (V1/V2) than stimuli presented at the trough of the cycle. Our results therefore show that phase of occipital alpha oscillations impacts the overall strength of a visually evoked response, as indexed by the BOLD signal. This observation complements existing evidence that alpha oscillations reflect periodic variations in cortical excitability and suggests that the phase of oscillations in postsynaptic potentials can serve as a mechanism of gain control for incoming neural activity. Finally, our findings provide a putative neural basis for observations of alpha phase dependence of visual perceptual performance. |
| Abstract: | The Visual Word Form Area (VWFA) is part of the left ventral visual stream that underlies the invariant identification of visual words. It remains debated whether this region is truly selective for words relative to common objects; why this particular part of the visual system is reproducibly engaged in reading; and whether reading expertise also relies on perceptual learning within earlier visual areas. In this fMRI study we matched written words and line-drawings of objects in luminance, contour length and number of features. We then compared them to control images made by scrambling procedures that kept local features intact. Greater responses to written words than to objects were found not only in the VWFA, but also in areas V1/V2 and V3v/V4. Furthermore, by contrasting stimuli reduced either to line junctions (vertices) or to line midsegments, we showed that the VWFA partially overlaps with regions of ventral visual cortex particularly sensitive to the presence of line junctions that are useful for object recognition. Our results indicate that preferential processing of written words can be observed at multiple levels of the visual system. It is possible that responses in early visual areas might be due to some remaining differences between words and controls not eliminated in the present stimuli. However, our results concur with recent comparisons of literates and illiterates and suggest that these early visual activations reflect the effects of perceptual learning under pressure for fast, parallel processing that is more prominent in reading than other visual cognitive processes. |
| Abstract: | In this study, we investigate whether multiple digits can be processed at a semantic level without awareness, either serially or in parallel. In two experiments, we presented participants with two successive sets of four simultaneous Arabic digits. The first set was masked and served as a subliminal prime for the second, visible target set. According to the instructions, participants had to extract from the target set either the mean or the sum of the digits, and to compare it with a reference value. Results showed that participants applied the requested instruction to the entire set of digits that was presented below the threshold of conscious perception, because their magnitudes jointly affected the participant's decision. Indeed, response decision could be accurately modeled as a sigmoid logistic function that pooled together the evidence provided by the four targets and, with lower weights, the four primes. In less than 800ms, participants successfully approximated the addition and mean tasks, although they tended to overweight the large numbers, particularly in the sum task. These findings extend previous observations on ensemble coding by showing that set statistics can be extracted from abstract symbolic stimuli rather than low-level perceptual stimuli, and that an ensemble code can be represented without awareness. |
| Abstract: | For a given physical duration, certain events can be experienced as subjectively longer in duration than others. Try this for yourself: take a quick glance at the second hand of a clock. Immediately, the tick will pause momentarily and appear to be longer than the subsequent ticks. Yet, they all last exactly 1?s. By and large, a deviant or an unexpected stimulus in a series of similar events (same duration, same features) can elicit a relative overestimation of subjective time (or "time dilation") but, as is shown here, this is not always the case. We conducted an event-related functional magnetic neuroimaging study on the time dilation effect. Participants were presented with a series of five visual discs, all static and of equal duration (standards) except for the fourth one, a looming or a receding target. The duration of the target was systematically varied and participants judged whether it was shorter or longer than all other standards in the sequence. Subjective time dilation was observed for the looming stimulus but not for the receding one, which was estimated to be of equal duration to the standards. The neural activation for targets (looming and receding) contrasted with the standards revealed an increased activation of the anterior insula and of the anterior cingulate cortex. Contrasting the looming with the receding targets (i.e., capturing the time dilation effect proper) revealed a specific activation of cortical midline structures. The implication of midline structures in the time dilation illusion is here interpreted in the context of self-referential processes. |
| Abstract: | According to hierarchical predictive coding models, the cortex constantly generates predictions of incoming stimuli at multiple levels of processing. Responses to auditory mismatches and omissions are interpreted as reflecting the prediction error when these predictions are violated. An alternative interpretation, however, is that neurons passively adapt to repeated stimuli. We separated these alternative interpretations by designing a hierarchical auditory novelty paradigm and recording human EEG and magnetoencephalographic (MEG) responses to mismatching or omitted stimuli. In the crucial condition, participants listened to frequent series of four identical tones followed by a fifth different tone, which generates a mismatch response. Because this response itself is frequent and expected, the hierarchical predictive coding hypothesis suggests that it should be cancelled out by a higher-order prediction. Three consequences ensue. First, the mismatch response should be larger when it is unexpected than when it is expected. Second, a perfectly monotonic sequence of five identical tones should now elicit a higher-order novelty response. Third, omitting the fifth tone should reveal the brain's hierarchical predictions. The rationale here is that, when a deviant tone is expected, its omission represents a violation of two expectations: a local prediction of a tone plus a hierarchically higher expectation of its deviancy. Thus, such an omission should induce a greater prediction error than when a standard tone is expected. Simultaneous EEE- magnetoencephalographic recordings verify those predictions and thus strongly support the predictive coding hypothesis. Higher-order predictions appear to be generated in multiple areas of frontal and associative cortices. |
| Abstract: | In recent years much has been learned about how a single computational processing step is implemented in the brain. By contrast, we still have surprisingly little knowledge of the neuronal mechanisms by which multiple such operations are sequentially assembled into mental algorithms. We outline a theory of how individual neural processing steps might be combined into serial programs. We propose a hybrid neuronal device: each step involves massively parallel computation that feeds a slow and serial production system. Production selection is mediated by a system of competing accumulator neurons that extends the role of these neurons beyond the selection of a motor action. Productions change the state of sensory and mnemonic neurons and iteration of such cycles provides a basis for mental programs. |
| 2010 |
| Abstract: | In the last decade, great progress has been made in characterizing the accumulation of neural information during simple unitary perceptual decisions. However, much less is known about how sequentially presented evidence is integrated over time for successful decision making. The aim of this study was to study the mechanisms of sequential decision making in humans. In a magnetoencephalography (MEG) study, we presented healthy volunteers with sequences of centrally presented arrows. Sequence length varied between one and five arrows, and the accumulated directions of the arrows informed the subject about which hand to use for a button press at the end of the sequence (e.g., LRLRR should result in a right-hand press). Mathematical modeling suggested that nonlinear accumulation was the rational strategy for performing this task in the presence of no or little noise, whereas quasilinear accumulation was optimal in the presence of substantial noise. MEG recordings showed a correlate of evidence integration over parietal and central cortex that was inversely related to the amount of accumulated evidence (i.e., when more evidence was accumulated, neural activity for new stimuli was attenuated). This modulation of activity likely reflects a top-down influence on sensory processing, effectively constraining the influence of sensory information on the decision variable over time. The results indicate that, when making decisions on the basis of sequential information, the human nervous system integrates evidence in a nonlinear manner, using the amount of previously accumulated information to constrain the accumulation of additional evidence. |
| Abstract: | Young children often make mirror errors when learning to read and write, for instance writing their first name from right to left in English. This competence vanishes in most adult readers, who typically cannot read mirror words but retain a strong competence for mirror recognition of images. We used fast behavioral and fMRI repetition priming to probe the brain mechanisms underlying mirror generalization and its absence for words in adult readers. In two groups of French and Japanese readers, we show that the left fusiform visual word form area, a major site of learning during reading acquisition, simultaneously shows a maximal effect of mirror priming for pictures and an absence of mirror priming for words. Thus, learning to read recruits an area which possesses a property of mirror invariance, seemingly present in all primates, which is deleterious for letter recognition and may explain children's transient mirror errors. |
| Abstract: | Does literacy improve brain function? Does it also entail losses? Using functional magnetic resonance imaging, we measured brain responses to spoken and written language, visual faces, houses, tools, and checkers in adults of variable literacy (10 were illiterate, 22 became literate as adults, and 31 became literate in childhood). As literacy enhanced the left fusiform activation evoked by writing, it induced a small competition with faces at this location but also broadly enhanced visual responses in fusiform and occipital cortex, extending to area V1. Literacy also enhanced phonological activation to speech in the planum temporale and afforded a top-down activation of orthography from spoken inputs. Most changes occurred even when literacy was acquired in adulthood, emphasizing that both childhood and adult education can profoundly refine cortical organization. |
| Abstract: | Understanding how language emerged in our species calls for a detailed investigation of the initial specialization of the human brain for speech processing. Our earlier research demonstrated that an adult-like left-lateralized network of perisylvian areas is already active when infants listen to sentences in their native language, but did not address the issue of the specialization of this network for speech processing. Here we used fMRI to study the organization of brain activity in two-month-old infants when listening to speech or to music. We also explored how infants react to their mother's voice relative to an unknown voice. The results indicate that the well-known structural asymmetry already present in the infants' posterior temporal areas has a functional counterpart: there is a left-hemisphere advantage for speech relative to music at the level of the planum temporale. The posterior temporal regions are thus differently sensitive to the auditory environment very early on, channelling speech inputs preferentially to the left side. Furthermore, when listening to the mother's voice, activation was modulated in several areas, including areas involved in emotional processing (amygdala, orbito-frontal cortex), but also, crucially, a large extent of the left posterior temporal lobe, suggesting that the mother's voice plays a special role in the early shaping of posterior language areas. Both results underscore the joint contributions of genetic constraints and environmental inputs in the fast emergence of an efficient cortical network for language processing in humans. |
| Abstract: | During the last trimester of human pregnancy, the cerebral cortex of foetuses becomes greatly and quickly gyrified, and post-mortem studies have demonstrated that hemispheres are already asymmetric at the level of Heschl gyrus, planum temporale and superior temporal sulcus (STS). Recently, magnetic resonance imaging (MRI) and dedicated post-processing tools enabled the quantitative study of brain development non-invasively in the preterm newborn. However, previous investigations were conducted either over the whole brain or in specific sulci. These approaches may consequently fail to highlight most cerebral sites, where anatomical landmarks are hard to delineate among individuals. In this cross-sectional study, we aimed to blindly and automatically map early asymmetries over the immature cortex. Voxel-based analyses of cortical and white matter masks were performed over a group of 25 newborns from 26 to 36weeks of gestational age. Inter-individual variations associated with increasing age were first detected in large cerebral regions, with a prevalence of the right hemisphere in comparison with the left. Asymmetries were further highlighted in three specific cortical regions. Confirming previous studies, we observed deeper STS on the right side and larger posterior region of the sylvian fissure on the left side, close to planum temporale. For the first time, we also detected larger anterior region of the sylvian fissure on the left side, close to Broca's region. This study demonstrated that perisylvian regions are the only regions to be asymmetric from early on, suggesting their anatomical specificity for the emergence of functional lateralization in language processing prior to language exposure. |
| Abstract: | Abstract When two displays are presented in close temporal succession at the same location, how does the brain assign them to one versus two conscious percepts? We investigate this issue using a novel reading paradigm in which the odd and even letters of a string are presented alternatively at a variable rate. The results reveal a window of temporal integration during reading, with a nonlinear boundary around approximately 80 msec of presentation duration. Below this limit, the oscillating stimulus is easily fused into a single percept, with all characteristics of normal reading. Above this limit, reading times are severely slowed and suffer from a word-length effect. ERPs indicate that, even at the fastest frequency, the oscillating stimulus elicits synchronous oscillations in posterior visual cortices, while late ERP components sensitive to lexical status vanish above the fusion threshold. Thus, the fusion/segregation dilemma is not resolved by retinal or subcortical filtering, but at cortical level by at most 300 msec. The results argue against theories of visual word recognition and letter binding that rely on temporal synchrony or other fine temporal codes. |
| Abstract: | Letter position dyslexia (LPD) is a deficit in the encoding of letter position within words. It is characterized by errors of letter migration within words, such as reading trail as trial and form as from. In order to examine whether LPD is domain-specific, and to assess the domain-specificity of the visual analysis system, this study explored whether LPD extends to number reading, by testing whether individuals who have letter migrations in word reading also show migrations while reading numbers. The reading of words and numbers of 12 Hebrew-speaking individuals with developmental LPD was assessed. Experiment 1 tested reading aloud of words and numbers, and Experiment 2 tested same-different decisions in words and numbers. The findings indicated that whereas the participants with developmental LPD showed a large number of migration errors in reading words, 10 of them read numbers well, without migration errors, and not differently from the control participants. A closer inspection of the pattern of errors in words and numbers of two individuals who had migrations in both numbers and words showed qualitative differences in the characteristics of migration errors in the two types of stimuli. In word reading, migration errors appeared predominantly in middle letters, whereas the errors in numbers occurred mainly in final (rightmost) digits. Migrations in numbers occurred almost exclusively in adjacent digits, but in words migrations occurred both in adjacent and in nonadjacent letters. The results thus indicate that words can be selectively impaired, without a parallel impairment in numbers, and that even when numbers are also impaired they show different error pattern. Thus, the visual analyzer is actually an orthographic visual analyzer, a module that is domain-specific for the analysis of words. |
| Abstract: | We explore the mechanisms sub-tending the re-organization and memorization of visual information by studying how these mechanisms fail in patients with schizophrenia. Several studies have suggested that patients have difficulties in organizing information in perception and memory. We explore to what extent prompting patients to group items influences memory performance. We distinguish automatic grouping from top-down grouping processes, which are especially involved in re-organizing information. The main task was to memorize pairs of figures. Following manipulation of proximity, pairs of figures were part of the same perceptual group (within-group pair, formed on the basis of automatic grouping) or belonged to different groups (between-group pairs, re-grouped through top-down processes). Prior to the memory task, subjects ran a perception task prompting them to prioritize either within-group or between-group pairs. Unlike patients, controls globally benefited from grouping by proximity in the memory task. In addition, the results showed that prioritizing between-group pairs had a deleterious effect in patients, but with a large decrement in memory performance in the case of within-group rather than between-group figures. This occurred despite preserved focalization on within-group figures, as shown by eye-movement recordings. The suggestion is that when patients are prompted to re-group separate items, they can do so, but the benefit derived from automatic grouping is then not only lost but also reversed. This suggests re-organizing visual information not only involves re-grouping separate items but also integrating these new groups in a unified representation, which is impaired in patients with schizophrenia. |
| Abstract: | Investigations of memory impairment in schizophrenia have frequently revealed a strategic processing deficit at encoding. The authors studied an early encoding process, refreshing (in this case, thinking of a stimulus that has just-previously been presented), and its impact on recognition memory in schizophrenia. Following simultaneous presentation of three words or a single word in the top, middle, or bottom position of the screen, 25 patients with schizophrenia and 25 control participants saw and read a new word (read condition), or a word presented on the previous screen (repeat condition), or saw a dot indicating that they should think of and say the last word to have appeared in that position (refresh condition). Later, on a surprise test, participants were asked to recognize words seen previously and give a Remember, Know, or Guess response according to whether they recognized each on the basis of conscious recollection, familiarity, or guessing. The cognitive operation of refreshing was impaired in schizophrenia: patients were slower on 1-word trials and less accurate on 3-word trials to refresh a word, and their Remember responses did not benefit from refreshing. |
| Abstract: | Perceptual decisions can be made when sensory input affords an inference about what generated that input. Here, we report findings from two independent perceptual experiments conducted during functional magnetic resonance imaging (fMRI) with a sparse event-related design. The first experiment, in the visual modality, involved forced-choice discrimination of coherence in random dot kinematograms that contained either subliminal or periliminal motion coherence. The second experiment, in the auditory domain, involved free response detection of (non-semantic) near-threshold acoustic stimuli. We analysed fluctuations in ongoing neural activity, as indexed by fMRI, and found that neuronal activity in sensory areas (extrastriate visual and early auditory cortex) biases perceptual decisions towards correct inference and not towards a specific percept. Hits (detection of near-threshold stimuli) were preceded by significantly higher activity than both misses of identical stimuli or false alarms, in which percepts arise in the absence of appropriate sensory input. In accord with predictive coding models and the free-energy principle, this observation suggests that cortical activity in sensory brain areas reflects the precision of prediction errors and not just the sensory evidence or prediction errors per se. |
| Abstract: | Last year, a study appeared that questioned the generally held assumption of a generic coupling between electrical and hemodynamic signs of neural activity (Sirotin and Das, 2009). Although the findings of that study can barely surprise the specialists in the field, it has caused a considerable confusion in the nonspecialist community due to the unwarranted claim of having discovered a "hitherto unknown signal." According to this claim, functional magnetic resonance imaging (fMRI) would pick up not only signals that reflect electrical brain activity but also purely hemodynamic signals that are not linked to neural activity. Here, we show that that study's failure to obtain significant electrophysiological responses to task structure is easily understood on the basis of findings reported for related functional paradigms. Ironically and counter its intention, the study by Sirotin and Das reminds us of the exquisite sensitivity of spatially pooled hemodynamic signals and the limitations of recording only very local samples of electrical activity by microelectrodes. We suggest that this sensitivity of hemodynamic signals should be converted into spatial resolution. In other words, hemodynamic signals should be used to create maps. Further, we suggest that electrical recordings should be obtained at systematically varying functional positions across these maps. And we speculate that under such appropriate experimental and analytical circumstances correspondence between the two modalities would be retrieved-at the expense of a novel signal lost in oblivion. |
| Abstract: | While the neural correlates of unconscious perception and subliminal priming have been largely studied for visual stimuli, little is known about their counterparts in the auditory modality. Here we used a subliminal speech priming method in combination with fMRI to investigate which regions of the cerebral network for language can respond in the absence of awareness. Participants performed a lexical decision task on target items preceded by subliminal primes, which were either phonetically identical or different from the target. Moreover, the prime and target could be spoken by the same speaker or by two different speakers. Word repetition reduced the activity in the insula and in the left superior temporal gyrus. Although the priming effect on reaction times was independent of voice manipulation, neural repetition suppression was modulated by speaker change in the superior temporal gyrus while the insula showed voice-independent priming. These results provide neuroimaging evidence of subliminal priming for spoken words and inform us on the first, unconscious stages of speech perception. |
| Abstract: | Psychologists often dismiss introspection as an inappropriate measure, yet subjects readily volunteer detailed descriptions of the time and effort that they spent on a task. Are such reports really so inaccurate? We asked subjects to perform a psychological refractory period experiment followed by extensive quantified introspection. On each trial, just after their objective responses, subjects provided no less than four subjective estimates of the timing of sensory, decision and response events. Based on these subjective variables, we reconstructed the phenomenology of an average trial and compared it to objective times and to predictions derived from the central interference model. Introspections of decision time were highly correlated with objective measures, but there was one point of drastic distortion: subjects were largely unaware that the second target was waiting while the first task was being completed, the psychological refractory period effect. Thus, conscious perception is systematically delayed and distorted while central processing resources are monopolized by another task. |
| Abstract: | ABSTRACT : BACKGROUND : Saccadic eye movements align the two eyes precisely to foveate a target. Trial-by-trial variance of eye movement is always observed within an identical experimental condition. This has often been treated as experimental error without addressing its significance. The present study examined statistical linkages between the two eyes' movements, namely interocular yoking, for the variance of eye position and velocity. METHODS : Horizontal saccadic movements were recorded from twelve right-eye-dominant subjects while they decided on saccade direction in Go-Only sessions and on both saccade execution and direction in Go/NoGo sessions. We used infrared corneal reflection to record simultaneously and independently the movement of each eye. Quantitative measures of yoking were provided by mutual information analysis of eye position or velocity, which is sensitive to both linear and non-linear relationships between the eyes' movements. Our mutual information analysis relied on the variance of the eyes movements in each experimental condition. The range of movements for each eye varies for different conditions so yoking was further studied by comparing GO-Only vs. Go/NoGo sessions, leftward vs. rightward saccades. RESULTS : Mutual information analysis showed that velocity yoking preceded positional yoking. Cognitive load increased trial variances of velocity with no increase in velocity yoking, suggesting that cognitive load may alter neural processes in areas to which oculomotor control is not tightly linked. The comparison between experimental conditions showed that interocular linkage in velocity variance of the right eye lagged that of the left eye during saccades. CONCLUSIONS : We conclude quantitative measure of interocular yoking based on trial-to-trial variance within a condition, as well as variance between conditions, provides a powerful tool for studying the binocular movement mechanism. |
| Abstract: | Reconstruction of neural current sources from magnetoencephalography (MEG) data provides two independent estimates of the instantaneous current modulus and its direction. Here, we explore how different information on the modulus and direction affects the inter-hemisphere connectivity of the human medial temporal complex (hMT+). Connectivity was quantified by mutual information values of paired time series of current moduli or directions, with the joint probability distribution estimated with an optimized Gaussian kernel. These time series were obtained from tomographic analysis of single-trial MEG responses to a visual motion stimulus. With a high-contrast stimulus, connectivity measures based on the modulus were relatively strong in the prestimulus period, continuing until 100ms after stimulus onset. The strongest modulus connectivity was produced with a long lag (19ms) of the right hMT+ after the left hMT+. On the other hand, connectivity measures based on direction were relatively strong after 100ms, with a short delay of less than 6ms. These results suggest that nonspecific and probably indirect communication between the homologous areas is turned, by the stimulus arrival, into more precise and direct communication through the corpus callosum. The orientation of the estimated current vector for the strong connectivity can be explained by the curvature of the active cortical sheet. The temporal patterns of modulus and directional connectivity were different at low contrast, but similar to those at high contrast. We conclude that the modulus and direction indicate distinct functional connectivity modes. |
| Abstract: | The physiological basis of human cerebral asymmetry for language remains mysterious. We have used simultaneous physiological and anatomical measurements to investigate the issue. Concentrating on neural oscillatory activity in speech-specific frequency bands and exploring interactions between gestural (motor) and auditory-evoked activity, we find, in the absence of language-related processing, that left auditory, somatosensory, articulatory motor, and inferior parietal cortices show specific, lateralized, speech-related physiological properties. With the addition of ecologically valid audiovisual stimulation, activity in auditory cortex synchronizes with left-dominant input from the motor cortex at frequencies corresponding to syllabic, but not phonemic, speech rhythms. Our results support theories of language lateralization that posit a major role for intrinsic, hardwired perceptuomotor processing in syllabic parsing and are compatible both with the evolutionary view that speech arose from a combination of syllable-sized vocalizations and meaningful hand gestures and with developmental observations suggesting phonemic analysis is a developmentally acquired process. |
| Abstract: | Most bilinguals understand their second language more slowly than their first. This behavioral asymmetry may arise from the perceptual, phonological, lexicosemantic, or strategic components of bilingual word processing. However, little is known about the neural source of such language dominance and how it is regulated in the bilingual brain. Using functional magnetic resonance imaging, we found that unconscious neural priming in bilingual word recognition is language nonselective in the left midfusiform gyrus but exhibits a preference for the dominant language in the left posterior middle temporal gyrus (MTG). These early-stage components of reading were located slightly upstream of the left midlateral MTG, which exhibited enhanced response during a conscious switch of language. Effective connectivity analysis revealed that this language switch is triggered by reentrant signals from inferior frontal cortex and not by bottom-up signals from occipitotemporal cortex. We further confirmed that magnetic stimulation of the same inferior frontal region interferes with conscious language control but does not disrupt unconscious priming by masked words. Collectively, our results demonstrate that the neural bottleneck in the bilingual brain is a cross-language asymmetry of form-meaning association in inferolateral temporal cortex, which is overcome by a top-down cognitive control for implementing a task schema in each language. |
| Abstract: | Hemispheric rivalry models of spatial neglect suggest that the left hemisphere becomes hyperactive following right-hemisphere lesions since the two hemispheres normally exert an inhibitory influence on each other via callosal connections. Using a masked hemifield priming paradigm, we investigated whether the putative change in hemispheric balance involves other, higher-order abstract representational systems in spatial neglect. Participants consisted of 12 neglect patients with right-hemisphere damage and three groups of control participants, i.e., 12 young healthy controls, 10 age-matched healthy controls and 10 right-hemisphere patients without spatial neglect. In each trial, participants made semantic categorization about a centrally presented target word which was preceded by a masked prime flashed either to the left or right visual field. All three control groups exhibited strong left-hemisphere advantage in inhibitory syllabic priming, consistent with the known left-hemisphere dominance in lexical inhibition during reading. By contrast, neglect patients exhibited a symmetrical pattern of priming between the left and right visual fields. These results suggest that (1) the neglected hemifield can rapidly extract abstract information even from weak and normally non-perceptible visual stimuli, but that (2) the normal left hemispheric dominance in reading is absent in neglect patients probably because of the generalized hyperactivity of the left hemisphere. Our results demonstrate a covert behavioral change in spatial neglect which may reflect the altered inter-hemispheric balance in the bilateral word recognition system encompassing lexico-semantic memory. |
| Abstract: | Attaching meaning to arbitrary symbols (i.e. words) is a complex and lengthy process. In the case of numbers, it was previously suggested that this process is grounded on two early pre-verbal systems for numerical quantification: the approximate number system (ANS or 'analogue magnitude'), and the object tracking system (OTS or 'parallel individuation'), which children are equipped with before symbolic learning. Each system is based on dedicated neural circuits, characterized by specific computational limits, and each undergoes a separate developmental trajectory. Here, I review the available cognitive and neuroscientific data and argue that the available evidence is more consistent with a crucial role for the ANS, rather than for the OTS, in the acquisition of abstract numerical concepts that are uniquely human. |
| Abstract: | Developmental dyscalculia is a learning disability that affects the acquisition of knowledge about numbers and arithmetic. It is widely assumed that numeracy is rooted on the "number sense", a core ability to grasp numerical quantities that humans share with other animals and deploy spontaneously at birth. To probe the links between number sense and dyscalculia, we used a psychophysical test to measure the Weber fraction for the numerosity of sets of dots, hereafter called number acuity. We show that number acuity improves with age in typically developing children. In dyscalculics, numerical acuity is severely impaired, with 10-year-old dyscalculics scoring at the level of 5-year-old normally developing children. Moreover, the severity of the number acuity impairment predicts the defective performance on tasks involving the manipulation of symbolic numbers. These results establish for the first time a clear association between dyscalculia and impaired "number sense", and they may open up new horizons for the early diagnosis and rehabilitation of mathematical learning deficits. |
| Abstract: | Abstract Language and arithmetic are both lateralized to the left hemisphere in the majority of right-handed adults. Yet, does this similar lateralization reflect a single overall constraint of brain organization, such an overall "dominance" of the left hemisphere for all linguistic and symbolic operations? Is it related to the lateralization of specific cerebral subregions? Or is it merely coincidental? To shed light on this issue, we performed a "colateralization analysis" over 209 healthy subjects: We investigated whether normal variations in the degree of left hemispheric asymmetry in areas involved in sentence listening and reading are mirrored in the asymmetry of areas involved in mental arithmetic. Within the language network, a region-of-interest analysis disclosed partially dissociated patterns of lateralization, inconsistent with an overall "dominance" model. Only two of these areas presented a lateralization during sentence listening and reading which correlated strongly with the lateralization of two regions active during calculation. Specifically, the profile of asymmetry in the posterior superior temporal sulcus during sentence processing covaried with the asymmetry of calculation-induced activation in the intraparietal sulcus, and a similar colateralization linked the middle frontal gyrus with the superior posterior parietal lobule. Given recent neuroimaging results suggesting a late emergence of hemispheric asymmetries for symbolic arithmetic during childhood, we speculate that these colateralizations might constitute developmental traces of how the acquisition of linguistic symbols affects the cerebral organization of the arithmetic network. |
| Abstract: | Expert readers exhibit a remarkable ability to recognize handwriting, in spite of enormous variability in character shape-a competence whose cerebral underpinnings are unknown. Subliminal priming, combined with neuroimaging, can reveal which brain areas automatically compute an invariant representation of visual stimuli. Here, we used behavioral and fMRI priming to study the areas involved in invariant handwritten word recognition. Compared to printed words, easily readable handwritten words caused additional activity in ventral occipitotemporal cortex, particularly in the right hemisphere, while difficult handwriting also mobilized an attentional parietofrontal network. Remarkably, however, subliminal repetition effects were observed across printed and handwritten styles, whether easy or difficult to read, both behaviorally and in the activation of the left visual word form area (VWFA). These results indicate that the left inferotemporal VWFA possesses an unsuspected degree of fast and automatic visual invariance for handwritten words, although surprisingly this invariance can be reflected both as repetition suppression and as repetition enhancement. |
| Abstract: | Eighty years ago, the Austrian neurologist Josef Gerstmann observed in a few patients a concomitant impairment in discriminating their own fingers, writing by hand, distinguishing left from right and performing calculations. He claimed that this tetrad of symptoms constituted a syndromal entity, assigned it to a lesion of the dominant parietal lobe and suggested that it was due to damage of a common functional denominator. Ever since, these claims have been debated and an astute synopsis and sceptical discussion was presented 40 years ago by MacDonald Critchley in this journal. Nonetheless, Gerstmann's syndrome has continued to intrigue both clinical neurologists and researchers in neuropsychology, and more frequently than not is described in textbooks as an example of parietal lobe damage. In this review, we revisit the chequered history of this syndrome, which can be seen as a case study of the dialectic evolution of concepts in neuropsychology. In light of several modern era findings of pure cases we conclude that it is legitimate to label the conjunction of symptoms first described by Gerstmann as a 'syndrome', but that it is very unlikely that damage to the same population of cortical neurons should account for all of the four symptoms. Instead, we propose that a pure form of Gerstmann's syndrome might arise from disconnection, via a lesion, to separate but co-localized fibre tracts in the subcortical parietal white matter, a hypothesis for which we have recently provided evidence using combined imaging of functional and structural organization in the healthy brain. |
| Abstract: | Trial-by-trial variability in perceptual performance on identical stimuli has been related to spontaneous fluctuations in ongoing activity of intrinsic functional connectivity networks (ICNs). In a paradigm requiring sustained vigilance for instance, we previously observed that higher prestimulus activity in a cingulo-insular-thalamic network facilitated subsequent perception. Here, we test our proposed interpretation that this network underpins maintenance of tonic alertness. We used simultaneous acquisition of functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) in the absence of any paradigm to test an ensuing hypothesis, namely that spontaneous fluctuations in this ICN's activity (as measured by fMRI) should show a positive correlation with the electrical signatures of tonic alertness (as recorded by concurrent EEG). We found in human subjects (19 male, 7 female) that activity in a network comprising dorsal anterior cingulate cortex, anterior insula, anterior prefrontal cortex and thalamus is positively correlated with global field power (GFP) of upper alpha band (10-12 Hz) oscillations, the most consistent electrical index of tonic alertness. Conversely, and in line with earlier findings, alpha band power was negatively correlated with activity in another ICN, the so-called dorsal attention network which is most prominently involved in selective spatial attention. We propose that the cingulo-insular-thalamic network serves maintaining tonic alertness through generalized expression of cortical alpha oscillations. Attention is mediated by activity in other systems, e.g., the dorsal attention network for space, selectively disrupts alertness-related suppression and hence manifests as local attenuation of alpha activity. |
| Abstract: | Anterior insular cortex is among the non-sensory brain regions most commonly found activated in functional brain imaging studies on visual and auditory perception. However, most of these studies do not explicitly address the functional role of this specific brain region in perception, but rather report its activation as a by-product. Here, we attempt to characterize the involvement of anterior insular cortex in various perceptual paradigms, including studies of visual awareness, perceptual decision making, cross-modal sensory processes and the role of spontaneous neural activity fluctuations in perception. We conclude that anterior insular cortex may be associated with perception in that it underpins heightened alertness of either stimulus- or task-driven origin, or both. Such a mechanism could integrate endogenous and exogenous functional demands under the joint criterion of whether they challenge an individual's homeostasis. |
| Abstract: | Spatial Independent Component Analysis (ICA) is an increasingly used data-driven method to analyze functional Magnetic Resonance Imaging (fMRI) data. To date, it has been used to extract sets of mutually correlated brain regions without prior information on the time course of these regions. Some of these sets of regions, interpreted as functional networks, have recently been used to provide markers of brain diseases and open the road to paradigm-free population comparisons. Such group studies raise the question of modeling subject variability within ICA: how can the patterns representative of a group be modeled and estimated via ICA for reliable inter-group comparisons? In this paper, we propose a hierarchical model for patterns in multi-subject fMRI datasets, akin to mixed-effect group models used in linear-model-based analysis. We introduce an estimation procedure, CanICA (Canonical ICA), based on i) probabilistic dimension reduction of the individual data, ii) canonical correlation analysis to identify a data subspace common to the group iii) ICA-based pattern extraction. In addition, we introduce a procedure based on cross-validation to quantify the stability of ICA patterns at the level of the group. We compare our method with state-of-the-art multi-subject fMRI ICA methods and show that the features extracted using our procedure are more reproducible at the group level on two datasets of 12 healthy controls: a resting-state and a functional localizer study. |
| Abstract: | An object moving towards an observer is subjectively perceived as longer in duration than the same object that is static or moving away. This "time dilation effect" has been shown for a number of stimuli that differ from standard events along different feature dimensions (e.g. color, size, and dynamics). We performed an event-related functional magnetic resonance imaging (fMRI) study, while subjects viewed a stream of five visual events, all of which were static and of identical duration except the fourth one, which was a deviant target consisting of either a looming or a receding disc. The duration of the target was systematically varied and participants judged whether the target was shorter or longer than all other events. A time dilation effect was observed only for looming targets. Relative to the static standards, the looming as well as the receding targets induced increased activation of the anterior insula and anterior cingulate cortices (the "core control network"). The decisive contrast between looming and receding targets representing the time dilation effect showed strong asymmetric activation and, specifically, activation of cortical midline structures (the "default network"). These results provide the first evidence that the illusion of temporal dilation is due to activation of areas that are important for cognitive control and subjective awareness. The involvement of midline structures in the temporal dilation illusion is interpreted as evidence that time perception is related to self-referential processing. |
| Abstract: | The human brain efficiently solves certain operations such as object recognition and categorization through a massively parallel network of dedicated processors. However, human cognition also relies on the ability to perform an arbitrarily large set of tasks by flexibly recombining different processors into a novel chain. This flexibility comes at the cost of a severe slowing down and a seriality of operations (100-500 ms per step). A limit on parallel processing is demonstrated in experimental setups such as the psychological refractory period (PRP) and the attentional blink (AB) in which the processing of an element either significantly delays (PRP) or impedes conscious access (AB) of a second, rapidly presented element. Here we present a spiking-neuron implementation of a cognitive architecture where a large number of local parallel processors assemble together to produce goal-driven behavior. The precise mapping of incoming sensory stimuli onto motor representations relies on a "router" network capable of flexibly interconnecting processors and rapidly changing its configuration from one task to another. Simulations show that, when presented with dual-task stimuli, the network exhibits parallel processing at peripheral sensory levels, a memory buffer capable of keeping the result of sensory processing on hold, and a slow serial performance at the router stage, resulting in a performance bottleneck. The network captures the detailed dynamics of human behavior during dual-task-performance, including both mean RTs and RT distributions, and establishes concrete predictions on neuronal dynamics during dual-task experiments in humans and non-human primates. |
| 2009 |
| Abstract: | Can conscious processing be inferred from neurophysiological measurements? Some models stipulate that the active maintenance of perceptual representations across time requires consciousness. Capitalizing on this assumption, we designed an auditory paradigm that evaluates cerebral responses to violations of temporal regularities that are either local in time or global across several seconds. Local violations led to an early response in auditory cortex, independent of attention or the presence of a concurrent visual task, whereas global violations led to a late and spatially distributed response that was only present when subjects were attentive and aware of the violations. We could detect the global effect in individual subjects using functional MRI and both scalp and intracerebral event-related potentials. Recordings from 8 noncommunicating patients with disorders of consciousness confirmed that only conscious individuals presented a global effect. Taken together these observations suggest that the presence of the global effect is a signature of conscious processing, although it can be absent in conscious subjects who are not aware of the global auditory regularities. This simple electrophysiological marker could thus serve as a useful clinical tool. |
| Abstract: | Previous studies reported impaired cerebral cortical gray matter (CGM) development and neurodevelopmental impairment after neonatal dexamethasone treatment for chronic lung disease (CLD) in preterm newborns. No long-term effects on neurocognitive outcome have yet been shown for hydrocortisone treatment. A prospective study was performed to evaluate the brain growth at term in preterm infants who did receive neonatal hydrocortisone for CLD. Thirty-eight preterm infants (n = 19 hydrocortisone, n = 19 controls) were matched for gestational age at birth. Gestational age and birth weight were 27.0+/- 1.4 versus 27.6+/- 1.1 wk (p = ns) and 826+/- 173 versus 1017+/- 202 g, respectively (p < 0.05). Infants were studied at term equivalent age. Hydrocortisone was started with a dose of 5 mg/kg/d for 1 wk, followed by a tapering course over 3 wk. A 3D-MRI technique was used to quantify cerebral tissue volumes: CGM, basal ganglia/thalami, unmyelinated white matter, myelinated white matter, cerebellum, and cerebrospinal fluid. Infants who were treated with hydrocortisone had more severe respiratory distress. There were no differences in cerebral tissue volumes between the two groups at term equivalent age. In conclusion, no effect on brain growth, measured at term equivalent age, was shown after treatment with hydrocortisone for CLD. |
| Abstract: | Abstract Speech is not a purely auditory signal. From around 2 months of age, infants are able to correctly match the vowel they hear with the appropriate articulating face. However, there is no behavioral evidence of integrated audiovisual perception until 4 months of age, at the earliest, when an illusory percept can be created by the fusion of the auditory stimulus and of the facial cues (McGurk effect). To understand how infants initially match the articulatory movements they see with the sounds they hear, we recorded high-density ERPs in response to auditory vowels that followed a congruent or incongruent silently articulating face in 10-week-old infants. In a first experiment, we determined that auditory-visual integration occurs during the early stages of perception as in adults. The mismatch response was similar in timing and in topography whether the preceding vowels were presented visually or aurally. In the second experiment, we studied audiovisual integration in the linguistic (vowel perception) and nonlinguistic (gender perception) domain. We observed a mismatch response for the both types of change at similar latencies. Their topographies were significantly different demonstrating that cross-modal integration of these features is computed in parallel by two different networks. Indeed, brain source modeling revealed that phoneme and gender computations were lateralized toward the left and toward the right hemisphere, respectively, suggesting that each hemisphere possesses an early processing bias. We also observed repetition suppression in temporal regions and repetition enhancement in frontal regions. These results underscore how complex and structured is the human cortical organization which sustains communication from the first weeks of life on |
| Abstract: | In order to learn an oral language, humans have to discover words from a continuous signal. Streams of artificial monotonous speech can be readily segmented based on the statistical analysis of the syllables' distribution. This parsing is considerably improved when acoustic cues, such as subliminal pauses, are added suggesting that a different mechanism is involved. Here we used a frequency-tagging approach to explore the neural mechanisms underlying word learning while listening to continuous speech. High-density EEG was recorded in adults listening to a concatenation of either random syllables or tri-syllabic artificial words, with or without subliminal pauses added every three syllables. Peaks in the EEG power spectrum at the frequencies of one and three syllables occurrence were used to tag the perception of a monosyllabic or tri-syllabic structure, respectively. Word streams elicited the suppression of a one-syllable frequency peak, steadily present during random streams, suggesting that syllables are no more perceived as isolated segments but bounded to adjacent syllables. Crucially, three-syllable frequency peaks were only observed during word streams with pauses, and were positively correlated to the explicit recall of the detected words. This result shows that pauses facilitate a fast, explicit and successful extraction of words from continuous speech, and that the frequency-tagging approach is a powerful tool to track brain responses to different hierarchical units of the speech structure |
| Abstract: | Abstract As literate adults, we appreciate numerical values as abstract entities that can be represented by a numeral, a word, a number of lines on a scorecard, or a sequence of chimes from a clock. This abstract, notation-independent appreciation of numbers develops gradually over the first several years of life. Here, using functional magnetic resonance imaging, we examine the brain mechanisms that 6- and 7-year-old children and adults recruit to solve numerical comparisons across different notation systems. The data reveal that when young children compare numerical values in symbolic and nonsymbolic notations, they invoke the same network of brain regions as adults including occipito-temporal and parietal cortex. However, children also recruit inferior frontal cortex during these numerical tasks to a much greater degree than adults. Our data lend additional support to an emerging consensus from adult neuroimaging, nonhuman primate neurophysiology, and computational modeling studies that a core neural system integrates notation-independent numerical representations throughout development but, early in development, higher-order brain mechanisms mediate this process. |
| Abstract: | Cohen Kadosh & Walsh (CK&W) neglect the solid empirical evidence for a convergence of notation-specific representations onto a shared representation of numerical magnitude. Subliminal priming reveals cross-notation and cross-modality effects, contrary to CK&W's prediction that automatic activation is modality and notation-specific. Notation effects may, however, emerge in the precision, speed, automaticity, and means by which the central magnitude representation is accessed. |
| Abstract: | Mathematicians frequently evoke their "intuition" when they are able to quickly and automatically solve a problem, with little introspection into their insight. Cognitive neuroscience research shows that mathematical intuition is a valid concept that can be studied in the laboratory in reduced paradigms, and that relates to the availability of "core knowledge" associated with evolutionarily ancient and specialized cerebral subsystems. As an illustration, I discuss the case of elementary arithmetic. Intuitions of numbers and their elementary transformations by addition and subtraction are present in all human cultures. They relate to a brain system, located in the intraparietal sulcus of both hemispheres, which extracts numerosity of sets and, in educated adults, maps back and forth between numerical symbols and the corresponding quantities. This system is available to animal species and to preverbal human infants. Its neuronal organization is increasingly being uncovered, leading to a precise mathematical theory of how we perform tasks of number comparison or number naming. The next challenge will be to understand how education changes our core intuitions of number. |
| Abstract: | Priming effects have been well documented in behavioral psycholinguistics experiments: The processing of a word or a sentence is typically facilitated when it shares lexico-semantic or syntactic features with a previously encountered stimulus. Here, we used fMRI priming to investigate which brain areas show adaptation to the repetition of a sentence's content or syntax. Participants read or listened to sentences organized in series which could or not share similar syntactic constructions and/or lexico-semantic content. The repetition of lexico-semantic content yielded adaptation in most of the temporal and frontal sentence processing network, both in the visual and the auditory modalities, even when the same lexico-semantic content was expressed using variable syntactic constructions. No fMRI adaptation effect was observed when the same syntactic construction was repeated. Yet behavioral priming was observed at both syntactic and semantic levels in a separate experiment where participants detected sentence endings. We discuss a number of possible explanations for the absence of syntactic priming in the fMRI experiments, including the possibility that the conglomerate of syntactic properties defining "a construction" is not an actual object assembled during parsing. |
| Abstract: | Both language capacity and strongly lateralized hand preference are among the most intriguing particularities of the human species. They are associated in the adult brain with functional and anatomical hemispheric asymmetries in the speech perception-production network and in the sensori-motor system. Only studies in early life can help us to understand how such asymmetries arise during brain development, and to which point structural left-right differences are the source or the consequence of functional lateralization. In this study, we aimed to provide new in vivo structural markers of hemispheric asymmetries in infants from 1 to 4 months of age, with diffusion tensor imaging. We used 3 complementary analysis methods based on local diffusion indices and spatial localizations of tracts. After a prospective approach over the whole brain, we demonstrated early leftward asymmetries in the arcuate fasciculus and in the cortico-spinal tract. These results suggest that the early macroscopic geometry, microscopic organization, and maturation of these white matter bundles are related to the development of later functional lateralization. |
| Abstract: | BACKGROUND: Neuropsychology and human functional neuroimaging have implicated human parietal cortex in numerical processing, and macaque electrophysiology has shown that intraparietal areas house neurons tuned to numerosity. Yet although the areas responding overall during numerical tasks have been well defined by neuroimaging, a direct demonstration of individual number coding by spatial patterns has thus far been elusive. RESULTS: We used multivariate pattern recognition on high-resolution functional imaging data to decode the information content of fine-scale signals evoked by different individual numbers. Parietal activation patterns for individual numerosities could be accurately discriminated and generalized across changes in low-level stimulus parameters. Distinct patterns were evoked by symbolic and nonsymbolic number formats, and individual digits were less accurately decoded (albeit still with significant accuracy) than numbers of dots. Interestingly, the numerosity of dot sets could be predicted above chance from the brain activation patterns evoked by digits, but not vice versa. Finally, number-evoked patterns changed in a gradual fashion as a function of numerical distance for the nonsymbolic notation, compatible with some degree of orderly layout of individual number representations. CONCLUSIONS: Our findings demonstrate partial format invariance of individual number codes that is compatible with more numerous but more broadly tuned populations for nonsymbolic than for symbolic numbers, as postulated by recent computational models. In more general terms, our results illustrate the potential of functional magnetic resonance imaging pattern recognition to understand the detailed format of representations within a single semantic category, and beyond sensory cortical areas for which columnar architectures are well established. |
| Abstract: | We compared conscious and nonconscious processing of briefly flashed words using a visual masking procedure while recording intracranial electroencephalogram (iEEG) in ten patients. Nonconscious processing of masked words was observed in multiple cortical areas, mostly within an early time window (,300 ms), accompanied by induced gamma- band activity, but without coherent long-distance neural activity, suggesting a quickly dissipating feedforward wave. In contrast, conscious processing of unmasked words was characterized by the convergence of four distinct neurophysiological markers: sustained voltage changes, particularly in prefrontal cortex, large increases in spectral power in the gamma band, increases in long-distance phase synchrony in the beta range, and increases in long-range Granger causality. We argue that all of those measures provide distinct windows into the same distributed state of conscious processing. These results have a direct impact on current theoretical discussions concerning the neural correlates of conscious access. |
| Abstract: | Blood oxygen level-dependent (BOLD) responses were measured in parts of primary visual cortex that represented unstimulated visual field regions at different distances from a stimulated central target location. The composition of the visual scene varied by the presence or absence of additional peripheral distracter stimuli. Bottom-up effects were assessed by comparing peripheral activity during central stimulation vs. no stimulation. Top-down effects were assessed by comparing active vs. passive conditions. In passive conditions subjects simply watched the central letter stimuli and in active conditions they had to report occurrence of pre-defined targets in a rapid serial letter stream. Onset of the central letter stream enhanced activity in V1 representations of the stimulated region. Within representations of the periphery activation decreased and finally turned into deactivation with increasing distance from the stimulated location. This pattern was most pronounced in the active conditions and during the presence of peripheral stimuli. Active search for a target did not lead to additional enhancement at areas representing the attentional focus but to a stronger deactivation in the vicinity. Suppressed neuronal activity was also found in the non distracter condition suggesting a top-down attention driven effect. Our observations suggest that BOLD signal decreases in primary visual cortex are modulated by bottom-up sensory-driven factors such as the presence of distracters in the visual field as well as by top-down attentional processes. |
| Abstract: | Although infants and animals respond to the approximate number of elements in visual, auditory, and tactile arrays, only human children and adults have been shown to possess abstract numerical representations that apply to entities of all kinds (e.g., 7 samurai, seas, or sins). Do abstract numerical concepts depend on language or culture, or do they form a part of humans' innate, core knowledge? Here we show that newborn infants spontaneously associate stationary, visual-spatial arrays of 4-18 objects with auditory sequences of events on the basis of number. Their performance provides evidence for abstract numerical representations at the start of postnatal experience. |
| Abstract: | Throughout the history of mathematics, concepts of number and space have been tightly intertwined. We tested the hypothesis that cortical circuits for spatial attention contribute to mental arithmetic. We trained a multivariate classifier to infer the direction of an eye movement, left or right, from the brain activation measured in posterior parietal cortex. Without further training, the classifier then generalized to an arithmetic task. Its left versus right classification could be used to sort out subtraction versus addition trials, whether performed with symbols or with sets of dots. These findings are consistent with the suggestion that mental arithmetic co-opts parietal circuitry associated with spatial coding. |
| Abstract: | When we add or subtract, do the corresponding quantities "move" along a mental number line? Does this internal movement lead to spatial biases? A new method was designed to investigate the psychophysics of approximate arithmetic. Addition and subtraction problems were presented either with sets of dots or with Arabic numerals, and subjects selected, from among seven choices, the most plausible result. In two experiments, the subjects selected larger numbers for addition than for subtraction problems, as if moving too far along the number line. This operational momentum effect was present in both notations and increased with the size of the outcome. Furthermore, we observed a new effect of spatial-numerical congruence, related to but distinct from the spatial numerical association of response codes effect: During nonsymbolic addition, the subjects preferentially selected numbers at the upper right location, whereas during subtraction, they were biased toward the upper left location. These findings suggest that approximate mental arithmetic involves dynamic shifts on a spatially organized mental representation of numbers. Supplemental materials for this study may be downloaded from app.psychonomic-journals.org/content/supplemental. |
| Abstract: | Whether masked number priming involves a low-level sensorimotor route or an amodal semantic level of processing remains highly debated. Several alternative interpretations have been put forward, proposing either that masked number priming is solely a byproduct of practice with numbers, or that stimulus awareness was underestimated. In a series of four experiments, we studied whether repetition and congruity priming for numbers reliably extend to novel (i.e., unpracticed) stimuli and whether priming transfers from a visual prime to an auditory target, even when carefully controlling for stimulus awareness. While we consistently observed cross-modal priming, the generalization to novel stimuli was weaker and reached significance only when considering the whole set of experiments. We conclude that number priming does involve an amodal, semantic level of processing, but is also modulated by task settings. |
| Abstract: | We report on a new framework to investigate the rapid brain development of newborns. It is based on the analysis of depth maps of the cortical surface through the study of a displacement field estimated by surfacic optical flow methods. This displacement field shows local evolution of sulci directly on the cortical surface. Detection of its critical points is performed with the Helmholtz decomposition which allows us to identify sources of the developmental process. They can be viewed as growth seeds or in other terms points around which the sulcal growth organizes itself. We show the reproducibility of such growth seeds across 4 neonates and make a link of this new concept to the "sulcal roots" one proposed to explain the variability of human brain anatomy. |
| Abstract: | Neuronal networks involved in second language (L2) processing vary between normal subjects. Patients with epilepsy may have ictal speech automatisms in their second language. To delineate the brain systems involved in L2 ictal speech, we combined functional MRI during bilingual tasks and ictal--interictal single-photon emission computed tomography in a patient who presented L2 ictal speech productions. These analyses showed that the networks activated by the seizure and those activated by L2 processing intersected in the right hippocampus. These results may provide some insights both into the pathophysiology of ictal speech and into the brain organization for L2. |
| Abstract: | Number symbols have allowed humans to develop superior mathematical skills that are a hallmark of technologically advanced cultures. Findings in animal cognition, developmental psychology, and anthropology indicate that these numerical skills are rooted in nonlinguistic biological primitives. Recent studies in human and nonhuman primates using a broad range of methodologies provide evidence that numerical information is represented and processed by regions of the prefrontal and posterior parietal lobes, with the intraparietal sulcus as a key node for the representation of the semantic aspect of numerical quantity. |
| Abstract: | Numerosity (the number of objects in a set), like color or movement, is a basic property of the environment. Animal and human brains have been endowed by evolution by mechanisms based on parietal circuitry for representing numerosity in an highly abstract, although approximate fashion. These mechanisms are functional at a very early age in humans and spontaneously deployed in the wild by animals of different species. The recent years have witnessed terrific advances in unveiling the neural code(s) underlying numerosity representations and showing similarities as well as differences across species. In humans, during development, with the introduction of symbols for numbers and the implementation of the counting routines, the parietal system undergoes profound (yet still largely mysterious) modifications, such that the neural machinery previously evolved to represent approximate numerosity gets partially "recycled" to support the representation of exact number. |
| Abstract: | Studies of endogenous (cue-directed) attention have traditionally assumed that such shifts must be volitional. However, recent behavioural experiments have shown that participants make automatic endogenous shifts of attention when presented with symbolic cues that are systematically associated with particular spatial directions, such as arrows and numerals, even when such cues were not behaviourally relevant. Here we used event-related potentials (ERPs) to test whether these automatic shifts of attention use the same mechanisms as volitional shifts of attention. We presented participants with non-predictive (50\% valid) task-irrelevant arrow and numeral cues while measuring cue- and target-locked ERPs. Although the cues were task-irrelevant, they elicited attention-related ERP components previously found in studies that used informative and/or task-relevant cues. These findings further substantiate the dissociation between endogenous and volitional attentional control, and suggest that the same fronto-parietal networks involved in volitional shifts of attention are also involved in reflexive endogenous shifts of attention. |
| Abstract: | Global neuronal workspace theory predicts that damage to long-distance white matter (WM) tracts should impair access to consciousness during the perception of brief stimuli. To address this issue, we studied visual backward masking in 18 patients at the very first clinical stage of multiple sclerosis (MS), a neurological disease characterized by extensive WM damage, and in 18 matched healthy subjects. In our masking paradigm, the visibility of a digit stimulus increases non-linearly as a function of the interval duration between this target and a subsequent mask. In order to characterize quantitatively, for each subject, the transition between non-conscious and conscious perception of the stimulus, we used non-linear regression to fit a sigmoid curve to objective performance and subjective visibility reports as a function of target-mask delay. The delay corresponding to the inflexion point of the sigmoid, where visibility suddenly increases, was termed the "non-linear transition threshold" and used as a summary measure of masking efficiency. Objective and subjective non-linear transition thresholds were highly correlated across subjects in both groups, and were higher in patients compared to controls. In patients, variations in the non-linear transition threshold were inversely correlated to the Magnetization transfer ratio (MTR) values inside the right dorsolateral prefrontal WM, the right occipito-frontal fasciculus and the left cerebellum. This study provides clinical evidence of a relationship between impairments of conscious access and integrity of large WM bundles, particularly involving prefrontal cortex, as predicted by global neuronal workspace theory |
| Abstract: | OBJECTIVE: To examine the functional neuroanatomy that could account for pure Gerstmann syndrome, which is the selective association of acalculia, finger agnosia, left-right disorientation, and agraphia. METHODS: We used structural and functional neuroimaging at high spatial resolution in healthy subjects to seek a shared cortical substrate of the Grundstrung posited by Gerstmann, ie, a common functional denominator accounting for this clinical tetrad. We construed a functional activation paradigm that mirrors each of the four clinical deficits in Gerstmann syndrome and determined cortical activation patterns. We then applied fiber tracking to diffusion tensor images and used cortical activation foci in the four functional domains as seed regions. RESULTS: None of the subjects showed parietal overlap of cortical activation patterns from the four cognitive domains. In every subject, however, the parietal activation patterns across all four domains consistently connected to a small region of subcortical parietal white matter at a location that is congruent with the lesion in a well-documented case of pure Gerstmann syndrome. INTERPRETATION: Our functional neuroimaging findings are not in agreement with Gerstmann's postulate of damage to a common cognitive function underpinning clinical semiology. Our evidence from intact functional neuroanatomy suggests that pure forms of Gerstmann's tetrad do not arise from lesion to a shared cortical substrate but from intraparietal disconnection after damage to a focal region of subcortical white matter. |
| Abstract: | A simple view, which dates back to Turing, proposes that complex cognitive operations are composed of serially arranged elementary operations, each passing intermediate results to the next. However, whether and how such serial processing is achieved with a brain composed of massively parallel processors, remains an open question. Here, we study the cognitive architecture for chained operations with an elementary arithmetic algorithm: we required participants to add (or subtract) two to a digit, and then compare the result with five. In four experiments, we probed the internal implementation of this task with chronometric analysis, the cued-response method, the priming method, and a subliminal forced-choice procedure. We found evidence for an approximately sequential processing, with an important qualification: the second operation in the algorithm appears to start before completion of the first operation. Furthermore, initially the second operation takes as input the stimulus number rather than the output of the first operation. Thus, operations that should be processed serially are in fact executed partially in parallel. Furthermore, although each elementary operation can proceed subliminally, their chaining does not occur in the absence of conscious perception. Overall, the results suggest that chaining is slow, effortful, imperfect (resulting partly in parallel rather than serial execution) and dependent on conscious control. |
| Abstract: | Recent studies have shown that ongoing activity fluctuations influence trial-by-trial perception of identical stimuli. Some brain systems seem to bias toward better perceptual performance and others toward worse. We tested whether these observations generalize to another as of yet unassessed sensory modality, audition, and a nonspatial but memory-dependent paradigm. In a sparse event-related functional magnetic resonance imaging design, we investigated detection of auditory near-threshold stimuli as a function of prestimulus baseline activity in early auditory cortex as well as several distributed networks that were defined on the basis of resting state functional connectivity. In accord with previous studies, hits were associated with higher prestimulus activity in related early sensory cortex as well as in a system comprising anterior insula, anterior cingulate, and thalamus, which other studies have related to processing salience and maintaining task set. In contrast to previous studies, however, higher prestimulus activity in the so-called dorsal attention system of frontal and parietal cortex biased toward misses, whereas higher activity in the so-called default mode network that includes posterior cingulate and precuneus biased toward hits. These results contradict a simple dichotomic view on the function of these two latter brain systems where higher ongoing activity in the dorsal attention network would facilitate perceptual performance, and higher activity in the default mode network would deteriorate perceptual performance. Instead, we show that the way in which ongoing activity fluctuations impact on perception depends on the specific sensory (i.e., nonspatial) and cognitive (i.e., mnemonic) context that is relevant. |
| Abstract: | Multistable perception is the spontaneous alternation between two or more perceptual states that occurs when sensory information is ambiguous. Multistable phenomena permit dissociation of neural activity related to conscious perception from that related to sensory stimulation, and therefore have been used extensively to study the neural correlates of consciousness. Here, we review recent work on the neural mechanisms underlying multistable perception and how such work has contributed to understanding the neural correlates of consciousness. Particular emphasis is put on the role of high-level brain mechanisms that are involved in actively selecting and interpreting sensory information, and their interactions with lower-level processes that are more directly concerned with the processing of sensory stimulus properties. |
| Abstract: | Object recognition relies heavily on invariant visual features such as the manner in which lines meet at vertices to form viewpoint-invariant junctions (e.g. T, L). We wondered whether these features also underlie readers' competence for fast recognition of printed words. Since reading is far too recent to have exerted any evolutionary pressure on brain evolution, visual word recognition might be based on pre-existing mechanisms common to all visual object recognition. In a naming task, we presented partially deleted pictures of objects and printed words in which either the vertices or the line midsegments were preserved. Subjects showed an identical pattern of behavior with both objects and words: they made fewer errors and were faster to respond when vertices were preserved. Our results suggest that vertex invariants are used for object recognition and that this evolutionarily ancient mechanism is being co-opted for reading. |
| Abstract: | How long did it take you to read this sentence? Chances are your response is a ball park estimate and its value depends on how fast you have scanned the text, how prepared you have been for this question, perhaps your mood or how much attention you have paid to these words. Time perception is here addressed in three sections. The first section summarizes theoretical difficulties in time perception research, specifically those pertaining to the representation of time and temporal processing. The second section reviews non-exhaustively temporal effects in multisensory perception. Sensory modalities interact in temporal judgement tasks, suggesting that (i) at some level of sensory analysis, the temporal properties across senses can be integrated in building a time percept and (ii) the representational format across senses is compatible for establishing such a percept. In the last section, a two-step analysis of temporal properties is sketched out. In the first step, it is proposed that temporal properties are automatically encoded at early stages of sensory analysis, thus providing the raw material for the building of a time percept; in the second step, time representations become available to perception through attentional gating of the raw temporal representations and via re-encoding into abstract representations. |
| Abstract: | Time research has been a neglected topic in the cognitive neurosciences of the last decades: how do humans perceive time? How and where in the brain is time processed? This introductory paper provides an overview of the empirical and theoretical papers on the psychological and neural basis of time perception collected in this theme issue. Contributors from the fields of cognitive psychology, psychiatry, neurology and neuroanatomy tackle this complex question with a variety of techniques ranging from psychophysical and behavioural experiments to pharmacological interventions and functional neuroimaging. Several (and some new) models of how and where in the brain time is processed are presented in this unique collection of recent research that covers experienced time intervals from milliseconds to minutes. We hope this volume to be conducive in developing a better understanding of the sense of time as part of complex set of brain-body factors that include cognitive, emotional and body states. |
| 2008 |
| Abstract: | Priming stimulations have shown powerful effects on motor cortex behavior. However, the effects over language areas have not been explored. We assessed the effects of different priming frequencies of repetitive transcranial magnetic stimulation (rTMS), 1 Hz rTMS or 50 Hz bursts of rTMS (theta burst stimulation [TBS]), on temporoparietal language areas (i.e., Wernicke's area) localized with functional magnetic resonance imaging. Functional maps were acquired during an auditory word-detection task with native or foreign language sentences in 14 healthy men. Frameless stereotaxy was used to guide the transcranial magnetic stimulation coil position over Wernicke's area. Active and placebo randomized sessions of priming stimulations (1 Hz rTMS or TBS) were applied at rest, and response times (RTs) were recorded during the auditory word-detection task performed subsequently with 1 Hz rTMS. Individual anatomofunctional maps localized activation in Wernicke's area. Repeated-measure analysis of variance for RTs revealed that priming with 1 Hz rTMS facilitated the detection of native words, whereas priming with TBS facilitated the detection of foreign words. Consistent with motor cortex studies, these findings suggest that priming frequency plays a crucial role in word detection in the auditory stream. |
| Abstract: | Fast, parallel word recognition, in expert readers, relies on sectors of the left ventral occipito-temporal pathway collectively known as the visual word form area. This expertise is thought to arise from perceptual learning mechanisms that extract informative features from the input strings. The perceptual expertise hypothesis leads to two predictions: (1) parallel word recognition, based on the ventral visual system, should be limited to words displayed in a familiar format (foveal horizontal words with normally spaced letters); (2) words displayed in formats outside this field of expertise should be read serially, under supervision of dorsal parietal attention systems. We presented adult readers with words that were progressively degraded in three different ways (word rotation, letter spacing, and displacement to the visual periphery). Behaviorally, we identified degradation thresholds above which reading difficulty increased non-linearly, with the concomitant emergence of a word length effect on reading latencies reflecting serial reading strategies. fMRI activations were correlated with reading difficulty in bilateral occipito-temporal and parietal regions, reflecting the strategies required to identify degraded words. A core region of the intraparietal cortex was engaged in all modes of degradation. Furthermore, in the ventral pathway, word degradation led to an amplification of activation in the posterior visual word form area, at a level thought to encode single letters. We also found an effect of word length restricted to highly degraded words in bilateral occipitoparietal regions. Those results clarify when and how the ventral parallel visual word form system needs to be supplemented by the deployment of dorsal serial reading strategies |
| Abstract: | Which cognitive processes are accessible to conscious report? To study the limits of conscious reportability, we designed a novel method of quantified introspection, in which subjects were asked, after each trial of a standard cognitive task, to estimate the time spent completing the task. We then applied classical mental-chronometry techniques, such as the additive-factors method, to analyze these introspective estimates of response time. We demonstrate that introspective response time can be a sensitive measure, tightly correlated with objective response time in a single-task context. In a psychological-refractory-period task, however, the objective processing delay resulting from interference by a second concurrent task is totally absent from introspective estimates. These results suggest that introspective estimates of time spent on a task tightly correlate with the period of availability of central processing resources |
| Abstract: | The mapping of numbers onto space is fundamental to measurement and to mathematics. Is this mapping a cultural invention or a universal intuition shared by all humans regardless of culture and education? We probed number-space mappings in the Mundurucu, an Amazonian indigene group with a reduced numerical lexicon and little or no formal education. At all ages, the Mundurucu mapped symbolic and nonsymbolic numbers onto a logarithmic scale, whereas Western adults used linear mapping with small or symbolic numbers and logarithmic mapping when numbers were presented nonsymbolically under conditions that discouraged counting. This indicates that the mapping of numbers onto space is a universal intuition and that this initial intuition of number is logarithmic. The concept of a linear number line appears to be a cultural invention that fails to develop in the absence of formal education |
| Abstract: | In the human brain, the morphology of cortical gyri and sulci is complex and variable among individuals, and it may reflect pathological functioning with specific abnormalities observed in certain developmental and neuropsychiatric disorders. Since cortical folding occurs early during brain development, these structural abnormalities might be present long before the appearance of functional symptoms. So far, the precise mechanisms responsible for such alteration in the convolution pattern during intra-uterine or post-natal development are still poorly understood. Here we compared anatomical and functional brain development in vivo among 45 premature newborns who experienced different intra-uterine environments: 22 normal singletons, 12 twins and 11 newborns with intrauterine growth restriction (IUGR). Using magnetic resonance imaging (MRI) and dedicated post-processing tools, we investigated early disturbances in cortical formation at birth, over the developmental period critical for the emergence of convolutions (26-36 weeks of gestational age), and defined early 'endophenotypes' of sulcal development. We demonstrated that twins have a delayed but harmonious maturation, with reduced surface and sulcation index compared to singletons, whereas the gyrification of IUGR newborns is discordant to the normal developmental trajectory, with a more pronounced reduction of surface in relation to the sulcation index compared to normal newborns. Furthermore, we showed that these structural measurements of the brain at birth are predictors of infants' outcome at term equivalent age, for MRI-based cerebral volumes and neurobehavioural development evaluated with the assessment of preterm infant's behaviour (APIB). |
| Abstract: | In the developing human brain, the cortical sulci formation is a complex process starting from 14 weeks of gestation onward. The potential influence of underlying mechanisms (genetic, epigenetic, mechanical or environmental) is still poorly understood, because reliable quantification in vivo of the early folding is lacking. In this study, we investigate the sulcal emergence noninvasively in 35 preterm newborns, by applying dedicated postprocessing tools to magnetic resonance images acquired shortly after birth over a developmental period critical for the human cortex maturation (26-36 weeks of age). Through the original three-dimensional reconstruction of the interface between developing cortex and white matter and correlation with volumetric measurements, we document early sulcation in vivo, and quantify changes with age, gender, and the presence of small white matter lesions. We observe a trend towards lower cortical surface, smaller cortex, and white matter volumes, but equivalent sulcation in females compared with males. By precisely mapping the sulci, we highlight interindividual variability in time appearance and interhemispherical asymmetries, with a larger right superior temporal sulcus than the left. Thus, such an approach, included in a longitudinal follow-up, may provide early indicators on the structural basis of cortical functional specialization and abnormalities induced by genetic and environmental factors. |
| Abstract: | Normal cognitive development in infants follows a well-known temporal sequence, which is assumed to be correlated with the structural maturation of underlying functional networks. Postmortem studies and, more recently, structural MR imaging studies have described qualitatively the heterogeneous spatiotemporal progression of white matter myelination. However, in vivo quantification of the maturation phases of fiber bundles is still lacking. We used noninvasive diffusion tensor MR imaging and tractography in twenty-three 1-4-month-old healthy infants to quantify the early maturation of the main cerebral fascicles. A specific maturation model, based on the respective roles of different maturational processes on the diffusion phenomena, was designed to highlight asynchronous maturation across bundles by evaluating the time-course of mean diffusivity and anisotropy changes over the considered developmental period. Using an original approach, a progression of maturation in four relative stages was determined in each tract by estimating the maturation state and speed, from the diffusion indices over the infants group compared with an adults group on one hand, and in each tract compared with the average over bundles on the other hand. Results were coherent with, and extended previous findings in 8 of 11 bundles, showing the anterior limb of the internal capsule and cingulum as the most immature, followed by the optic radiations, arcuate and inferior longitudinal fascicles, then the spinothalamic tract and fornix, and finally the corticospinal tract as the most mature bundle. Thus, this approach provides new quantitative landmarks for further noninvasive research on brain-behavior relationships during normal and abnormal development |
| Abstract: | The development of cognitive functions during childhood relies on several neuroanatomical maturation processes. Among these processes is myelination of the white matter pathways, which speeds up electrical conduction. Quantitative indices of such structural processes can be obtained in vivo with diffusion tensor imaging (DTI), but their physiological significance remains uncertain. Here, we investigated the microstructural correlates of early functional development by combining DTI and visual event-related potentials (VEPs) in 15 one- to 4-month-old healthy infants. Interindividual variations of the apparent conduction speed, computed from the latency of the first positive VEP wave (P1), were significantly correlated with the infants' age and DTI indices measured in the optic radiations. This demonstrates that fractional anisotropy and transverse diffusivity are structural markers of functionally efficient myelination. Moreover, these indices computed along the optic radiations showed an early wave of maturation in the anterior region, with the posterior region catching up later in development, which suggests two asynchronous fronts of myelination in both the geniculocortical and corticogeniculate fibers. Thus, in addition to microstructural information, DTI provides noninvasive exquisite information on the functional development of the brain in human infants |
| Abstract: | Both language capacity and strongly lateralized hand preference are among the most intriguing particularities of the human species. They are associated in the adult brain with functional and anatomical hemispheric asymmetries in the speech perception-production network and in the sensori-motor system. Only studies in early life can help us to understand how such asymmetries arise during brain development, and to which point structural left-right differences are the source or the consequence of functional lateralization. In this study, we aimed to provide new in vivo structural markers of hemispheric asymmetries in infants from 1 to 4 months of age, with diffusion tensor imaging. We used 3 complementary analysis methods based on local diffusion indices and spatial localizations of tracts. After a prospective approach over the whole brain, we demonstrated early leftward asymmetries in the arcuate fasciculus and in the cortico-spinal tract. These results suggest that the early macroscopic geometry, microscopic organization, and maturation of these white matter bundles are related to the development of later functional lateralization |
| Abstract: | A central issue for understanding visual object recognition is how the cortical hierarchy represents incoming sensory information and transforms it across successive processing stages. The format of object representation in the human brain has thus far mostly been studied using adaptation paradigms because the neuronal layout of object selectivities was thought to be beyond the resolution of conventional functional MRI (fMRI). Recently, however, multivariate pattern recognition succeeded in discriminating fMRI responses of object-selective cortex to different object exemplars within a given category. Here, we use increased spatial fMRI resolution to explore size sensitivity and tolerance to size change of response patterns evoked by object exemplars across a range of three sizes. Results from Support Vector Classification on responses of the human lateral occipital complex (LOC) show that discrimination of size (for a given object) and discrimination of objects across changes in size depended on the amount of size difference. Even across the largest amount of size change, accuracy for generalization was still significant in LOC, whereas the same comparison was at chance performance in early visual (calcarine) cortex. Analyzing subregions, we further found an anterior-posterior gradient in the degree of size sensitivity and size generalization within the posterior-dorsal and anterior-ventral parts of LOC. These results speak against fully size-invariant representation of object information in human LOC and are hence congruent with findings in monkeys showing object identity and size information in population activity of inferotemporal cortex. Moreover, these results provide evidence for a fine-grained functional heterogeneity within human LOC beyond the commonly used LO/fusiform subdivision. |
| Abstract: | Functional neuroimaging and studies of brain-damaged patients made it possible to delineate the main components of the cerebral system for word reading. However, the anatomical connections subtending the flow of information within this network are still poorly defined. Here we study the connectivity of the Visual Word Form Area (VWFA), a pivotal component of the reading network achieving the invariant identification of letter strings, and reproducibly located in the left lateral occipitotemporal sulcus. Diffusion images and functional imaging data were gathered in a patient who developed pure alexia following a small surgical lesion in the vicinity of his VWFA. We had a unique opportunity to compare images obtained before, early after, and late after surgery. Analysis of diffusion images with white matter tractography and voxel-based morphometry showed that the VWFA was mainly linked to the occipital cortex through the inferior longitudinal fasciculus (ILF), and to perisylvian language areas (supramarginal gyrus) through the arcuate fasciculus. After surgery, we observed the progressive and selective degeneration of the ILF, while the VWFA was anatomically intact. This allowed us to establish the critical causal role of this fiber tract in normal reading, and to show that its disruption is one pathophysiological mechanism of pure alexia, thus clarifying a long-standing debate on the role of disconnection in neurocognitive disorders. |
| Abstract: | Ratings for age of acquisition (AoA) and subjective frequency were collected for the 1,493 monosyllabic French words that were most known to French students. AoA ratings were collected by asking participants to estimate in years the age at which they learned each word. Subjective frequency ratings were collected on a 7-point scale, ranging from never encountered to encountered several times daily. The results were analyzed to address the relationship between AoA and subjective frequency ratings with other psycholinguistic variables (objective frequency, imageability, number of letters, and number of orthographic neighbors). The results showed high reliability ratings with other databases. Supplementary materials for this study may be downloaded from the Psychonomic Society's Archive of Norms, Stimuli, and Data, www.psychonomic.org/archive. |
| Abstract: | Abstract The attentional blink (AB) documents a particularly strong case of visual attentional competition, in which subjects' ability to identify a second target (T2) is significantly impaired when it is presented with a short SOA after a first target (T1). We used functional magnetic resonance imaging to investigate the impact of the AB on visual activity in individually defined retinotopic representations of the target stimuli. Our results show reduction of neural response in V3 and marginally in V2 and V1, paralleling the behavioral AB effect. Reduction of visual activity was accompanied by reduced neural response in the inferior parietal cortex. This indicates that attentional competition modulates activity in higher-order parietal regions and the early visual cortex, providing a plausible neural basis of the behavioral AB effect. |
| Abstract: | Neural variability in responding to identical repeated stimuli has been related to trial-by-trial fluctuations in ongoing activity, yet the neural and perceptual consequences of these fluctuations remain poorly understood. Using functional neuroimaging, we recorded brain activity in subjects who reported perceptual decisions on an ambiguous figure, Rubin's vase-faces picture, which was briefly presented at variable intervals of > or = 20 s. Prestimulus activity in the fusiform face area, a cortical region preferentially responding to faces, was higher when subjects subsequently perceived faces instead of the vase. This finding suggests that endogenous variations in prestimulus neuronal activity biased subsequent perceptual inference. Furnishing evidence that evoked sensory responses, we then went on to show that the pre- and poststimulus activity interact in a nonlinear way and the ensuing perceptual decisions depend upon the prestimulus context in which they occur. |
| Abstract: | We have recently shown that intrinsic fluctuations of ongoing activity during baseline have an impact on perceptual decisions reported for an ambiguous visual stimulus (Hesselmann et al., 2008). To test whether this result generalizes from the visual object domain to other perceptual and neural systems, the current study investigated the effect of ongoing signal fluctuations in motion-sensitive brain regions on the perception of coherent visual motion. We determined motion coherence thresholds individually for each subject using a dynamic random dot display. During functional magnetic resonance imaging (fMRI), brief events of subliminal, supraliminal, and periliminal coherent motion were presented with long and variable interstimulus intervals between them. On each trial, subjects reported whether they had perceived "coherent" or "random" motion, and fMRI signal time courses were analyzed separately as a function of stimulus and percept type. In the right motion-sensitive occipito-temporal cortex (hMT+), coherent percepts of periliminal stimuli yielded a larger stimulus-evoked response than random percepts. Prestimulus baseline activity in this region was also significantly higher in these coherent trials than in random trials. As in our previous study, however, the relation between ongoing and evoked activity was not additive but interacted with perceptual outcome. Our data thus suggest that endogenous fluctuations in baseline activity have a generic effect on subsequent perceptual decisions. Although mainstream analytical techniques used in functional neuroimaging do not capture this nonadditive effect of baseline on evoked response, it is in accord with postulates from theoretical frameworks as, for instance, predictive coding. |
| Abstract: | Human adults are thought to possess two dissociable systems to represent numbers: an approximate quantity system akin to a mental number line, and a verbal system capable of representing numbers exactly. Here, we study the interface between these two systems using an estimation task. Observers were asked to estimate the approximate numerosity of dot arrays. We show that, in the absence of calibration, estimates are largely inaccurate: responses increase monotonically with numerosity, but underestimate the actual numerosity. However, insertion of a few inducer trials, in which participants are explicitly (and sometimes misleadingly) told that a given display contains 30 dots, is sufficient to calibrate their estimates on the whole range of stimuli. Based on these empirical results, we develop a model of the mapping between the numerical symbols and the representations of numerosity on the number line |
| Abstract: | All humans, regardless of their culture and education, possess an intuitive understanding of number. Behavioural evidence suggests that numerical competence may be present early on in infancy. Here, we present brain-imaging evidence for distinct cerebral coding of number and object identity in 3-mo-old infants. We compared the visual event-related potentials evoked by unforeseen changes either in the identity of objects forming a set, or in the cardinal of this set. In adults and 4-y-old children, number sense relies on a dorsal system of bilateral intraparietal areas, different from the ventral occipitotemporal system sensitive to object identity. Scalp voltage topographies and cortical source modelling revealed a similar distinction in 3-mo-olds, with changes in object identity activating ventral temporal areas, whereas changes in number involved an additional right parietoprefrontal network. These results underscore the developmental continuity of number sense by pointing to early functional biases in brain organization that may channel subsequent learning to restricted brain areas |
| Abstract: | There is a current debate whether the human brain possesses a shared representation for various types of magnitude such as numerical quantities, physical size, or loudness. Here, we critically review evidence from chronometric, neuroimaging, developmental and comparative fields, and supplement it with a meta-analysis of the neuroimaging data. Together, based on such an integrative overview, we discuss limitations inherent in each approach, and the possibility whether shared, or distinct magnitude representation, or both representations exist. |
| Abstract: | Simultaneous recording of brain activity by different neurophysiological modalities can yield insights that reach beyond those obtained by each technique individually, even when compared to those from the post-hoc integration of results from each technique recorded sequentially. Success in the endeavour of real-time multimodal experiments requires special hardware and software as well as purpose-tailored experimental design and analysis strategies. Here, we review the key methodological issues in recording electrophysiological data in humans simultaneously with magnetic resonance imaging (MRI), focusing on recent technical and analytical advances in the field. Examples are derived from simultaneous electroencephalography (EEG) and electromyography (EMG) during functional MRI in cognitive and systems neuroscience as well as in clinical neurology, in particular in epilepsy and movement disorders. We conclude with an outlook on current and future efforts to achieve true integration of electrical and haemodynamic measures of neuronal activity using data fusion models. |
| Abstract: | Within-subject analysis in fMRI essentially addresses two problems, i.e., the detection of activated brain regions in response to an experimental task and the estimation of the underlying dynamics, also known as the characterisation of Hemodynamic response function (HRF). So far, both issues have been treated sequentially while it is known that the HRF model has a dramatic impact on the localisation of activations and that the HRF shape may vary from one region to another. In this paper, we conciliate both issues in a region-based joint detection-estimation framework that we develop in the Bayesian formalism. Instead of considering function basis to account for spatial variability, spatially adaptive General Linear Models are built upon region-based non-parametric estimation of brain dynamics. Regions are first identified as functionally homogeneous parcels in the mask of the grey matter using a specific procedure [Thirion, B., Flandin, G., Philippe Pinel, Roche, A., Ciuciu, P., Poline, J.-B., August 2006. Dealing with the shortcomings of spatial normalization: Multi-subject parcellation of fMRI datasets. Hum. Brain Mapp. 27 (8), 678-693.]. Then, in each parcel, prior information is embedded to constrain this estimation. Detection is achieved by modelling activating, deactivating and non-activating voxels through mixture models within each parcel. From the posterior distribution, we infer upon the model parameters using Markov Chain Monte Carlo (MCMC) techniques. Bayesian model comparison allows us to emphasize on artificial datasets first that inhomogeneous gamma-Gaussian mixture models outperform Gaussian mixtures in terms of sensitivity/specificity trade-off and second that it is worthwhile modelling serial correlation through an AR(1) noise process at low signal-to-noise (SNR) ratio. Our approach is then validated on an fMRI experiment that studies habituation to auditory sentence repetition. This phenomenon is clearly recovered as well as the hierarchical temporal organisation of the superior temporal sulcus, which is directly derived from the parcel-based HRF estimates |
| Abstract: | Speech perception consists of a set of computations that take continuously varying acoustic waveforms as input and generate discrete representations that make contact with the lexical representations stored in long-term memory as output. Because the perceptual objects that are recognized by the speech perception enter into subsequent linguistic computation, the format that is used for lexical representation and processing fundamentally constrains the speech perceptual processes. Consequently, theories of speech perception must, at some level, be tightly linked to theories of lexical representation. Minimally, speech perception must yield representations that smoothly and rapidly interface with stored lexical items. Adopting the perspective of Marr, we argue and provide neurobiological and psychophysical evidence for the following research programme. First, at the implementational level, speech perception is a multi-time resolution process, with perceptual analyses occurring concurrently on at least two time scales (approx. 20-80 ms, approx. 150-300 ms), commensurate with (sub)segmental and syllabic analyses, respectively. Second, at the algorithmic level, we suggest that perception proceeds on the basis of internal forward models, or uses an 'analysis-by-synthesis' approach. Third, at the computational level (in the sense of Marr), the theory of lexical representation that we adopt is principally informed by phonological research and assumes that words are represented in the mental lexicon in terms of sequences of discrete segments composed of distinctive features. One important goal of the research programme is to develop linking hypotheses between putative neurobiological primitives (e.g. temporal primitives) and those primitives derived from linguistic inquiry, to arrive ultimately at a biologically sensible and theoretically satisfying model of representation and computation in speech. |
| Abstract: | Diffusion magnetic resonance imaging (dMRI) has become an established research tool for the investigation of tissue structure and orientation. In this paper, we present a method for real-time processing of diffusion tensor and Q-ball imaging. The basic idea is to use Kalman filtering framework to fit either the linear tensor or Q-ball model. Because the Kalman filter is designed to be an incremental algorithm, it naturally enables updating the model estimate after the acquisition of any new diffusion-weighted volume. Processing diffusion models and maps during ongoing scans provides a new useful tool for clinicians, especially when it is not possible to predict how long a subject may remain still in the magnet. First, we introduce the general linear models corresponding to the two diffusion tensor and analytical Q-ball models of interest. Then, we present the Kalman filtering framework and we focus on the optimization of the diffusion orientation sets in order to speed up the convergence of the online processing. Last, we give some results on a healthy volunteer for the online tensor and the Q-ball model, and we make some comparisons with the conventional offline techniques used in the literature. We could achieve full real-time for diffusion tensor imaging and deferred time for Q-ball imaging, using a single workstation. |
| Abstract: | PURPOSE: To combine parallel imaging with 3D single-shot acquisition (echo volumar imaging, EVI) in order to acquire high temporal resolution volumar functional MRI (fMRI) data. MATERIALS AND METHODS: An improved EVI sequence was associated with parallel acquisition and field of view reduction in order to acquire a large brain volume in 200 msec. Temporal stability and functional sensitivity were increased through optimization of all imaging parameters and Tikhonov regularization of parallel reconstruction. Two human volunteers were scanned with parallel EVI in a 1.5T whole-body MR system, while submitted to a slow event-related auditory paradigm. RESULTS: Thanks to parallel acquisition, the EVI volumes display a low level of geometric distortions and signal losses. After removal of low-frequency drifts and physiological artifacts, activations were detected in the temporal lobes of both volunteers and voxelwise hemodynamic response functions (HRF) could be computed. On these HRF different habituation behaviors in response to sentence repetition could be identified. CONCLUSION: This work demonstrates the feasibility of high temporal resolution 3D fMRI with parallel EVI. Combined with advanced estimation tools, this acquisition method should prove useful to measure neural activity timing differences or study the nonlinearities and nonstationarities of the BOLD response |
| Abstract: | Subitizing is the rapid and accurate enumeration of small sets (up to 3-4 items). Although subitizing has been studied extensively since its first description about 100 years ago, its underlying mechanisms remain debated. One hypothesis proposes that subitizing results from numerical estimation mechanisms that, according to Weber's law, operate with high precision for small numbers. Alternatively, subitizing might rely on a distinct process dedicated to small numerosities. In this study, we tested the hypothesis that there is a shared estimation system for small and large quantities in human adults, using a masked forced-choice paradigm in which participants named the numerosity of displays taken from sets matched for discrimination difficulty; one set ranged from 1 through 8 items, and the other ranged from 10 through 80 items. Results showed a clear violation of Weber's law (much higher precision over numerosities 1-4 than over numerosities 10-40), thus refuting the single-estimation-system hypothesis and supporting the notion of a dedicated mechanism for apprehending small numerosities |
| Abstract: | Patients with frontal lobe damage have been shown to produce implausible answers in cognitive estimation, a task requiring approximate answers to quantity-related questions of general semantic knowledge. We investigated a patient with frontal lobe damage who presented executive deficits and difficulties in cognitive estimation. The patient also showed difficulties in verbal numerosity estimation (approximately evaluating the quantity of visually presented sets of items), as he produced extreme answers well outside healthy participants' range of answers. A series of tasks evidenced intact number processing and well preserved semantic representation of numbers. Detailed investigation of estimation processes suggested a deficit at the level of translation from an intact semantic representation of numbers to output, whether verbal or non-symbolic. This case study allows disentangling different processes involved in estimation and contributes to a better understanding of the cognitive estimation deficits frequently reported for patients with frontal lesions. |
| Abstract: | While neglected stimuli can still be processed, few studies have directly addressed the issue of the unconscious access to semantics. In order to clarify this issue, we engaged four patients with unilateral left spatial neglect in a number comparison task. Each target number was preceded by a lateralized number prime, either in the intact or neglected hemifield (HF). Both group analyses and the intensive study of a single patient show that left (neglected) as well as right (consciously perceived) number primes affect performance: primes representing quantities that fall on the same side of the reference as the target lead to faster categorization. This congruency effect is highly suggestive of numerical semantic processing of neglected stimuli. Absence of conscious perception of neglected primes was evaluated using a combination of subjective and objective measures of performance in forced-choice tasks. |
| Abstract: | The psychological refractory period (PRP) refers to the fact that humans typically cannot perform two tasks at once. Behavioral experiments have led to the proposal that, in fact, peripheral perceptual and motor stages continue to operate in parallel, and that only a central decision stage imposes a serial bottleneck. We tested this model using neuroimaging methods combined with innovative time-sensitive analysis tools. Subjects performed a dual-task visual-auditory paradigm in which a delay of 300 ms was injected into the auditory task either within or outside of the dual-task interference period. Event-related potentials indicated that the first approximately 250 ms of processing were insensitive to dual-task interference, and that the PRP was mainly reflected in a delayed global component. By a clustering analysis based on time-resolved functional magnetic resonance imaging, we identified networks with qualitatively different timing properties: sensory areas tracked the objective time of stimulus presentation, a bilateral parietoprefrontal network correlated with the PRP delay, and an extended bilateral network that included bilateral posterior parietal cortex, premotor cortex, supplementary motor area, anterior part of the insula, and cerebellum was shared by both tasks during the extent of dual-task performance. The results provide physiological evidence for the coexistence of serial and parallel processes within a cognitive task |
| Abstract: | A briefly presented target shape can be made invisible by the subsequent presentation of a mask that replaces the target. While varying the targetâ??mask interval in order to investigate perception near the consciousness threshold,we discovered a novel visual illusion. At some intervals,the target is clearly visible,but its location is misperceived. By manipulating the mask's size and target's position,we demonstrate that the perceived target location is always displaced to the boundary of a virtual surface defined by the mask contours. Thus,mutual exclusion of surfaces appears as a cause of masking |
| Abstract: | The ability to estimate the passage of time is of fundamental importance for perceptual and cognitive processes. One experience of time is the perception of duration, which is not isomorphic to physical duration and can be distorted by a number of factors. Yet, the critical features generating these perceptual shifts in subjective duration are not understood.We used prospective duration judgments within and across sensory modalities to examine the effect of stimulus predictability and feature change on the perception of duration. First, we found robust distortions of perceived duration in auditory, visual and auditory-visual presentations despite the predictability of the feature changes in the stimuli. For example, a looming disc embedded in a series of steady discs led to time dilation, whereas a steady disc embedded in a series of looming discs led to time compression. Second, we addressed whether visual (auditory) inputs could alter the perception of duration of auditory (visual) inputs. When participants were presented with incongruent audio-visual stimuli, the perceived duration of auditory events could be shortened or lengthened by the presence of conflicting visual information; however, the perceived duration of visual events was seldom distorted by the presence of auditory information and was never perceived shorter than their actual durations.These results support the existence of multisensory interactions in the perception of duration and, importantly, suggest that vision can modify auditory temporal perception in a pure timing task. Insofar as distortions in subjective duration can neither be accounted for by the unpredictability of an auditory, visual or auditory-visual event, we propose that it is the intrinsic features of the stimulus that critically affect subjective time distortions. |
| Abstract: | Human face-to-face communication is essentially audiovisual. Typically, people talk to us face-to-face, providing concurrent auditory and visual input. Understanding someone is easier when there is visual input, because visual cues like mouth and tongue movements provide complementary information about speech content. Here, we hypothesized that, even in the absence of visual input, the brain optimizes both auditory-only speech and speaker recognition by harvesting speaker-specific predictions and constraints from distinct visual face-processing areas. To test this hypothesis, we performed behavioral and neuroimaging experiments in two groups: subjects with a face recognition deficit (prosopagnosia) and matched controls. The results show that observing a specific person talking for 2 min improves subsequent auditory-only speech and speaker recognition for this person. In both prosopagnosics and controls, behavioral improvement in auditory-only speech recognition was based on an area typically involved in face-movement processing. Improvement in speaker recognition was only present in controls and was based on an area involved in face-identity processing. These findings challenge current unisensory models of speech processing, because they show that, in auditory-only speech, the brain exploits previously encoded audiovisual correlations to optimize communication. We suggest that this optimization is based on speaker-specific audiovisual internal models, which are used to simulate a talking face. |
| 2007 |
| Abstract: | In this chapter we review the possible biological bases for developmental dyscalculia, which is a disorder in mathematical abilities presumed to be due to impaired brain function. By reviewing what is known about the localization of numerical cognition functions in the adult brain, the causes of acquired dyscalculia, and the normal development of numerical cognition, we propose several hypotheses for causes of developmental dyscalculia, including that of a core deficit of "number sense" related to an impairment in the horizontal intra- parietal sulcus (HIPS) area. We then discuss research on dyscalculia, including the contribution of recent imaging results in special populations, and evaluate to what extent this research supports our hypotheses. We conclude that there is promising preliminary evidence for a core deficit of number sense in dyscalculia, but we also emphasize that more research is needed to test the hypothesis of multiple types of dyscalculia, particularly in the area of dyscalculia subtyping. We complete the chapter with a discussion of future directions to be taken, the implications for education, and the construction of number sense remediation software in our laboratory |
| Abstract: | Part of human cortex is specialized for cultural domains such as reading and arithmetic, whose invention is too recent to have influenced the evolution of our species. Representations of letter strings and of numbers occupy reproducible locations within large-scale macromaps, respectively in the left occipito-temporal and bilateral intraparietal cortex. Furthermore, recent fMRI studies reveal a systematic architecture within these areas. To explain this paradoxical cerebral invariance of cultural maps, we propose a neuronal recycling hypothesis, according to which cultural inventions invade evolutionarily older brain circuits and inherit many of their structural constraints |
| Abstract: | When a flashed stimulus is followed by a backward mask, subjects fail to perceive it unless the target-mask interval exceeds a threshold duration of about 50 ms. Models of conscious access postulate that this threshold is associated with the time needed to establish sustained activity in recurrent cortical loops, but the brain areas involved and their timing remain debated. We used high-density recordings of event-related potentials (ERPs) and cortical source reconstruction to assess the time course of human brain activity evoked by masked stimuli and to determine neural events during which brain activity correlates with conscious reports. Target-mask stimulus onset asynchrony (SOA) was varied in small steps, allowing us to ask which ERP events show the characteristic nonlinear dependence with SOA seen in subjective and objective reports. The results separate distinct stages in mask-target interactions, indicating that a considerable amount of subliminal processing can occur early on in the occipito-temporal pathway (<250 ms) and pointing to a late (>270 ms) and highly distributed fronto-parieto-temporal activation as a correlate of conscious reportability |
| Abstract: | Cognitive processes are often attributed to statistical or symbolic general-purpose mechanisms. Here we show that some spontaneous generalizations are driven by specialized, highly constrained symbolic operations. We explore how two types of artificial grammars are acquired, one based on repetitions and the other on characteristic relations between tones ("ordinal" grammars). Whereas participants readily acquire repetition-based grammars, displaying early electrophysiological responses to grammar violations, they perform poorly with ordinal grammars, displaying no such electrophysiological responses. This outcome is problematic for both general symbolic and statistical models, which predict that both types of grammars should be processed equally easily. This suggests that some simple grammars are acquired using perceptual primitives rather than general-purpose mechanisms; such primitives may be elements of a "toolbox" of specialized computational heuristics, which may ultimately allow constructing a psychological theory of symbol manipulation. |
| Abstract: | Unconscious mental representations elicited by subliminal stimuli are marked by their fleeting lifetimes, usually below 1 s. Can such evanescent subliminal stimuli, nevertheless, lead to long-lasting learning? To date, evidence suggesting a long-term influence of briefly perceived stimuli on behaviour or brain activity is scarce and questionable. In this study, we used intracranial recordings to provide the first direct demonstration that unconsciously perceived subliminal words could exert long-lasting effects on neuronal signals. When repeating subliminal words over long interstimulus intervals, we observed electrophysiological repetition effects. These unconscious repetition effects suggest that the single presentation of a masked word can durably affect neural architecture |
| Abstract: | Across multiple timescales, acoustic regularities of speech match rhythmic properties of both the auditory and motor systems. Syllabic rate corresponds to natural jaw-associated oscillatory rhythms, and phonemic length could reflect endogenous oscillatory auditory cortical properties. Hemispheric lateralization for speech could result from an asymmetry of cortical tuning, with left and right auditory areas differentially sensitive to spectro-temporal features of speech. Using simultaneous electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) recordings from humans, we show that spontaneous EEG power variations within the gamma range (phonemic rate) correlate best with left auditory cortical synaptic activity, while fluctuations within the theta range correlate best with that in the right. Power fluctuations in both ranges correlate with activity in the mouth premotor region, indicating coupling between temporal properties of speech perception and production. These data show that endogenous cortical rhythms provide temporal and spatial constraints on the neuronal mechanisms underlying speech perception and production |
| Abstract: | Previous work has shown a relationship between parietal lobe anatomy and nonnative speech sound learning. We scanned a new group of phonetic learners using structural magnetic resonance imaging and diffusion tensor imaging. Voxel-based morphometry indicated higher white matter (WM) density in left Heschl's gyrus (HG) in faster compared with slower learners, and manual segmentation of this structure confirmed that the WM volume of left HG is larger in the former compared with the latter group. This finding was replicated in a reanalysis of the original groups tested in Golestani and others (2002, Anatomical correlates of learning novel speech sounds. Neuron 35:997-1010). We also found that faster learners have a greater asymmetry (left > right) in parietal lobe volumes than slower learners and that the right insula and HG are more superiorly located in slower compared with faster learners. These results suggest that left auditory cortex WM anatomy, which likely reflects auditory processing efficiency, partly predicts individual differences in an aspect of language learning that relies on rapid temporal processing. It also appears that a global displacement of components of a right hemispheric language network, possibly reflecting individual differences in the functional anatomy and lateralization of language processing, is predictive of speech sound learning |
| Abstract: | Previous work has shown a relationship between brain anatomy and how quickly adults learn to perceive foreign speech sounds. Faster learners have greater asymmetry (left > right) in parietal lobe white matter (WM) volumes and larger WM volumes of left Heschl's gyrus than slower learners. Here, we tested native French speakers who were previously scanned using high-resolution anatomical magnetic resonance imaging. We asked them to pronounce a Persian consonant that does not exist in French but which can easily be distinguished from French speech sounds, the voiced uvular stop. Two judges scored the goodness of the utterances. Voxel-based morphometry revealed that individuals who more accurately pronounce the foreign sound have higher WM density in the left insula/prefrontal cortex and in the inferior parietal cortices bilaterally compared with poorer producers. Results suggest that WM anatomy in brain regions previously implicated in articulation and phonological working memory, or the size/shape of these or adjacent regions, is in part predictive of the accuracy of speech sound pronunciation |
| Abstract: | Why is it hard to divide attention between dissimilar activities, such as reading and listening to a conversation? We used functional magnetic resonance imaging (fMRI) to study interference between simple auditory and visual decisions, independently of motor competition. Overlapping activity for auditory and visual tasks performed in isolation was found in lateral prefrontal regions, middle temporal cortex and parietal cortex. When the visual stimulus occurred during the processing of the tone, its activation in prefrontal and middle temporal cortex was suppressed. Additionally, reduced activity was seen in modality-specific visual cortex. These results paralleled impaired awareness of the visual event. Even without competing motor responses, a simple auditory decision interferes with visual processing on different neural levels, including prefrontal cortex, middle temporal cortex and visual regions |
| Abstract: | Synesthesia is an experience in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream. Although synesthesia is often referred to as a "neurological condition," it is not listed in the DSM IV or the ICD classifications, as it generally does not interfere with normal daily functioning. However, its high prevalence rate (one in 23) means that synesthesia may be reported by patients who present with other psychiatric symptoms. In this review, I focus on recent research examining the neural basis of the two most intensively studied forms of synesthesia, grapheme --> color synesthesia and tone --> color synesthesia. These data suggest that these forms of synesthesia are elicited through anomalous activation of color-selective areas, perhaps in concert with hyperbinding mediated by the parietal cortex. I then turn to questions for future research and the implications of these models for other forms of synesthesia. |
| Abstract: | This comment challenges the dichtotomy that Kriegeskorte and Bandettini (this issue) propose to exist between "activation-based" and "information-based" approaches to fMRI analyses and argues that multi-variate analyses are just a special case within the overall repertoire of methods for analyzing paradigm-related BOLD signal variations. Moreover, this comment argues that using multi-variate approaches comes at a price, trading-off spatial resolution for sensitivity, and thus partially cancels potential benefits from high-field fMRI. Paradoxically, this comment thus concludes that pattern analyses provide a powerful complement to existing methods but not the complement that will actually permit to map functional architecture at mesoscopic resolution, i.e., one of the most interesting applications of high-field fMRI |
| Abstract: | Understanding the extent and limits of non-conscious processing is an important step on the road to a thorough understanding of the cognitive and cerebral correlates of conscious perception. In this article, we present a critical review of research on subliminal perception during masking and other related experimental conditions. Although initially controversial, the possibility that a broad variety of processes can be activated by a non-reportable stimulus is now well established. Behavioural findings of subliminal priming indicate that a masked word or digit can have an influence on perceptual, lexical and semantic levels, while neuroimaging directly visualizes the brain activation that it evokes in several cortical areas. This activation is often attenuated under subliminal presentation conditions compared to consciously reportable conditions, but there are sufficiently many exceptions, in paradigms such as the attentional blink, to indicate that high activation, per se, is not a sufficient condition for conscious access to occur. We conclude by arguing that for a stimulus to reach consciousness, two factors are jointly needed: (i) the input stimulus must have enough strength (which can be prevented by masking) and (ii) it must receive top-down attention (which can be prevented by drawing attention to another stimulus or task). This view leads to a distinction between two types of non-conscious processes, which we call subliminal and preconscious. According to us, maintaining this distinction is essential in order to make sense of the growing neuroimaging data on the neural correlates of consciousness |
| Abstract: | Several studies have investigated the neural correlates of conscious perception by contrasting functional magnetic resonance imaging (fMRI) activation to conscious and nonconscious visual stimuli. The results often reveal an amplification of posterior occipito-temporal activation and its extension into a parieto-frontal network. However, some of these effects might be due to a greater deployment of attentional or strategical processes in the conscious condition. Here, we examined the brain activity evoked by visible and invisible stimuli, both of which were irrelevant to the task. We collected fMRI data in a masking paradigm in which subliminal versus supraliminal letter strings were presented as primes while subjects focused attention on another subsequent, highly visible target word. Under those conditions, prime visibility was associated with greater activity confined to bilateral posterior occipito-temporal cortices, without extension into frontal and parietal cortices. However, supraliminal primes, compared with subliminal primes, evoked more extensive repetition suppression in a widely distributed set of parieto-frontal areas. Furthermore, only supraliminal primes caused phonological repetition enhancement in left inferior frontal and anterior insular cortex. Those results suggest a 2-stage view of conscious access: Relative to masked stimuli, unmasked stimuli elicit increased occipito-temporal activity, thus allowing them to compete for global conscious access and to induce priming in multiple distant areas. In the absence of attention, however, their access to a second stage of distributed parieto-frontal processing may remain blocked. |
| Abstract: | We used behavioral and functional magnetic resonance imaging (fMRI) methods to probe the cerebral organization of a simple logical deduction process. Subjects were engaged in a motor trial-and-error learning task, in which they had to infer the identity of an unknown 4-key code. The design of the task allowed subjects to base their inferences not only on the feedback they received but also on the internal deductions that it afforded (autoevaluation). fMRI analysis revealed a large bilateral parietal, prefrontal, cingulate, and striatal network that activated suddenly during search periods and collapsed during ensuing periods of sequence repetition. Fine-grained analyses of the temporal dynamics of this search network indicated that it operates according to near-optimal rules that include 1) computation of the difference between expected and obtained rewards and 2) anticipatory deductions that predate the actual reception of positive reward. In summary, the dynamics of effortful mental deduction can be tracked with fMRI and relate to a distributed network engaging prefrontal cortex and its interconnected cortical and subcortical regions |
| Abstract: | Can human adults perform arithmetic operations with large approximate numbers, and what effect, if any, does an internal spatial-numerical representation of numerical magnitude have on their responses? We conducted a psychophysical study in which subjects viewed several hundred short videos of sets of objects being added or subtracted from one another and judged whether the final numerosity was correct or incorrect. Over a wide range of possible outcomes, the subjects' responses peaked at the approximate location of the true numerical outcome and gradually tapered off as a function of the ratio of the true and proposed outcomes (Weber's law). Furthermore, an operational momentum effect was observed, whereby addition problems were overestimated and subtraction problems were underestimated. The results show that approximate arithmetic operates according to precise quantitative rules, perhaps analogous to those characterizing movement on an internal continuum |
| Abstract: | A stimulus that suddenly appears in the corner of the eye inevitably captures our attention, and this in turn leads to faster detection of a second stimulus presented at the same position shortly thereafter. After about 250 msec, however, this effect reverses and the second stimulus is detected faster when it appears far away from the first. Here, we report a potential physiological correlate of this time-dependent attentional facilitation and inhibition. We measured the activity in visual cortex representations of the second (target) stimulus' location depending on the stimulus onset asynchrony (SOA) and spatial distance that separated the target from the preceding cue stimulus. At an SOA of 100 msec, the target yielded larger responses when it was presented near to than far away from the cue. At an SOA of 850 msec, however, the response to the target was more pronounced when it appeared far away from the cue. Our data show how the neural substrate of visual orienting is guided by immediately preceding sensory experience and how a fast-reacting brain system modulates sensory processing by briefly increasing and subsequently decreasing responsiveness in parts of the visual cortex. We propose these activity modulations as the neural correlate of the sequence of perceptual facilitation and inhibition after attentional capture |
| Abstract: | We explored the impact of task context on subliminal neural priming using functional magnetic resonance imaging. The repetition of words during semantic categorization produced activation reduction in the left middle temporal gyrus previously associated with semantic-level representation and dorsal premotor cortex. By contrast, reading aloud produced repetition enhancement in the left inferior parietal lobe associated with print-to-sound conversion and ventral premotor cortex. Analyses of effective connectivity revealed that the task set for reading generated reciprocal excitatory connections between the left inferior parietal and superior temporal regions, reflecting the audiovisual integration required for vocalization, whereas categorization did not produce such backward projection to posterior regions. Thus, masked repetition priming involves two distinct components in the task-specific neural streams, one in the parietotemporal cortex for task-specific word processing and the other in the premotor cortex for behavioral response preparation. The top-down influence of task sets further changes the directions of the unconscious priming in the entire cerebral circuitry for reading. |
| Abstract: | Activation of the horizontal segment of the intraparietal sulcus (hIPS) has been observed in various number-processing tasks, whether numbers were conveyed by symbolic numerals (digits, number words) or by nonsymbolic displays (dot patterns). This suggests an abstract coding of numerical magnitude. Here, we critically tested this hypothesis using fMRI adaptation to demonstrate notation-independent coding of numerical quantity in the hIPS. Once subjects were adapted either to dot patterns or to Arabic digits, activation in the hIPS and in frontal regions recovered in a distance-dependent fashion whenever a new number was presented, irrespective of notation changes. This remained unchanged when analyzing the hIPS peaks from an independent localizer scan of mental calculation. These results suggest an abstract coding of approximate number common to dots, digits, and number words. They support the idea that symbols acquire meaning by linking neural populations coding symbol shapes to those holding nonsymbolic representations of quantities |
| Abstract: | BACKGROUND: Although cognitive processes such as reading and calculation are associated with reproducible cerebral networks, inter-individual variability is considerable. Understanding the origins of this variability will require the elaboration of large multimodal databases compiling behavioral, anatomical, genetic and functional neuroimaging data over hundreds of subjects. With this goal in mind, we designed a simple and fast acquisition procedure based on a 5-minute functional magnetic resonance imaging (fMRI) sequence that can be run as easily and as systematically as an anatomical scan, and is therefore used in every subject undergoing fMRI in our laboratory. This protocol captures the cerebral bases of auditory and visual perception, motor actions, reading, language comprehension and mental calculation at an individual level. RESULTS: 81 subjects were successfully scanned. Before describing inter-individual variability, we demonstrated in the present study the reliability of individual functional data obtained with this short protocol. Considering the anatomical variability, we then needed to correctly describe individual functional networks in a voxel-free space. We applied then non-voxel based methods that automatically extract main features of individual patterns of activation: group analyses performed on these individual data not only converge to those reported with a more conventional voxel-based random effect analysis, but also keep information concerning variance in location and degrees of activation across subjects. CONCLUSION: This collection of individual fMRI data will help to describe the cerebral inter-subject variability of the correlates of some language, calculation and sensorimotor tasks. In association with demographic, anatomical, behavioral and genetic data, this protocol will serve as the cornerstone to establish a hybrid database of hundreds of subjects suitable to study the range and causes of variation in the cerebral bases of numerous mental processes. |
| Abstract: | Magnetic resonance diffusion imaging (dMRI) has become an established research tool for the investigation of tissue structure and orientation. In this paper, we present a method for real time processing of diffusion tensor and Q-ball imaging. The basic idea is to use Kalman filtering framework to fit either the linear tensor or Q-ball model. Because the Kalman filter is designed to be an incremental algorithm, it naturally enables updating the model estimate after the acquisition of any new diffusion-weighted volume. Processing diffusion models and maps during ongoing scans provides a new useful tool for clinicians, especially when it is not possible to predict how long a subject may remain still in the magnet. |
| Abstract: | Periventricular white matter damage affecting large bundles connecting distant cortical areas may constitute the main neuronal mechanism for the deficit of controlled information processing observed in patients with early multiple sclerosis (MS). Visual backward masking has been demonstrated to affect late stages of conscious perception involving long-range interactions between visual perceptual areas and higher level integrative cortices while leaving intact early feed-forward visual processing and even complex processing such as object recognition or semantic processing. We therefore hypothesized that patients with early MS would have an elevated masking threshold, because of an impairment of conscious perception whereas subliminal processing of masked stimuli would be preserved. Twenty-two patients with early MS and 22 normal controls performed two backward-masking experiments. We used Arabic digits as stimuli and varied quasi-continuously the temporal interval with a subsequent mask, thus allowing us to progressively "unmask" the stimuli. We finely quantified the visibility of the masked stimuli using both objective and subjective measures, thus obtaining accurate estimates of the threshold duration for access to consciousness. We also studied the priming effect caused by the variably masked numbers on a comparison task performed on a subsequently presented and highly visible target number. The threshold for access to consciousness of masked stimuli was elevated in MS patients compared to controls, whereas non-conscious processing of these stimuli, as measured by priming, was preserved. These findings suggest that conscious access to masked stimuli depends on the integrity of large-scale cortical integrative processes, which involve long-distance white matter projections, and are impaired due to diffuse demyelinating injury in patients with early MS |
| Abstract: | In temporal lobe epilepsy (TLE) due to hippocampal sclerosis (HS), ictal discharge spread to the frontal and insulo-perisylvian cortex is commonly observed. The implication of white matter pathways in this propagation has not been investigated. We compared diffusion tensor imaging (DTI) measurements along the uncinate fasciculus (UF), a major tract connecting the frontal and temporal lobes, in patients and controls. Ten right-handed patients referred for intractable TLE due to a right HS were investigated on a 1.5-T MR scanner including a DTI sequence. All patients had interictal fluorodeoxyglucose PET showing an ipsilateral temporal hypometabolism associated with insular and frontal or perisylvian hypometabolism. The controls consisted of ten right-handed healthy subjects. UF fiber tracking was performed, and its fractional anisotropy (FA) values were compared between patients and controls, separately for the right and left UF. The left-minus-right FA UF asymmetry index was computed to test for intergroup differences. Asymmetries were found in the control group with right-greater-than-left FA. This asymmetrical pattern was lost in the patient group. Right FA values were lower in patients with right HS versus controls. Although preliminary, these findings may be related to the preferential pathway of seizure spread from the mesial temporal lobe to frontal and insulo-perisylvian areas |
| Abstract: | Identifying the sequence of computations which constitute a cognitive task is a fundamental problem in neuroscience. Here we show, using functional magnetic resonance imaging (fMRI), that we can parse, at the time scale of about 100 ms, the different stages of brain activations which compose a complex sequential task. To identify timing information from the slow blood oxygen level-dependent (BOLD) signal response, we use a simple analytic method, based on periodic stimulation and an analysis of covariation of the spectral parameters (phase and power spectrum at the stimulation frequency) with the different experimental conditions. We implement this strategy in a sequential task, where the onset and duration of different stages are under experimental control. We are able to detect changes in onset latency and in the duration of the response, in an invariant fashion across different brain regions, and reconstruct the stream of activations consistent with five distinct stages of processing of the task. Sensory and motor clusters activate in the expected order and for the expected duration. The timing of sensory activations is more precise than the timing of motor activation. We also parse in time the reading-verbal network: visual extrastriate and phonological access regions (supramarginal gyrus) activate at the time of word presentation, while the inferior frontal gyrus, the anterior cingulate and the supplementary motor area are activated during the rehearsal period |
| Abstract: | When looking at ambiguous visual stimuli, the observer experiences frequent spontaneous transitions between two competing percepts while physical stimulation remains unchanged. Despite recent advances in understanding the neural processes underlying such perceptual rivalry, a key question has remained unresolved: Does perceptual rivalry result merely from local bistability of neural activity patterns in sensory stimulus representations, or do higher-order areas play a causal role by shifting inference and, thus, initiating perceptual changes? We used functional MRI to measure brain activity while human observers reported successive spontaneous changes in perceived direction for an ambiguous apparent motion stimulus. In a control condition, the individual sequences of spontaneous perceptual switches during bistability were replayed by using a disambiguated version of the stimulus. Greater activations during spontaneous compared with stimulus-driven switches were observed in inferior frontal cortex bilaterally. Subsequent chronometric analyses of event-related signal time courses showed that, relative to activations in motion-sensitive extrastriate visual cortex, right inferior frontal cortex activation occurred earlier during spontaneous than during stimulus-driven perceptual changes. The temporal precedence of right inferior frontal activations suggests that this region participates in initiating spontaneous switches in perception during constant physical stimulation. Our findings can thus be seen as a signature of when and where the brain "makes up its mind" about competing perceptual interpretations of a given sensory input pattern |
| Abstract: | The goal of this study was to determine whether brain regions implicated in emotion processing show structural alterations in adolescents with conduct disorder (CD). Using an optimized voxel-based morphometry protocol, we compared grey matter volume in 12 patients with CD and 12 age-, sex-, and intelligence-matched control subjects. Grey matter volume in bilateral anterior insular cortex and the left amygdala was significantly reduced in CD patients compared to healthy control subjects. The insular grey matter abnormalities could be attributed to aggressive behaviour. Moreover, bilateral anterior insular grey matter volume in CD patients correlated significantly with empathy scores. These novel findings point at a joint neuroanatomical substrate underpinning aggressive behaviour and impaired capacity of empathy and suggest a critical role for the anterior insula in regulating social behaviour |
| Abstract: | The aim of group fMRI studies is to relate contrasts of tasks or stimuli to regional brain activity increases. These studies typically involve 10 to 16 subjects. The average regional activity statistical significance is assessed using the subject to subject variability of the effect (random effects analyses). Because of the relatively small number of subjects included, the sensitivity and reliability of these analyses is questionable and hard to investigate. In this work, we use a very large number of subject (more than 80) to investigate this issue. We take advantage of this large cohort to study the statistical properties of the inter-subject activity and focus on the notion of reproducibility by bootstrapping. We asked simple but important methodological questions: Is there, from the point of view of reliability, an optimal statistical threshold for activity maps? How many subjects should be included in group studies? What method should be preferred for inference? Our results suggest that i) optimal thresholds can indeed be found, and are rather lower than usual corrected for multiple comparison thresholds, ii) 20 subjects or more should be included in functional neuroimaging studies in order to have sufficient reliability, iii) non-parametric significance assessment should be preferred to parametric methods, iv) cluster-level thresholding is more reliable than voxel-based thresholding, and v) mixed effects tests are much more reliable than random effects tests. Moreover, our study shows that inter-subject variability plays a prominent role in the relatively low sensitivity and reliability of group studies |
| Abstract: | Visual word recognition has been proposed to rely on a hierarchy of increasingly complex neuronal detectors, from individual letters to bigrams and morphemes. We used fMRI to test whether such a hierarchy is present in the left occipitotemporal cortex, at the site of the visual word-form area, and with an anterior-to-posterior progression. We exposed adult readers to (1) false-font strings; (2) strings of infrequent letters; (3) strings of frequent letters but rare bigrams; (4) strings with frequent bigrams but rare quadrigrams; (5) strings with frequent quadrigrams; (6) real words. A gradient of selectivity was observed through the entire span of the occipitotemporal cortex, with activation becoming more selective for higher-level stimuli toward the anterior fusiform region. A similar gradient was also seen in left inferior frontoinsular cortex. Those gradients were asymmetrical in favor of the left hemisphere. We conclude that the left occipitotemporal visual word-form area, far from being a homogeneous structure, presents a high degree of functional and spatial hierarchical organization which must result from a tuning process during reading acquisition |
| 2006 |
| Abstract: | L'homme manipule les nombres en utilisant les mots de sa langue. De plus, tout un ensemble de données convergent pour indiquer qu'il dispose aussi d'un système non-verbal pour représenter la numérosité des ensembles, système hérité du monde animal. Nous avons abordé la question des interactions entre représentations numériques verbales et non-verbales, en étudiant trois populations différentes : des adultes occidentaux, des bébés de trois mois, ainsi que des Indiens d'Amazonie, les Mundu! rucus, peuple dont la langue possède un lexique numérique très restreint. Nos recherches s'articulent autour des trois axes suivants : 1. Tout d'abord nous avons cherché à donner une caractérisation fine des représentations non-verbales de la numérosité, à l'aide d'un modèle mathématique, qui postule que les numérosités sont représentées sur un continuum interne, la ligne numérique interne. Les prédictions! du modèle s'accordent avec un ensemble de mesures expérimentales, sur des tâches de comparaison, d'addition et de soustraction de numérosités. Enfin, en confrontant les prédictions du modèle aux résultats d'une tâche d'estimation de numérosité que nous avons développée, nous avons pu conclure que la ligne numérique interne est compressive. 2. Par ailleurs, nos travaux sur l'estimation abordent la question des liens entre les représentations de numérosités et les numéraux de la langue. De manière spontanée, les sujets ont une tendance marquée à sous-estimer la numérosité des stimuli, mais la donnée d'un indice suffit à modifier radicalement la manière dont ils sont calibrés. De plus, le processus de calibration agit de manière globale sur toute la ligne numérique. 3. Enfin, que se passe-t-il en l'absence de représentations verbales pour les nombres ? A l'aide de la technique des potentiels évoqués (ERPs), nous avons montré que les bébés âgés de trois mois sont déjà sensibles à la numérosité. Par ailleurs, nos expériences chez les Indiens Mundurucus montrent qu'en l'absence d'un lexique pour les grands nombres, ceux-ci déploient les mêmes compétences que des occidentaux dans des tâches d'arithmétique sur la numérosité, tant qu'on n'exige qu'une réponse approximative. De plus, les Mundurucus possèdent un concept d'égalité exacte, transcendant leurs représentations non-verbales approximatives de numérosité, mais se trouvent limités dans la plupart des tâches d'arithmétique exacte, de fait qu'il leur manque un outil cognitif (analogue à notre procédure de comptage) pour évaluer! la numérosité exacte d'un ensemble. Ces résultats nous éclairent sur les influences respectives de notre bagage biologique et de la culture dans le développement de la cognition numérique |
| Abstract: | The supplementary motor area (SMA) is a key structure for behavioral planning and execution. Recent research on motor control conducted with monkeys and humans has put to light an anatomical and functional distinction between pre-SMA and SMA-proper. According to this view, the pre-SMA would be involved in higher level processes while the SMA-proper would be more closely tied to motor output. We extended this general framework to the verbal domain, in order to investigate the role of the SMA in speech production. We conducted two speech production experiments with fMRI where we manipulated parameters such as familiarity, complexity or constraints on word selection. The results reveal a parcellation of the SMA into three distinct regions, according to their involvement in different aspects of word production. More specifically, following a rostrocaudal gradient, we observed differential activations related to lexical selection, linear sequence encoding and control of motor output. A parallel organization was observed in the dorsolateral frontal cortex. By refining its anatomical and functional parcellation, these results clarify the roles of the SMA in speech production |
| Abstract: | Repetitive transcranial magnetic stimulation (rTMS) can interfere with linguistic performance when delivered over language areas. At low frequency (1 Hz), rTMS is assumed to decrease cortical excitability; however, the degree of TMS effect on cortical language areas may depend on the localization of the stimulation coil with respect to the inter-individual anatomo-functional variations. Hence, we aimed at investigating individual brain areas involved in semantic and phonological auditory processes. We hypothesized that active rTMS targeted over Wernicke's area might modify the performance during a language-fragment-detection task. Sentences in native or foreign languages were presented to 12 right-handed male healthy volunteers during functional magnetic resonance imaging (fMRI). 3D-functional maps localized the posterior temporal activation (Wernicke) in each subject and MRI anatomical cortical landmarks were used to define Broca's pars opercularis (F3Op). A frameless stereotaxy system was used to guide the TMS coil position over Wernicke's and F3Op areas in each subject. Active and placebo randomized rTMS sessions were applied at 1 Hz, 110\% of motor threshold, during the same language-fragment-detection task. Accuracy and response time (RT) were recorded. RT was significantly decreased by active rTMS compared to placebo over Wernicke's area, and was more decreased for native than for foreign languages. No significant RT change was observed for F3Op area. rTMS conditions did not impair participants' accuracy. Thus, low-frequency rTMS over Wernicke's area can speed-up the response to a task tapping on native language perception in healthy volunteers. This individually-guided stimulation study confirms that facilitatory effects are not confined to high-frequency rTMS |
| Abstract: | OBJECTIVE: Altered anterior cingulate cortex activity has been consistently detected by functional imaging in schizophrenia patients. In the present study, we hypothesized that the detection of such local hypoactivity varies when the subjects' local gyrification is monitored. Using a group-statistical approach, we investigated whether the presence or absence of a paracingulate sulcus (PCS) does influence the detection of the activation patterns in the cognitive division of the anterior cingulate cortex (ACcd). METHOD: fMRI data were acquired using an event-related paradigm during a task involving both priming and interference between stimuli. In the fMRI dataset collected from 13 schizophrenia patients and 16 healthy subjects, subgroups were defined according to the presence or absence of a PCS. Regional activations during interference between stimuli were examined in the ACcd of each hemisphere, using for each region of interest both voxel-based random-effects and non-parametric analyses. RESULTS: ACcd activation was left-sided in healthy subjects with a PCS, and right-sided in healthy subjects devoid of a PCS. ACcd activations were detected bilaterally in schizophrenia patients with a PCS, whereas left ACcd was deactivated in patients without a PCS. Subgroup comparisons revealed no difference between healthy subjects with a PCS and patients with a PCS, whereas in the subgroups devoid of PCS, the patients exhibited a bilateral ACcd hypoactivation relative to healthy subjects. CONCLUSIONS: PCS presence or absence influences the detection of ACcd activations in group-analysis of schizophrenia patients |
| Abstract: | Previous studies have shown that processing information in one sensory modality can either be enhanced or attenuated by concurrent stimulation of another modality. Here, we reconcile these apparently contradictory results by showing that the sign of cross-modal interactions depends on whether the content of two modalities is associated or not. When concurrently presented auditory and visual stimuli are paired by chance, cue-induced preparatory neural activity is strongly enhanced in the task-relevant sensory system and suppressed in the irrelevant system. Conversely, when information in the two modalities is reliably associated, activity is enhanced in both systems regardless of which modality is task relevant. Our findings illustrate an ecologically optimal flexibility of the neural mechanisms that govern multisensory processing: facilitation occurs when integration is expected, and suppression occurs when distraction is expected. Because thalamic structures were more active when the senses needed to operate separately, we propose them to serve gatekeeper functions in early cross-modal interactions |
| Abstract: | Five experiments investigated whether adults and preschool children can perform simple arithmetic calculations on non-symbolic numerosities. Previous research has demonstrated that human adults, human infants, and non-human animals can process numerical quantities through approximate representations of their magnitudes. Here we consider whether these non-symbolic numerical representations might serve as a building block of uniquely human, learned mathematics. Both adults and children with no training in arithmetic successfully performed approximate arithmetic on large sets of elements. Success at these tasks did not depend on non-numerical continuous quantities, modality-specific quantity information, the adoption of alternative non-arithmetic strategies, or learned symbolic arithmetic knowledge. Abstract numerical quantity representations therefore are computationally functional and may provide a foundation for formal mathematics |
| Abstract: | Of the many brain events evoked by a visual stimulus, which are specifically associated with conscious perception, and which merely reflect non-conscious processing? Several recent neuroimaging studies have contrasted conscious and non-conscious visual processing, but their results appear inconsistent. Some support a correlation of conscious perception with early occipital events, others with late parieto-frontal activity. Here we attempt to make sense of these dissenting results. On the basis of the global neuronal workspace hypothesis, we propose a taxonomy that distinguishes between vigilance and access to conscious report, as well as between subliminal, preconscious and conscious processing. We suggest that these distinctions map onto different neural mechanisms, and that conscious perception is systematically associated with surges of parieto-frontal activity causing top-down amplification |
| Abstract: | Does geometry constitute a core set of intuitions present in all humans, regardless of their language or schooling? We used two nonverbal tests to probe the conceptual primitives of geometry in the Munduruku, an isolated Amazonian indigene group. Munduruku children and adults spontaneously made use of basic geometric concepts such as points, lines, parallelism, or right angles to detect intruders in simple pictures, and they used distance, angle, and sense relationships in geometrical maps to locate hidden objects. Our results provide evidence for geometrical intuitions in the absence of schooling, experience with graphic symbols or maps, or a rich language of geometrical terms |
| Abstract: | Subliminal words cause behavioral priming, yet the depth of their processing remains debated. Using transcranial magnetic stimulation (TMS), Nakamura et al. demonstrate in this issue of Neuron that this subliminal priming effect can be selectively disrupted. Distinct TMS sites disrupt priming in lexical decision and pronunciation tasks, suggesting that task set influences subliminal processing |
| Abstract: | The functional organization of the perisylvian language network was examined using a functional MRI (fMRI) adaptation paradigm with spoken sentences. In Experiment 1, a given sentence was presented every 14.4 s and repeated two, three, or four times in a row. The study of the temporal properties of the BOLD response revealed a temporal gradient along the dorsal-ventral and rostral-caudal directions: From Heschl's gyrus, where the fastest responses were recorded, responses became increasingly slower toward the posterior part of the superior temporal gyrus and toward the temporal poles and the left inferior frontal gyrus, where the slowest responses were observed. Repetition induced a decrease in amplitude and a speeding up of the BOLD response in the superior temporal sulcus (STS), while the most superior temporal regions were not affected. In Experiment 2, small blocks of six sentences were presented in which either the speaker voice or the linguistic content of the sentence, or both, were repeated. Data analyses revealed a clear asymmetry: While two clusters in the left superior temporal sulcus showed identical repetition suppression whether the sentences were produced by the same speaker or different speakers, the homologous right regions were sensitive to sentence repetition only when the speaker voice remained constant. Thus, hemispheric left regions encode linguistic content while homologous right regions encode more details about extralinguistic features like speaker voice. The results demonstrate the feasibility of using sentence-level adaptation to probe the functional organization of cortical language areas. Hum Brain Mapp, 2006. (c) 2006 Wiley-Liss, Inc |
| Abstract: | BACKGROUND: Studies of visual backward masking have frequently revealed an elevated masking threshold in schizophrenia. This finding has frequently been interpreted as indicating a low-level visual deficit. However, more recent models suggest that masking may also involve late and higher-level integrative processes, while leaving intact early bottom-up visual processing. OBJECTIVE: To test the hypothesis that the backward-masking deficit in schizophrenia corresponds to a deficit in the late stages of conscious perception, whereas the subliminal processing of masked stimuli is fully preserved. DESIGN: Twenty-eight patients with schizophrenia and 28 normal control subjects performed 2 backward-masking experiments. We used Arabic digits as stimuli and varied quasi-continuously the interval with a subsequent mask, thus allowing us to progressively unmask the stimuli. We finely quantified their degree of visibility using objective and subjective measures to evaluate the threshold duration for access to consciousness. We also studied the priming effect caused by the variably masked numbers in a comparison task performed on a subsequently presented and highly visible target number. RESULTS: The threshold delay between the digit and mask necessary for the conscious perception of the masked stimulus was longer in patients compared with controls. This higher consciousness threshold in patients was confirmed by an objective and a subjective measure, and both measures were highly correlated for the patients and controls. However, subliminal priming of masked numbers was effective and identical in patients and controls. CONCLUSIONS: Access to conscious report of masked stimuli is impaired in schizophrenia, whereas fast bottom-up processing of the same stimuli, as assessed by subliminal priming, is preserved. These findings suggest a high-level origin of the masking deficit in schizophrenia, although they leave open for further research its exact relation to previously identified bottom-up visual processing abnormalities |
| Abstract: | Mammals acquire much of their sensory information by actively moving their sensory organs. Rats, in particular, scan their surrounding environment with their whiskers. This form of active sensing induces specific patterns of temporal encoding of sensory information, which are based on a conversion of space into time via sensor movement. We investigate the ways in which object location is encoded by the whiskers and decoded by the brain. We recorded from first-order neurons located in the trigeminal ganglion (TG) of anaesthetized rats during epochs of artificial whisking induced by electrical stimulation of the facial motor nerve. We found that TG neurons encode the three positional coordinates with different codes. The horizontal coordinate (along the backward-forward axis) is encoded by two encoding schemes, both relying on the firing times of one type of TG neuron, the 'contact cell'. The radial coordinate (from face outward) is encoded primarily by the firing magnitude of another type of TG neurons, the 'pressure cell'. The vertical coordinate (from ground up) is encoded by the identity of activated neurons. The decoding schemes of at least some of these sensory cues, our data suggest, are also active: cortical representations are generated by a thalamic comparison of cortical expectations with incoming sensory data. |
| Abstract: | The human infant is particularly immature at birth and brain maturation, with the myelination of white matter fibers, is protracted until adulthood. Diffusion tensor imaging offers the possibility to describe non invasively the fascicles spatial organization at an early stage and to follow the cerebral maturation with quantitative parameters that might be correlated with behavioral development. Here, we assessed the feasibility to study the organization and maturation of major white matter bundles in eighteen 1- to 4-month-old healthy infants, using a specific acquisition protocol customized to the immature brain (with 15 orientations of the diffusion gradients and a 700 s mm(-2)b factor). We were able to track most of the main fascicles described at later ages despite the low anisotropy of the infant white matter, using the FACT algorithm. This mapping allows us to propose a new method of quantification based on reconstructed tracts, split between specific regions, which should be more sensitive to specific changes in a bundle than the conventional approach, based on regions-of-interest. We observed variations in fractional anisotropy and mean diffusivity over the considered developmental period in most bundles (corpus callosum, cerebellar peduncles, cortico-spinal tract, spino-thalamic tract, capsules, radiations, longitudinal and uncinate fascicles, cingulum). The results are in good agreement with the known stages of white matter maturation and myelination, and the proposed approach might provide important insights on brain development |
| Abstract: | OBJECT: A method is proposed for generating schemes of diffusion gradient orientations which allow the diffusion tensor to be reconstructed from partial data sets in clinical DT-MRI, should the acquisition be corrupted or terminated before completion because of patient motion. MATERIALS AND METHODS: A general energy-minimization electrostatic model was developed in which the interactions between orientations are weighted according to their temporal order during acquisition. In this report, two corruption scenarios were specifically considered for generating relatively uniform schemes of 18 and 60 orientations, with useful subsets of 6 and 15 orientations. The sets and subsets were compared to conventional sets through their energy, condition number and rotational invariance. Schemes of 18 orientations were tested on a volunteer. RESULTS: The optimized sets were similar to uniform sets in terms of energy, condition number and rotational invariance, whether the complete set or only a subset was considered. Diffusion maps obtained in vivo were close to those for uniform sets whatever the acquisition time was. This was not the case with conventional schemes, whose subset uniformity was insufficient. CONCLUSION: With the proposed approach, sets of orientations responding to several corruption scenarios can be generated, which is potentially useful for imaging uncooperative patients or infants. |
| Abstract: | Whether masked words can be processed at a semantic level remains a controversial issue in cognitive psychology. Although recent behavioral studies have demonstrated masked semantic priming for number words, attempts to generalize this finding to other categories of words have failed. Here, as an alternative to subliminal priming, we introduce a sensitive behavioral method to detect nonconscious semantic processing of words. The logic of this method consists of presenting words close to the threshold for conscious perception and examining whether their semantic content modulates performance in objective and subjective tasks. Our results disclose two independent sources of modulation of the threshold for access to consciousness. First, prior conscious perception of words increases the detection rate of the same words when they are subsequently presented with stronger masking. Second, the threshold for conscious access is lower for emotional words than for neutral ones, even for words that have not been previously consciously perceived, thus implying that written words can receive nonconscious semantic processing |
| Abstract: | Models of the "visual word form system" postulate that a left occipitotemporal region implements the automatic visual word recognition required for efficient reading. This theory was assessed in a patient in whom reading was explored with behavioral measures, fMRI, and intracranial local field potentials. Prior to surgery, when reading was normal, fMRI revealed a normal mosaic of ventral visual selectivity for words, faces, houses, and tools. Intracranial recordings demonstrated that the left occipitotemporal cortex responded with a short latency to conscious but also to subliminal words. Surgery removed a small portion of word-responsive occipitotemporal cortex overlapping with the word-specific fMRI activation. The patient developed a marked reading deficit, while recognition of other visual categories remained intact. Furthermore, in the post-surgery fMRI map of visual cortex, only word-specific activations disappeared. Altogether, these results provide direct evidence for the causal role of the left occipitotemporal cortex in the recognition of visual words |
| Abstract: | Do infants perceive visual cues as diverse as frontal-view faces, profiles or bodies as being different aspects of the same object, a fellow human? If that is the case, visual exposure to one such cue should facilitate the subsequent processing of the others. To verify this hypothesis, we recorded event-related responses in 4-month-old infants and in adults. Pictures of eyes were interleaved amongst images belonging to three human contexts (frontal-view faces, profiles or bodies) or non-human contexts (houses, cars or pliers). In adults, both profile and frontal-face contexts elicited suppression of the N170 response to eye pictures, indicating an access to a view-invariant representation of faces. In infants, a response suppression of the N290 component was recorded only in the context of frontal faces, while profile context induces a different effect (i.e., a P400 enhancement) on eye processing. This dissociation suggests that the view-invariant representation of faces is learned, as it is for other 3-D objects and needs more than 4 months of exposure to be established. In a follow-up study, where infants were exposed to a short movie showing people rotating their heads, the profile-induced P400 effect was speeded up, indicating that exposure to successive views of the same object is probably a way to build up adult-like face representations |
| Abstract: | We used fMRI to examine the functional correlates of syntactical processing in the first (L1) and second (L2) languages of non-proficient, late bilinguals. Subjects either covertly read words or produced sentences from them. Syntactical production during sentence production activated regions including left inferior frontal (LIFG) gyrus and the supplementary motor area in both languages. Analyses performed on the LIFG activation identified on a subject-by-subject basis revealed greater activation in L2 compared to L1 during sentence production and during word reading, consistent with previous work suggesting that greater cognitive effort may be subserved by less well-tuned neural representations that require greater neuronal activity. Remarkably, there was a greater separation in the LIFG activations in L1 versus L2 in less compared to more proficient bilinguals during syntax production, suggesting a functional reorganisation of regions involved in syntactical production as a function of syntactical proficiency |
| Abstract: | We present a hypothetical neurocomputational model that combines a set of neural circuits at the molecular, cellular, and system levels and accounts for several neurobiological and behavioral processes leading to nicotine addiction. We propose that combining changes in the nicotinic receptor response, expressed by mesolimbic dopaminergic neurons, with dopamine-gated learning in action-selection circuits, suffices to capture the acquisition of nicotine addiction. We show that an opponent process enhanced by persistent nicotine-taking renders self-administration rigid and habitual by inhibiting the learning process, resulting in long-term impairments in the absence of the drug. The model implies distinct thresholds on the dosage and duration for the acquisition and persistence of nicotine addiction. Our hypothesis unites a number of prevalent ideas on nicotine action into a coherent formal network for further understanding of compulsive drug addiction |
| Abstract: | At the intersection of two intensely belabored fields, primary visual cortex (V1) function and neural mechanisms of cognitive control, Jack et al. (in this issue of Neuron) report a neural signal that is neither related to stimulus representation nor spatial attention. Instead, this endogenous signal correlates with task structure and raises new questions |
| Abstract: | PURPOSE OF REVIEW: To discuss whether recent functional neuroimaging results can account for clinical phenomenology in visual associative agnosias. RECENT FINDINGS: Functional neuroimaging studies in healthy human subjects have identified only two regions of ventral occipitotemporal cortex that invariantly respond to individual faces and visual words, respectively. The signature of face identity coding in the fusiform neural response was shown to be missing in a patient with prosopagnosia. Another case study established that a surgical lesion close to the region sensitive to visual words can result in pure alexia. SUMMARY: Evidence is increasing that functional specialization for processing face identity and visual word forms is restricted to two specialized sensory modules in the occipitotemporal cortex. A structural or functional lesion to face-sensitive and word-sensitive regions in the ventral occipitotemporal cortex can provide the most parsimonious account for the clinical syndromes of prosopagnosia and agnosic alexia. This review suggests that functional specialization should be considered in terms of whether exclusively one brain region (instead of many) underpins a defined function and not as whether this brain region underpins exclusively one cognitive function. Such functional specialization seems to exist for at least two higher-order visual perceptual functions, face and word identification |
| Abstract: | Previous studies using simultaneous EEG and fMRI recordings have yielded discrepant results regarding the topography of brain activity in relation to spontaneous power fluctuations in the alpha band of the EEG during eyes-closed rest. Here, we explore several possible explanations for this discrepancy by re-analyzing in detail our previously reported data. Using single subject analyses as a starting point, we found that alpha power decreases are associated with fMRI signal increases that mostly follow two distinct patterns: either 'visual' areas in the occipital lobe or 'attentional' areas in the frontal and parietal lobe. On examination of the EEG spectra corresponding to these two fMRI patterns, we found greater relative theta power in sessions yielding the 'visual' fMRI pattern during alpha desynchronization and greater relative beta power in sessions yielding the 'attentional' fMRI pattern. The few sessions that fell into neither pattern featured the overall lowest theta and highest beta power. We conclude that the pattern of brain activation observed during spontaneous power reduction in the alpha band depends on the general level of brain activity as indexed over a broader spectral range in the EEG. Finally, we relate these findings to the concepts of 'resting state' and 'default mode' and discuss how - as for sleep - EEG-based criteria might be used for staging brain activity during wakefulness |
| Abstract: | In group average analyses, we generalize the classical one-sample t test to account for heterogeneous within-subject uncertainties associated with the estimated effects. Our test statistic is defined as the maximum likelihood ratio corresponding to a Gaussian mixed-effect model. The test's significance level is calibrated using the same sign permutation framework as in Holmes et al., allowing for exact specificity control under a mild symmetry assumption about the subjects' distribution. Because our likelihood ratio test does not rely on homoscedasticity, it is potentially more sensitive than both the standard t test and its permutation-based version. We present results from the Functional Imaging Analysis Contest 2005 dataset to support this claim. Hum Brain Mapp 27:402-410, 2006. (c) 2006 Wiley-Liss, Inc |
| Abstract: | The visual perception of words is known to activate the auditory representation of their spoken forms automatically. We examined the neural mechanism for this phonological activation using transcranial magnetic stimulation (TMS) with a masked priming paradigm. The stimulation sites (left superior temporal gyrus [L-STG] and inferior parietal lobe [L-IPL]), modality of targets (visual and auditory), and task (pronunciation and lexical decision) were manipulated independently. For both within- and cross-modal conditions, the repetition priming during pronunciation was eliminated when TMS was applied to the L-IPL, but not when applied to the L-STG, whereas the priming during lexical decision was eliminated when the L-STG, but not the L-IPL, was stimulated. The observed double dissociation suggests that the conscious task instruction modulates the stimulus-driven activation of the lateral temporal cortex for lexico-phonological activation and the inferior parietal cortex for spoken word production, and thereby engages a different neural network for generating the appropriate behavioral response. |
| Abstract: | In the present study, we reexamined the effect of word length (number of letters in a word) on lexical decision. Using the English Lexicon Project, which is based on a large data set of over 40,481 words (Balota et al., 2002), we performed simultaneous multiple regression analyses on a selection of 33,006 English words (ranging from 3 to 13 letters in length). Our analyses revealed an unexpected pattern of results taking the form of a U-shaped curve. The effect of number of letters was facilitatory for words of 3-5 letters, null for words of 5-8 letters, and inhibitory for words of 8-13 letters. We also showed that printed frequency, number of syllables, and number of orthographic neighbors all made independent contributions. The length effects were replicated in a new analysis of a subset of 3,833 monomorphemic nouns (ranging from 3 to 10 letters), and also in another analysis based on 12,987 bisyllabic items (ranging from 3 to 9 letters). These effects were independent of printed frequency, number of syllables, and number of orthographic neighbors. Furthermore, we also observed robust linear inhibitory effects of number of syllables. Implications for models of visual word recognition are discussed |
| Abstract: | Human adults can assess the number of objects in a set (numerosity) by approximate estimation or by exact counting. There is evidence suggesting that numerosity estimation depends on a dedicated mechanism that is a-modal and non-verbal. By contrast, counting requires the coordination between the pre-existing numerosity estimation abilities with language and one-to-one correspondence principles. In this paper we investigate with fMRI the neural correlates of numerosity estimation and counting in human adults, using both visual and auditory stimuli. Results show that attending to approximate numerosity correlates with increased activity of a right lateralized fronto-parietal cortical network, and that this activity is independent of the stimuli presentation's modality. Counting activates additional left prefrontal, parietal, and bilateral premotor areas, again independently from stimulus modality. These results dissociate two neuronal systems that underlie different numerosity judgements |
| Abstract: | There is a universal and often unconscious tendency to mentally associate the number sequence with a spatial continuum (the mental number line). Here we study one individual who reports a strong and vivid sense of space when processing numbers. For him, the number sequence has a precise spatial form: a curvilinear right-to-left oriented line. We used various tasks to demonstrate that this numerical - spatial association is not a mere figment of his imagination, but a constrained experiential phenomenon consistent across sessions and automatically triggered by the visual presentation of numbers. We also show that this idiosyncratic representation can coexist with another implicit association, the SNARC effect (Spatial?Numerical Association of Response Codes, where small numbers are associated with the left side of space). This effect is present in individuals without explicit number forms and is not affected in the present subject in spite of his reversed subjective representation |
| Abstract: | The left superior temporal cortex, which supports linguistic functions, has consistently been reported to activate during auditory-verbal hallucinations in schizophrenia patients. It has been suggested that auditory hallucinations and the processing of normal external speech compete for common neurophysiological resources. We tested the hypothesis of a negative relationship between the clinical severity of hallucinations and local brain activity in posterior linguistic regions while patients were listening to external speech. Fifteen right-handed patients with schizophrenia and daily auditory hallucinations for at least 3 months were studied with event-related fMRI while listening to sentences in French or to silence. Severity of hallucinations, assessed using the auditory hallucination subscales of the Psychotic Symptom Rating Scales (PSYRATS) and of the Scale for the Assessment of Positive Symptoms (SAPS-AH), negatively correlated with activation in the left temporal superior region in the French minus silence condition. This finding supports the hypothesis that auditory hallucinations compete with normal external speech for processing sites within the temporal cortex in schizophrenia |
| Abstract: | The Functional Imaging Analysis Contest (FIAC) culminated in the FIAC Workshop held at the 11th Annual Meeting of the Organization for Human Brain Mapping in Toronto in 2005. This special issue summarizes various analyses used by contestants with a single functional magnetic resonance imaging (fMRI) study, a cortical-language study using sentence repetition. The results from the cognitive neuroscientists who developed the test-base language study, and report their data analysis, are complemented by expert analyses of the same test-base data by most of the major groups actively developing fMRI software packages. Analyses include many variants of the general linear model (GLM), cutting-edge spatial- and temporal-wavelets, permutation-based, and ICA approaches. A number of authors also include surface-based approaches. Several articles describe the important emerging areas of diagnostics for GLM analysis, multivariate predictive modeling, and functional connectivity analysis. While the FIAC did not achieve all of its goals, it helped identify new activation regions in the test-base data, and more important, through this special issue it illustrates the significant methods-driven variability that potentially exists in the literature. Variable results from different methods reported here should provide a cautionary note and motivate the Human Brain Mapping community to explore more thoroughly the methodologies they use for analyzing fMRI data. Hum Brain Mapp 27:351-359, 2006. (c) 2006 Wiley-Liss, Inc |
| Abstract: | Rats explore their environment by actively moving their whiskers. Recently, we described how object location in the horizontal (front-back) axis is encoded by first-order neurons in the trigeminal ganglion (TG) by spike timing. Here we show how TG neurons encode object location along the radial coordinate, i.e., from the snout outward. Using extracellular recordings from urethane-anesthetized rats and electrically induced whisking, we found that TG neurons encode radial distance primarily by the number of spikes fired. When an object was positioned closer to the whisker root, all touch-selective neurons recorded fired more spikes. Some of these cells responded exclusively to objects located near the base of whiskers, signaling proximal touch by an identity (labeled-line) code. A number of tonic touch-selective neurons also decreased delays from touch to the first spike and decreased interspike intervals for closer object positions. Information theory analysis revealed that near-certainty discrimination between two objects separated by 30\% of the length of whiskers was possible for some single cells. However, encoding reliability was usually lower as a result of large trial-by-trial response variability. Our current findings, together with the identity coding suggested by anatomy for the vertical dimension and the temporal coding of the horizontal dimension, suggest that object location is encoded by separate neuronal variables along the three spatial dimensions: temporal for the horizontal, spatial for the vertical, and spike rate for the radial dimension. |
| Abstract: | Traditional inference in neuroimaging consists in describing brain activations elicited and modulated by different kinds of stimuli. Recently, however, paradigms have been studied in which the converse operation is performed, thus inferring behavioral or mental states associated with activation images. Here, we use the well-known retinotopy of the visual cortex to infer the visual content of real or imaginary scenes from the brain activation patterns that they elicit. We present two decoding algorithms: an explicit technique, based on the current knowledge of the retinotopic structure of the visual areas, and an implicit technique, based on supervised classifiers. Both algorithms predicted the stimulus identity with significant accuracy. Furthermore, we extend this principle to mental imagery data: in five data sets, our algorithms could reconstruct and predict with significant accuracy a pattern imagined by the subjects |
| Abstract: | The visual system of literate adults develops a remarkable perceptual expertise for printed words. To delineate the aspects of this competence intrinsic to the occipitotemporal "what" pathway, we studied a patient with bilateral lesions of the occipitoparietal "where" pathway. Depending on critical geometric features of the display (rotation angle, letter spacing, mirror reversal, etc.), she switched from a good performance, when her intact ventral pathway was sufficient to encode words, to severely impaired reading, when her parietal lesions prevented the use of alternative reading strategies as a result of spatial and attentional impairments. In particular, reading was disrupted (a) by rotating word by more than 50 degrees , providing an approximation of the invariance range for words encoding in the ventral pathway; (b) by separating letters with double spaces, revealing the limits of letter grouping into perceptual wholes; (c) by mirror-reversing words, showing that words escape the default mirror-invariant representation of visual objects in the ventral pathway. Moreover, because of her parietal lesions, she was unable to discriminate mirror images of common objects, although she was excellent with reversible pseudowords, confirming that the breaking of mirror symmetry was intrinsic to the occipitotemporal cortex. Thus, charting the display conditions associated with preserved or impaired performance allowed us to infer properties of word coding in the normal ventral pathway and to delineate the roles of the parietal lobes in single-word recognition |
| Abstract: | ABSTRACT: BACKGROUND: Adaptive game software has been successful in remediation of dyslexia. Here we describe the cognitive and algorithmic principles underlying the development of similar software for dyscalculia. Our software is based on current understanding of the cerebral representation of number and the hypotheses that dyscalculia is due to a "core deficit" in number sense or in the link between number sense and symbolic number representations. METHODS: "The Number Race" software trains children on an entertaining numerical comparison task, by presenting problems adapted to the performance level of the individual child. We report full mathematical specifications of the algorithm used, which relies on an internal model of the child's knowledge in a multidimensional "learning space" consisting of three difficulty dimensions: numerical distance, response deadline, and conceptual complexity (from non-symbolic numerosity processing to increasingly complex symbolic operations). RESULTS: The performance of the software was evaluated both by mathematical simulations and by five weeks of use by nine children with mathematical learning difficulties. The results indicate that the software adapts well to varying levels of initial knowledge and learning speeds. Feedback from children, parents and teachers was positive. A companion article 1 describes the evolution of number sense and arithmetic scores before and after training. CONCLUSION: The software, open-source and freely available online, is designed for learning disabled children aged 5-8, and may also be useful for general instruction of normal preschool children. The learning algorithm reported is highly general, and may be applied in other domains |
| Abstract: | ABSTRACT: BACKGROUND: In a companion article 1, we described the development and evaluation of software designed to remediate dyscalculia. This software is based on the hypothesis that dyscalculia is due to a "core deficit" in number sense or in its access via symbolic information. Here we review the evidence for this hypothesis, and present results from an initial open-trial test of the software in a sample of nine 7-9 year old children with mathematical difficulties. METHODS: Children completed adaptive training on numerical comparison for half an hour a day, four days a week over a period of five-weeks. They were tested before and after intervention on their performance in core numerical tasks: counting, transcoding, base-10 comprehension, enumeration, addition, subtraction, and symbolic and non-symbolic numerical comparison. RESULTS: Children showed specific increases in performance on core number sense tasks. Speed of subitizing and numerical comparison increased by several hundred msec. Subtraction accuracy increased by an average of 23\%. Performance on addition and base-10 comprehension tasks did not improve over the period of the study. CONCLUSION: Initial open-trial testing showed promising results, and suggested that the software was successful in increasing number sense over the short period of the study. However these results need to be followed up with larger, controlled studies. The issues of transfer to higher-level tasks, and of the best developmental time window for intervention also need to be addressed |
| 2005 |
| Abstract: | L'exposition à une langue pendant l'enfance laisse-t-elle des traces indélébiles dans le cerveau ? Est-ce possible qu'une seconde langue (L2) "remplace" une première langue (L1), dans des circonstances particulières ? L'hypothèse de la période critique pour l'acquisition du langage prédit que les aires langagières du cerveau perdent la plasticité avec l'âge. Une conséquence de cette hypothèse est donc que l'exposition à une langue donnée pendant les premières années de vie laissera des traces permanentes dans le cerveau. Ceci implique que la perte complète d'une langue maternelle ne pourrait avoir lieu. L'autre conséquence de la perte de plasticité est la difficulté croissante en fonction de l'âge de l'apprenant. Des études de l'acquisition d'une deuxième langue et des cas de personnes privées de langage pendant les premières années de vie (les "enfants-loups" et parfois les sourds nés dans des familles d' entendants) portent des preuves en faveur de la perte de plasticité. Dans cette thèse nous explorons les traces éventuelles de la L1 apprise et `perdue'pendant l'enfance, et la compétence dans la L2 apprise plus tard dans l'enfance. Nous avons choisi d'étudier des adoptés d'origine étrangère pour examiner ces questions. Notre étude concerne des adultes d'origine coréenne adoptés par des familles francophones entre l'âge de 3 et 10 ans et ayant été complètement isolés de leur langue et culture d'origine depuis leur arrivé en France il y a 15 à 30 ans. Cette thèse comporte trois parties : 1.) la recherche de traces de mémoire linguistique et autres (reconnaissance de séries numériques, de mots, de séries de jours de la semaine, de morphologie faciale) ; 2.) la recherche de traces éventuelles de la phonologie du coréen (discrimination de phonèmes, entra?nement aux sons du coréen) et 3.) l'évaluation de certains aspects du français (genre grammatical , phonotactique). Nous avons comparé les résultats des adoptés à ceux de francophones sans connaissances du coréen dans chaque expérience, et à ceux de Coréens natifs résidant en France pour certaines expériences. Les expériences de mémoire ont révélé l'existence de très peu de traces du coréen et d'autres types de souvenirs. De la même façon, les expériences de phonologie du coréen ont montré un comportement similaire à celui des francophones, et ceci indépendamment de la réexposition de certains adoptés à la langue coréenne lors de séjours de courte durée en Corée. Ces résultats nous suggèrent soit une perte du coréen, soit une inaccessibilité importante à cette langue par les adoptés. Les performances des adoptés sur les tests de français sont également semblables à celles des francophones et diffèrent de celle des Coréens natifs, indiquant une bonne ma?trise des aspects du français difficiles pour des Coréens. L'ensemble des données montre que les adoptés coréens sont devenus comme des francophones natifs dans leur traitement langagier. L'important rôle joué par la plasticité du système langagier chez les adoptés est corroboré par nos résultats, qui suggèrent qu'une langue maternelle peut être facilement remplacée par une autre langue pendant l'enfance. Il y a une convergence entre nos résultats et ceux d'études de cas d'attrition langagière chez des jeunes enfants adoptés montrant que la L1 est rapidement oubliée (Nicoladis \& Grabois, 2002 ; Isurin, 2000), alors que la L2 est vite assimilée |
| Abstract: | Even in the absence of sensory inputs, cortical and thalamic neurons can show structured patterns of ongoing spontaneous activity, whose origins and functional significance are not well understood. We use computer simulations to explore the conditions under which spontaneous activity emerges from a simplified model of multiple interconnected thalamocortical columns linked by long-range, top-down excitatory axons, and to examine its interactions with stimulus-induced activation. Simulations help characterize two main states of activity. First, spontaneous gamma-band oscillations emerge at a precise threshold controlled by ascending neuromodulator systems. Second, within a spontaneously active network, we observe the sudden "ignition" of one out of many possible coherent states of high-level activity amidst cortical neurons with long-distance projections. During such an ignited state, spontaneous activity can block external sensory processing. We relate those properties to experimental observations on the neural bases of endogenous states of consciousness, and particularly the blocking of access to consciousness that occurs in the psychophysical phenomenon of "inattentional blindness," in which normal subjects intensely engaged in mental activity fail to notice salient but irrelevant sensory stimuli. Although highly simplified, the generic properties of a minimal network may help clarify some of the basic cerebral phenomena underlying the autonomy of consciousness |
| Abstract: | How is reading, a cultural invention, coded by neural populations in the human brain? The neural code for written words must be abstract, because we can recognize words regardless of their location, font and size. Yet it must also be exquisitely sensitive to letter identity and letter order. Most existing coding schemes are insufficiently invariant or incompatible with the constraints of the visual system. We propose a tentative neuronal model according to which part of the occipito-temporal 'what' pathway is tuned to writing and forms a hierarchy of local combination detectors sensitive to increasingly larger fragments of words. Our proposal can explain why the detection of 'open bigrams' (ordered pairs of letters) constitutes an important stage in visual word recognition |
| Abstract: | Many people exposed to sinewave analogues of speech first report hearing them as electronic glissando and, later, when they switch into a 'speech mode', hearing them as syllables. This perceptual switch modifies their discrimination abilities, enhancing perception of differences that cross phonemic boundaries while diminishing perception of differences within phonemic categories. Using high-density evoked potentials and fMRI in a discrimination paradigm, we studied the changes in brain activity that are related to this change in perception. With ERPs, we observed that phonemic coding is faster than acoustic coding: The electrophysiological mismatch response (MMR) occurred earlier for a phonemic change than for an equivalent acoustic change. The MMR topography was also more asymmetric for a phonemic change than for an acoustic change. In fMRI, activations were also significantly asymmetric, favoring the left hemisphere in both perception modes. Furthermore, switching to the speech mode significantly enhanced activation in the posterior parts of the left superior gyrus and sulcus relative to the non-speech mode. When responses to a change of stimulus were studied, a cluster of voxels in the supramarginal gyrus was activated significantly more by a phonemic change than by an acoustic change. These results demonstrate that phoneme perception in adults relies on a specific and highly efficient left-hemispheric network, which can be activated in top-down fashion when processing ambiguous speech/non-speech stimuli |
| Abstract: | This paper introduces a method to study the variation of brain functional connectivity networks with respect to experimental conditions in fMRI data. It is related to the psychophysiological interaction technique introduced by Friston et al. and extends to networks of correlation modulation (CM networks). Extended networks containing several dozens of nodes are determined in which the links correspond to consistent correlation modulation across subjects. In addition, we assess inter-subject variability and determine networks in which the condition-dependent functional interactions can be explained by a subject-dependent variable. We applied the technique to data from a study on syntactical production in bilinguals and analysed functional interactions differentially across tasks (word reading or sentence production) and across languages. We find an extended network of consistent functional interaction modulation across tasks, whereas the network comparing languages shows fewer links. Interestingly, there is evidence for a specific network in which the differences in functional interaction across subjects can be explained by differences in the subjects' syntactical proficiency. Specifically, we find that regions, including ones that have previously been shown to be involved in syntax and in language production, such as the left inferior frontal gyrus, putamen, insula, precentral gyrus, as well as the supplementary motor area, are more functionally linked during sentence production in the second, compared with the first, language in syntactically more proficient bilinguals than in syntactically less proficient ones. Our approach extends conventional activation analyses to the notion of networks, emphasizing functional interactions between regions independently of whether or not they are activated. On the one hand, it gives rise to testable hypotheses and allows an interpretation of the results in terms of the previous literature, and on the other hand, it provides a basis for studying the structure of functional interactions as a whole, and hence represents a further step towards the notion of large-scale networks in functional imaging |
| Abstract: | A novel mechanism of MRI contrast enhancement, based on the detection by a balanced steady-state free precession (SSFP) sequence of the proton resonance frequency shift induced by bulk magnetic susceptibility (BMS) contrast agents, was investigated. The potential for this contrast mechanism to image blood vessels was explored. The relaxation time and the frequency shift effects of gadolinium- and dysprosium-DOTA on SSFP signal was first simulated and evaluated on a water phantom at 1.5 T. In vitro, a 5-mM concentration in contrast agent induced a 20-Hz frequency shift, leading to a signal increase of 92\% for Dy-DOTA, and a 10-Hz frequency shift, leading to a signal increase of 58\% for Gd-DOTA at the reference frequency, taking into account the nonlinear SSFP signal response on frequency offset. The concept was then evaluated in vivo on anesthetized rabbits. Low doses of dysprosium-DOTA were injected in their vascular system, and imaging was performed at the level of neck vessels. Following a bolus injection, mean signal changes of 31\%, 20\% and 14\% were observed in the carotid arteries, the vertebral veins and the jugular veins, respectively. The bolus peak times in arteries and veins were consistent with the rabbit vascular circulation. This frequency-shift based contrast mechanism presents interesting potential for contrast-enhanced MR angiography (CE-MRA) compared to usual relaxation-based contrast, but further investigations on reproducibility will be necessary. |
| Abstract: | Most studies on visual perception of human beings have focused on perception of faces. However, bodies are another important visual element, which help us to identify a member of our species in the visual scene. In order to study whether similar configural information processing is used in body and face perception, we recorded high-density even-related potentials (ERPs) to normal and distorted faces and bodies in adults and 3-month-old infants. In adults, the N1 responses evoked by bodies and faces were similar in amplitude but differed slightly in latency. The voltage topography of N1 also differed in concordance with fMRI data showing that two distinct areas are involved in face and body perception. Distortion affected ERPs to faces and bodies similarly from N1 on, although the effect was significant earlier for bodies than for faces. These results suggest that fast processing of configural information is not specific to faces but it also occurs for bodies. In 3-month-old infants, distortion decreased the amplitude of P400 around 450 msec, showing no interaction with image category. This result demonstrates that infants are not only able to recognize the normal configuration of faces, but also that of bodies. This could either be related to an innate knowledge of this particular type of biological object, or to fast learning through intense exposure during the first months of life |
| Abstract: | Lesions affecting the ventral cortex of the left temporal lobe commonly yield a selective reading impairment known as pure alexia. It is thought to result from the disruption or deafferentation of the Visual Word Form Area (VWFA), a region in the left lateral occipitotemporal sulcus activated whenever normal subjects are viewing alphabetic strings. Most pure alexic patients retain the ability to identify single letters, and develop a strategy of letter-by-letter (LBL) reading. We recently studied fMRI activations in LBL readers and clarified the underlying mechanisms. However, LBL reading is a dynamic process which may improve over months or years of practice, although the cerebral bases of this continuing improvement are currently unknown. We had the opportunity to run the same behavioural testing and fMRI experiment a second time in an alexic patient, 8 months after collecting the data reported by Cohen et al. [Cohen, L., Henry, C., Dehaene, S., Molko, N., Lehericy, S., Martinaud, O., Lemer, C., & Ferrieux, S. (2004). The pathophysiology of letter-by-letter reading. Neuropsychologia, 42, 1768-1780]. We analyze the changes that occurred over this period in the pattern of reading-related activations, while the patient's LBL reading improved. The activation level decreased in most of the overall network between the two sessions. This general trend contrasted with a focal increase restricted to specific left frontal and parietal areas. When studying the contrast between words and consonant strings, which may be taken as a correlate of LBL reading, we also found a general decrease, except for similar left frontal and parietal regions, which showed a significant increase. We suggest that the pattern of evolution fits with the minimal hypothesis of normal strategic abilities and skill learning, associated with perceptual tuning in right-hemispheric structures able to substitute the disrupted VWFA |
| Abstract: | Since the time of Pythagoras, numerical and spatial representations have been inextricably linked. We suggest that the relationship between the two is deeply rooted in the brain's organization for these capacities. Many behavioural and patient studies have shown that numerical-spatial interactions run far deeper than simply cultural constructions, and, instead, influence behaviour at several levels. By combining two previously independent lines of research, neuroimaging studies of numerical cognition in humans, and physiological studies of spatial cognition in monkeys, we propose that these numerical-spatial interactions arise from common parietal circuits for attention to external space and internal representations of numbers |
| Abstract: | Recruitment of executive attention is normally associated to a subjective feeling of mental effort. Here we investigate the nature of this coupling in a patient with a left mesio-frontal cortex lesion including the anterior cingulate cortex (ACC), and in a group of comparison subjects using a Stroop paradigm. We show that in normal subjects, subjective increases in effort associated with executive control correlate with higher skin-conductance responses (SCRs). However, our patient experienced no conscious feeling of mental effort and showed no SCR, in spite of exhibiting normal executive control, and residual right anterior cingulate activity measured with event-related potentials (ERPs). Finally, this patient demonstrated a pattern of impaired behavior and SCRs in the Iowa gambling task-elaborated by Damasio, Bechara and colleagues-replicating the findings reported by these authors for other patients with mesio-frontal lesions. Taken together, these results call for a theoretical refinement by revealing a decoupling between conscious cognitive control and consciously reportable feelings. Moreover, they reveal a fundamental distinction, observed here within the same patient, between the cognitive operations which are depending on normal somatic marker processing, and those which are withstanding to impairments of this system |
| Abstract: | A classical but still open issue in cognitive psychology concerns the depth of subliminal processing. Can the meaning of undetected words be accessed in the absence of consciousness? Subliminal priming experiments in normal subjects have revealed only small effects whose interpretation remains controversial. Here, we provide a direct demonstration of semantic access for unseen masked words. In three epileptic patients with intracranial electrodes, we recorded brain potentials from the amygdala, a neural structure that responds to fearful or threatening stimuli presented in various modalities, including written words. We show that the subliminal presentation of emotional words modulates the activity of the amygdala at a long latency (>800 ms). Our result indicates that subliminal words can trigger long-lasting cerebral processes, including semantic access to emotional valence |
| Abstract: | Recent evidence has suggested that the human occipitotemporal region comprises several subregions, each sensitive to a distinct processing level of visual words. To further explore the functional architecture of visual word recognition, we employed a subliminal priming method with functional magnetic resonance imaging (fMRI) during semantic judgments of words presented in two different Japanese scripts, Kanji and Kana. Each target word was preceded by a subliminal presentation of either the same or a different word, and in the same or a different script. Behaviorally, word repetition produced significant priming regardless of whether the words were presented in the same or different script. At the neural level, this cross-script priming was associated with repetition suppression in the left inferior temporal cortex anterior and dorsal to the visual word form area hypothesized for alphabetical writing systems, suggesting that cross-script convergence occurred at a semantic level. fMRI also evidenced a shared visual occipito-temporal activation for words in the two scripts, with slightly more mesial and right-predominant activation for Kanji and with greater occipital activation for Kana. These results thus allow us to separate script-specific and script-independent regions in the posterior temporal lobe, while demonstrating that both can be activated subliminally |
| Abstract: | Abstract-Early experience with faces of a given racial type facilitates visual recognition for this type of face relative to others. To assess whether this so-called other-race effect can be reversed by subsequent experience with new types of faces, we tested adults of Korean origin who were adopted by European Caucasian families when they were between the ages of 3 to 9. The adoptees performed a face recognition task with photographs of Caucasian and Asian faces. They performed exactly like a control group of French participants, identifying the Caucasian faces better than the Asiatic ones. In contrast, a control group of Koreans showed the reverse pattern. This result indicates that the face recognition system remains plastic enough during childhood to reverse the other-race effect |
| Abstract: | In the phenomenon of attentional blink, identical visual stimuli are sometimes fully perceived and sometimes not detected at all. This phenomenon thus provides an optimal situation to study the fate of stimuli not consciously perceived and the differences between conscious and nonconscious processing. We correlated behavioral visibility ratings and recordings of event-related potentials to study the temporal dynamics of access to consciousness. Intact early potentials (P1 and N1) were evoked by unseen words, suggesting that these brain events are not the primary correlates of conscious perception. However, we observed a rapid divergence around 270 ms, after which several brain events were evoked solely by seen words. Thus, we suggest that the transition toward access to consciousness relates to the optional triggering of a late wave of activation that spreads through a distributed network of cortical association areas |
| Abstract: | Parsing a mental operation into components, characterizing the parallel or serial nature of this flow, and understanding what each process ultimately contributes to response time are fundamental questions in cognitive neuroscience. Here we show how a simple theoretical model leads to an extended set of predictions concerning the distribution of response time and its alteration by simultaneous performance of another task. The model provides a synthesis of psychological refractory period and random-walk models of response time. It merely assumes that a task consists of three consecutive stages-perception, decision based on noisy integration of evidence, and response-and that the perceptual and motor stages can operate simultaneously with stages of another task, while the central decision process constitutes a bottleneck. We designed a number-comparison task that provided a thorough test of the model by allowing independent variations in number notation, numerical distance, response complexity, and temporal asynchrony relative to an interfering probe task of tone discrimination. The results revealed a parsing of the comparison task in which each variable affects only one stage. Numerical distance affects the integration process, which is the only step that cannot proceed in parallel and has a major contribution to response time variability. The other stages, mapping the numeral to an internal quantity and executing the motor response, can be carried out in parallel with another task. Changing the duration of these processes has no significant effect on the variance |
| Abstract: | BACKGROUND: Failure to resist chronic obsessive-compulsive symptoms may denote an altered state of cognitive control. We searched for the cerebral regions engaged in this dysfunction. METHOD: Differences in brain regional activity were examined by event-related functional magnetic regional imaging (fMRI) in a group of adolescents or young adults (n = 12) with childhood-onset obsessive-compulsive disorder (OCD), relative to healthy subjects. Subjects performed a conflict task involving the presentation of two consecutive and possibly conflicting prime and target numbers. Patients' image dataset was further analysed according to resistance or non-resistance to symptoms during the scans. RESULTS: Using volume correction based on a priori hypotheses, an exploratory analysis revealed that, within the prime-target repetition condition, the OCD subjects activated more than healthy subjects a subregion of the anterior cingulate gyrus and the left parietal lobe. Furthermore, compared with 'resistant' patients, the 'non-resistant' OCD subjects activated a bilateral network including the precuneus, pulvinar and paracentral lobules. CONCLUSIONS: Higher regional activations suggest an abnormal amplification process in OCD subjects during the discrimination of repetitive visual stimuli. The regional distribution of functional changes may vary with the patients' ability to resist obsessions |
| 2004 |
| Abstract: | Understanding of the functional neurobiology of the rodent whisker system would be advanced by neurobehavioral studies in awake, behaving animals that combine unit recording from structures at various levels of the system with quantitative characterization of the kinematics and temporal organization of whisking. Such studies require the solution of a number of methodological problems. These include: chronic recording procedures ensuring unit isolation, stability and maximum yield, monitoring and display of unit activity and whisker movements within the same (ms) timeframe and behavioral paradigms which bring whisking movement parameters under the control of the experimenter rather than the rat. Here we describe a head-fixed rodent preparation which makes possible chronic recording of unit activity in the awake, whisking rat, combined with real-time, high resolution monitoring of whisker and pad movements in two dimensions and under behavioral control. While the head-fixed "whisking" preparation has some inherent limitations, it may be used to address a number of important neurobehavioral problems. We suggest that it should contribute significantly to understanding the functional neurobiology of the whisker system. |
| Abstract: | Several lines of evidence suggest the importance of phonological working memory (PWM) in language acquisition. We investigated the neural correlates of PWM in young adults who were under compelling social pressure to be bilingual. Equal bilinguals had high proficiency in English and Chinese as measured by a standardized examination, whereas unequal bilinguals were proficient in English but not Chinese. Both groups were matched on several measures of nonverbal intelligence and working memory. In-scanner behavioral results did not show between-group differences. Of the regions showing load-dependent increments in activation, the left insula showed greater activation in equal bilinguals. Unequal bilinguals showed greater task-related deactivation in the anterior medial frontal region and greater anterior cingulate activation. Although unequal bilinguals kept apace with equal bilinguals in the simple PWM task, the differential cortical activations suggest that more optimal engagement of PWM in the latter may correlate with better second-language attainment |
| Abstract: | Using anatomical and functional magnetic resonance imaging, we studied the pattern of brain lateralization during spoken and written language tasks, in an 11-year-old girl who underwent a left occipitotemporal resection for a Sturge-Weber angioma at the age of 4 years, that is, after the development of speech but before the acquisition of reading. We observed a selective and successful shift to the right hemisphere of the visual component of reading, particularly the Visual Word Form Area, whereas the verbal components remained strongly left-lateralized. This emphasizes the potential utility of a precise functional and developmental cartography of language for the surgical treatment of focal brain lesions in children. Ann Neurol 2004;56:890-894. |
| Abstract: | Fluent readers recognize visual words across changes in case and retinal location, while maintaining a high sensitivity to the arrangement of letters. To evaluate the automaticity and functional anatomy of invariant word recognition, we measured brain activity during subliminal masked priming. By preceding target words with an unrelated prime, a repeated prime, or an anagram made of the same letters, we separated letter-level and whole-word codes. By changing the case and the retinal location of primes and targets, we evaluated the invariance of those codes. Our results indicate that an invariant binding of letters into words is achieved unconsciously through a series of increasingly invariant stages in the left occipito-temporal pathway. |
| Abstract: | What representations underlie the ability to think and reason about number? Whereas certain numerical concepts, such as the real numbers, are only ever represented by a subset of human adults, other numerical abilities are widespread and can be observed in adults, infants and other animal species. We review recent behavioral and neuropsychological evidence that these ontogenetically and phylogenetically shared abilities rest on two core systems for representing number. Performance signatures common across development and across species implicate one system for representing large, approximate numerical magnitudes, and a second system for the precise representation of small numbers of individual objects. These systems account for our basic numerical intuitions, and serve as the foundation for the more sophisticated numerical concepts that are uniquely human |
| Abstract: | In this study, we used event-related functional magnetic resonance imaging to investigate whether visual mental images retinotopically activate early visual cortex. Six participants were instructed to visualize or view horizontally or vertically oriented flashing bow-tie shaped stimuli. When compared to baseline, imagery globally activated Area V1. When the activation evoked by the stimuli at the different orientations was directly compared, distinct spatial activation patterns were obtained for each orientation in most participants. Not only was the topography of the activation patterns from imagery similar to the topography obtained with a corresponding visual perception task, but it closely matched the individual cortical representation of either the horizontal or the vertical visual field meridians. These findings strongly support that visual imagery and perception share low-level anatomical substrate and functional processes. Binding of spatial features is suggested as one possible mechanism. |
| Abstract: | Analysis of brain structure in Turner syndrome (TS) provides the opportunity to identify the consequences of the loss of one X chromosome on brain anatomy and to characterize the neural bases underlying the specific cognitive profile of TS subjects which includes deficits in spatial-numerical processing and social cognition. Fourteen subjects with TS and fourteen controls were investigated using voxel-based analysis of high resolution anatomical and diffusion tensor images and using sulcal morphometry. The analysis of anatomical images provided evidence for macroscopical changes in cortical regions involved in social cognition such as the left superior temporal sulcus and orbito-frontal cortex and in a region involved in spatial and numerical cognition such as the right intraparietal sulcus. Diffusion tensor images showed a displacement of the grey-white matter interface of the left and right superior temporal sulcus and revealed bilateral microstructural anomalies in the temporal white matter. The analysis of fiber orientation suggests specific alterations of fiber tracts connecting posterior to anterior temporal regions. Last, sulcal morphometry confirmed the anomalies of the left and right superior temporal sulci and of the right intraparietal sulcus. Our results thus provide converging evidence of regionally specific structural changes in TS that are highly consistent with the hallmark symptoms associated with TS |
| Abstract: | In this article, we present a new lexical database for French: Lexique. In addition to classical word information such as gender, number, and grammatical category, Lexique includes a series of interesting new characteristics. First, word frequencies are based on two cues: a contemporary corpus of texts and the number of Web pages containing the word. Second, the database is split into a graphemic table with all the relevant frequencies, a table structured around lemmas (particularly interesting for the study of the inflectional family), and a table about surface frequency cues. Third, Lexique is distributed under a GNU-like license, allowing people to contribute to it. Finally, a metasearch engine, Open Lexique, has been developed so that new databases can be added very easily to the existing ones. Lexique can either be downloaded or interrogated freely from http://www.lexique.org |
| Abstract: |
Number, like color or movement, is a basic property of the environment. Recently, single neurons tuned to number have been observed in animals. We used both psychophysics and neuroimaging to examine whether a similar neural coding scheme is present in humans. When participants viewed sets of items with a variable number, the bilateral intraparietal sulci responded selectively to number change. Functionally, the shape of this response indicates that humans, like other animal species, encode approximate number on a compressed internal scale. Anatomically, the intraparietal site coding for number in humans is compatible with that observed in macaque monkeys. Our results therefore suggest an evolutionary basis for human elementary arithmetic Check this document for detailed explantations about the creation of the stimuli. |
| Abstract: | Is calculation possible without language? Or is the human ability for arithmetic dependent on the language faculty? To clarify the relation between language and arithmetic, we studied numerical cognition in speakers of Munduruku, an Amazonian language with a very small lexicon of number words. Although the Munduruku lack words for numbers beyond 5, they are able to compare and add large approximate numbers that are far beyond their naming range. However, they fail in exact arithmetic with numbers larger than 4 or 5. Our results imply a distinction between a nonverbal system of number approximation and a language-based counting system for exact number and arithmetic |
| Abstract: | How are comparative judgments performed in the human brain? We scanned subjects with fMRI while they compared stimuli for size, luminance, or number. Regions involved in comparative judgments were identified using three criteria: task-related activation, presence of a distance effect, and interference of one dimension onto the other. We observed considerable overlap in the neural substrates of the three comparison tasks. Interestingly, the amount of overlap predicted the amount of cross-dimensional interference: in both behavior and fMRI, number interfered with size, and size with luminance, but number did not interfere with luminance. The results suggest that during comparative judgments, the relevant continuous quantities are represented in distributed and overlapping neural populations, with number and size engaging a common parietal spatial code, while size and luminance engage shared occipito-temporal perceptual representations |
| Abstract: | One striking property of perception is that it can be achieved in two seemingly different ways: either consciously or non-consciously. What distinguishes these two types of processing at the neural level? So far, empirical findings suggest that conscious perception is associated with an increase in activity at the sensory level, the specific involvement of a fronto-parietal network and an increase in long-distance functional connectivity and synchrony within a broad network of areas. We interpret these data in the framework of the global neuronal workspace model which proposes that the neural basis of conscious access is a sudden self-amplifying process leading to a global brain-scale pattern of activity. In contradiction with several theories which assume that there is a continuum of perception, associated with a gradual change in the intensity of brain activation, the model predicts a sharp non-linear transition between non-conscious and conscious processing. |
| Abstract: | Human functional MRI studies frequently reveal the joint activation of parietal and of lateral and mesial frontal areas during various cognitive tasks. To analyze the geometrical organization of those networks, we used an automatized clustering algorithm that parcels out sets of areas based on their similar profile of task-related activations or deactivations. This algorithm allowed us to reanalyze published fMRI data (Simon, O., Mangin, J.F., Cohen, L., Le Bihan, D., Dehaene, S., 2002. Topographical layout of hand, eye, calculation, and language-related areas in the human parietal lobe. Neuron 33, 475-487) and to reproduce the previously observed geometrical organization of activations for saccades, attention, grasping, pointing, calculation, and language processing in the parietal lobe. Further, we show that this organization extends to lateral and mesial prefrontal regions. Relative to the parietal lobe, the prefrontal functional geometry is characterized by a partially symmetrical anteroposterior ordering of activations, a decreased representation of effector-specific tasks, and a greater emphasis on higher cognitive functions of attention, higher-order spatial representation, calculation, and language. Anatomically, our results in humans are closely homologous to the known connectivity of parietal and frontal regions in the macaque monkey |
| Abstract: | Does early exposure to a language leave permanent traces in the brain? We examine this issue by testing a group of native Koreans who were adopted by French-speaking families and have stopped using their ?rst language for many years. Previous results suggest that they are not able to recognize Korean sentences, nor to identify Korean words (Pallier et al. 2003). In the present study, we focus on the possible remnants of L1 phonology, by assessing the adoptees? capacity to discriminate Korean voiceless consonants which are dif?cult to perceive by native French speakers. Data from groups of adoptees, native speakers of French, and native speakers of Korean, show that the adoptees do not perceive the differences between Korean phonemes better than native French speakers previously unexposed to Korean. Also, adoptees having been reexposed to Korean and those without reexposure perform similarly on this task. These results demonstrate that the Korean adoptees do not have easy access to the phonetic categories of the Korean language |
| 2003 |
| Abstract: | Languages differ depending on the set of basic sounds they use (the inventory of consonants and vowels) and on the way in which these sounds can be combined to make up words and phrases (phonological grammar). Previous research has shown that our inventory of consonants and vowels affects the way in which our brains decode foreign sounds (Goto, 1971; Näätänen et al., 1997; Kuhl, 2000). Here, we show that phonological grammar has an equally potent effect. We build on previous research, which shows that stimuli that are phonologically ungrammatical are assimilated to the closest grammatical form in the language (Dupoux et al., 1999). In a cross-linguistic design using French and Japanese participants and a fast event-related functional magnetic resonance imaging (fMRI) paradigm, we show that phonological grammar involves the left superior temporal and the left anterior supramarginal gyri, two regions previously associated with the processing of human vocal sounds |
| Abstract: | Theories of perception have proposed a basic distinction between parallel pre-attentive and serial attentive modes of processing. However, chronometric measures are often ambiguous in separating parallel and serial processes. We have used the activity of attention-related regions of the human brain, measured with functional magnetic resonance imaging, to separate parallel from serial processes at the single-trial level in a visual quantification task. In this task, some have suggested the deployment of two qualitatively different processes, a fast parallel 'subitizing' for sets of one, two or three objects and a slow serial counting for larger sets. Our results indicate that attention-related regions of the posterior parietal and frontal cortices show a sudden increase in activity only from numerosity four onwards, confirming the parallel-serial dichotomy of subitizing and counting. Moreover, using the presence or absence of attentional shifts, as inferred from the activation of posterior parietal regions, we successfully predict whether, on a given trial, subjects deployed a serial exploration of the display or a parallel apprehension. Beyond the subitizing/counting debate, this approach may prove useful to probe the attentional demands of other cognitive tasks |
| Abstract: | Mammals acquire much of their sensory information by actively moving their sensory organs. Yet, the principles of encoding by active sensing are not known. Here we investigated the encoding principles of active touch by rat whiskers (vibrissae). We induced artificial whisking in anesthetized rats and recorded from first-order neurons in the trigeminal ganglion. During active touch, first-order trigeminal neurons presented a rich repertoire of responses, which could not be inferred from their responses to passive deflection stimuli. Individual neurons encoded four specific events: whisking, contact with object, pressure against object, and detachment from object. Whisking-responsive neurons fired at specific deflection angles, reporting the actual whiskers' position with high precision. Touch-responsive neurons encoded the horizontal coordinate of objects' position by spike timing. These findings suggest two specific encoding-decoding schemes for horizontal object position in the vibrissal system. |
| 2002 |
| Abstract: | We present a general method-denoted MoDef-to help specify (or define) the model used to analyze brain imaging data. This method is based on the use of the multivariate linear model on a training data set. We show that when the a priori knowledge about the expected brain response is not too precise, the method allows for the specification of a model that yields a better sensitivity in the statistical results. This obviously relies on the validity of the a priori information, in our case the representativity of the training set, an issue addressed using a cross-validation technique. We propose a fast implementation that allows the use of the method on large data sets as found with functional Magnetic Resonance Images. An example of application is given on an experimental fMRI data set that includes nine subjects who performed a mental computation task. Results show that the method increases the statistical sensitivity of fMRI analyses. |
| Abstract: | We present a general method-denoted MoDef-to help specify (or define) the model used to analyze brain imaging data. This method is based on the use of the multivariate linear model on a training data set. We show that when the a priori knowledge about the expected brain response is not too precise, the method allows for the specification of a model that yields a better sensitivity in the statistical results. This obviously relies on the validity of the a priori information, in our case the representativity of the training set, an issue addressed using a cross-validation technique. We propose a fast implementation that allows the use of the method on large data sets as found with functional Magnetic Resonance Images. An example of application is given on an experimental fMRI data set that includes nine subjects who performed a mental computation task. Results show that the method increases the statistical sensitivity of fMRI analyses |
| Abstract: | The lexicon consists of a set of word meanings and their semantic relationships. A systematic representation of the English lexicon based in psycholinguistic considerations has been put together in the database Wordnet in a long-term collaborative effort. We present here a quantitative study of the graph structure of Wordnet to understand the global organization of the lexicon. Semantic links follow power-law, scale-invariant behaviors typical of self-organizing networks. Polysemy (the ambiguity of an individual word) is one of the links in the semantic network, relating the different meanings of a common word. Polysemous links have a profound impact in the organization of the semantic graph, conforming it as a small world network, with clusters of high traffic (hubs) representing abstract concepts such as line, head, or circle. Our results show that: (i) Wordnet has global properties common to many self-organized systems, and (ii) polysemy organizes the semantic graph in a compact and categorical representation, in a way that may explain the ubiquity of polysemy across languages |
| 2001 |
| Abstract: | Perceptual learning is a lifelong process. We begin by encoding information about the basic structure of the natural world and continue to assimilate information about specific patterns with which we become familiar. The specificity of the learning suggests that all areas of the cerebral cortex are plastic and can represent various aspects of learned information. The neural substrate of perceptual learning relates to the nature of the neural code itself, including changes in cortical maps, in the temporal characteristics of neuronal responses, and in modulation of contextual influences. Top-down control of these representations suggests that learning involves an interaction between multiple cortical areas |
| Abstract: | To understand how the human visual system analyzes images, it is essential to know the structure of the visual environment. In particular, natural images display consistent statistical properties that distinguish them from random luminance distributions. We have studied the geometric regularities of oriented elements (edges or line segments) present in an ensemble of visual scenes, asking how much information the presence of a segment in a particular location of the visual scene carries about the presence of a second segment at different relative positions and orientations. We observed strong long-range correlations in the distribution of oriented segments that extend over the whole visual field. We further show that a very simple geometric rule, cocircularity, predicts the arrangement of segments in natural scenes, and that different geometrical arrangements show relevant differences in their scaling properties. Our results show similarities to geometric features of previous physiological and psychophysical studies. We discuss the implications of these findings for theories of early vision |
| 2000 |
| Abstract: | Neuronal responses are conspicuously variable. We focus on one particular aspect of that variability: the precision of action potential timing. We show that for common models of noisy spike generation, elementary considerations imply that such variability is a function of the input, and can be made arbitrarily large or small by a suitable choice of inputs. Our considerations are expected to extend to virtually any mechanism of spike generation, and we illustrate them with data from the visual pathway. Thus, a simplification usually made in the application of information theory to neural processing is violated: noise is not independent of the message. However, we also show the existence of error-correcting topologies, which can achieve better timing reliability than their components |
| Abstract: | Neuronal network models have been proposed for the organization of evaluation and decision processes in prefrontal circuitry and their putative neuronal and molecular bases. The models all include an implementation and simulation of an elementary reward mechanism. Their central hypothesis is that tentative rules of behavior, which are coded by clusters of active neurons in prefrontal cortex, are selected or rejected based on an evaluation by this reward signal, which may be conveyed, for instance, by the mesencephalic dopaminergic neurons with which the prefrontal cortex is densely interconnected. At the molecular level, the reward signal is postulated to be a neurotransmitter such as dopamine, which exerts a global modulatory action on prefrontal synaptic efficacies, either via volume transmission or via targeted synaptic triads. Negative reinforcement has the effect of destabilizing the currently active rule-coding clusters; subsequently, spontaneous activity varies again from one cluster to another, giving the organism the chance to discover and learn a new rule. Thus, reward signals function as effective selection signals that either maintain or suppress currently active prefrontal representations as a function of their current adequacy. Simulations of this variation-selection have successfully accounted for the main features of several major tasks that depend on prefrontal cortex integrity, such as the delayed-response test, the Wisconsin card sorting test, the Tower of London test and the Stroop test. For the more complex tasks, we have found it necessary to supplement the external reward input with a second mechanism that supplies an internal reward; it consists of an auto-evaluation loop which short-circuits the reward input from the exterior. This allows for an internal evaluation of covert motor intentions without actualizing them as behaviors, by simply testing them covertly by comparison with memorized former experiences. This element of architecture gives access to enhanced rates of learning via an elementary process of internal or covert mental simulation. We have recently applied these ideas to a new model, developed with M. Kerszberg, which hypothesizes that prefrontal cortex and its reward-related connections contribute crucially to conscious effortful tasks. This model distinguishes two main computational spaces within the human brain: a unique global workspace composed of distributed and heavily interconnected neurons with long-range axons, and a set of specialized and modular perceptual, motor, memory, evaluative and attentional processors. We postulate that workspace neurons are mobilized in effortful tasks for which the specialized processors do not suffice; they selectively mobilize or suppress, through descending connections, the contribution of specific processor neurons. In the course of task performance, workspace neurons become spontaneously co-activated, forming discrete though variable spatio-temporal patterns subject to modulation by vigilance signals and to selection by reward signals. A computer simulation of the Stroop task shows workspace activation to increase during acquisition of a novel task, effortful execution, and after errors. This model makes predictions concerning the spatio-temporal activation patterns during brain imaging of cognitive tasks, particularly concerning the conditions of activation of dorsolateral prefrontal cortex and anterior cingulate, their relation to reward mechanisms, and their specific reaction during error processing |
| Abstract: | This study was designed to test the Fundamental Difference Hypoth- esis (Bley-Vroman, 1988), which states that, whereas children are known to learn language almost completely through (implicit) do- main-specific mechanisms, adults have largely lost the ability to learn a language without reflecting on its structure and have to use alternative mechanisms, drawing especially on their problem-solving capacities, to learn a second language. The hypothesis implies that only adults with a high level of verbal analytical ability will reach near-native competence in their second language, but that this ability will not be a significant predictor of success for childhood second language acquisition. A study with 57 adult Hungarian-speaking immigrants confirmed the hypothesis in the sense that very few adult immigrants scored within the range of child arrivals on a grammaticality judgment test, and that the few who did had high levels of verbal analytical ability; this ability was not a significant predictor for childhood arrivals. This study replicates the findings of Johnson and Newport (1989) and provides an explanation for the apparent exceptions in their study. These findings lead to a reconceptualization of the Critical Period Hypothesis: If the scope of this hypothesis is limited to implicit learning mechanisms, then it appears that there may be no exceptions to the age effects that the hypothesis seeks to explain |
| Abstract: | We studied the transition of stimuli from novel to familiar in visual search and in the guidance of attention to a particular object. Ability to identify an object improved dramatically over several days of training. The learning was specific for the object's position in the visual field, orientation and configuration. Improvement was initially localized to one or two positions near the fixation spot and then expanded radially to include the full area of the stimulus array. Characteristics of this learning process may reflect a shift in the cortical representation of complex features toward earlier stages in the visual pathway |
| Abstract: | Progression of inflammatory processes correlates with the release of cell-derived mediators from the local site of inflammation. These mediators, including cytokines of the IL-1 and IL-6 families, act on host cells and exert their action by activating their signal transduction pathways leading to specific target gene activation. Parthenolide, a sesquiterpene lactone found in many medical plants, is an inhibitor of IL-1-type cytokine signaling that blocks the activation of NF-kappaB. Here we show that parthenolide is also an effective inhibitor of IL-6-type cytokines. It inhibits IL-6-type cytokine-induced gene expression by blocking STAT3 phosphorylation on Tyr705. This prevents STAT3 dimerization necessary for its nuclear translocation and consequently STAT3-dependent gene expression. This is a new molecular mechanism of parthenolide action that additionally explains its anti-inflammatory activities. |
| 1999 |
| 1998 |
| 1997 |
| Abstract: | It is well attested that we perceive speech through the filter of our native language: a classic example is that of Japanese listeners who cannot discriminate between the American /l/ and /r/ and identify both as their own /r/ phoneme (Goto. H., 1971. Neuropsychologia 9, 317-323.). Studies in the laboratory have shown, however, that perception of non-native speech sounds can be learned through training (Lively, S.E., Pisoni, D.B., Yamada, R.A., Tohkura, Y.I., Yamada, T., 1994. Journal of the Acoustical Society of America 96 (4), 2076-2087). This is consistent with neurophysiological evidence showing considerable experience-dependent plasticity in the brain at the first levels of sensory processing (Edeline, J.-M., Weinberger, N.M., 1993. Behavioral Neuroscience 107, 82-103; Merzenich, M.M., Sameshima, K., 1993. Current Opinion in Neurobiology 3, 187-196; Weinberger, N.M., 1993. Current Opinion in Neurobiology 3, 577-579; Kraus, N., McGee, T., Carrel, T.D., King, C., Tremblay, K., Nicol, T., 1995. Journal of Cognitive Neuroscience 7 (1), 25-32). Outside of the laboratory, however, the situation seems to differ: we here report is study involving Spanish-Catalan bilingual subjects who have had the best opportunities to learn a new contrast but did not do it. Our study demonstrates a striking lack of behavioral plasticity: early and extensive exposure to a second language is not sufficient to attain the ultimate phonological competence of native speakers |
| Abstract: | EXPE is a DOS program for the design and running of experiments that involve the presentation of audio or visual stimuli and the collection of on-line or off-line behavioral responses. Its flexibility makes it also a very useful tool for the rapid design of protocols for testing neuropsychological patients. EXPE provides a powerful scripting language which allows the user to specify all the components of an experiment in a human readable file. Subjects? responses are saved in a user-specified format, also in readable ASCII files. A remarkable feature of EXPE is that the user can easily add new commands to the language: all the instructions are calls to functions written in independent Borland Pascal units. Thus, users can link their own pascal procedures to EXPE to meet any special need. This makes it possible, for example, to adapt EXPE to new hardware, such as new sound or video boards |
| 1996 |
| 1995 |
| Abstract: | Quels sont les mécanismes cérébraux qui permettent l'accès à la conscience ? Cette interrogation naît d'une découverte étonnante : notre cerveau est capable d'extraire une grande quantité d'information sur des stimuli extérieurs, même lorsque nous sommes incapables de rapporter explicitement leur présence. La perception pourrait donc fonctionner selon deux modes : conscient et non-conscient. Dans ce travail de thèse, nous avons tenté de préciser les relations entre ces deux modes de traitement perceptif en utilisant le protocole de clignement attentionnel (attentional blink), dans lequel on observe classiquement un déficit d'identification ou même de détection d'une cible visuelle qui est pourtant présentée au centre de notre champ visuel et pour des durées relativement longues. Dans une série d'expériences comportementales, nous avons demandé à des observateurs d'évaluer la visibilité d'une cible visuelle (un mot) sur une échelle continue. Bien que cette échelle offre la possibilité de rapporter des changements fins et graduels dans la perception de la cible, nous avons observé, dans la condition de clignement attentionnel, une dissociation entre deux types d'essais : pour des stimulations identiques, dans certains essais la cible était aussi bien perçue que lorsqu'elle était présentée en dehors du clignement attentionnel, tandis que dans les autres essais, les sujets utilisaient l'échelle comme si aucune cible n'avait été présentée. Ceci suggère que le traitement non-conscient ne correspond pas à une simple dégradation des même processus qui sont impliqués dans le traitement conscient. Ces résultats semblent plutôt indiquer l'existence d'une dynamique discontinue conduisant à l'accès à la conscience. Une étude d'électro-encéphalographie utilisant le même protocole nous a permis de préciser la dynamique temporelle des processus cérébraux sous-tendant la dissociation perceptive observée en comportement. Nous avons comparé, à stimulation identique, toute la séquence de potentiels cérébraux évoqués par le mot cible, selon qu'il était consciemment perçu ou non. Le traitement visuel précoce (P1, N1) était totalement préservé pour les cibles non-conscientes. Cependant, à la suite d'une période de transition rapide (entre 200 et 300 ms après la présentation de la cible), les cibles conscientes évoquaient une série de potentiels tardifs (N3, P3a, P3b), totalement absents pour des cibles non-conscientes. Nous proposons que l'accès à la conscience repose sur la mise en jeu optionnelle d'un réseau d'aires parieto-fronto-cingulaires permettant d'amplifier et de maintenir les représentations sensorielles et conceptuelles extraites du stimulus, et de mettre en communication des aires cérébrales éloignées. Ces mécanismes neurophysiologiques pourraient rendre compte des possibilités offertes par le traitement conscient qui se manifestent en comportement, depuis la possibilité de rapporter le stimulus jusqu'à l'utilisation stratégique des informations extraites de ce stimulus. |
| 1994 |
| Abstract: | Recent advances in the field of speech processing indicate that speakers of differing languages process speech relying on units that are appropriate to the rhythmical properties of their maternal tongue. Studies with young infants suggest that the acquisition of these processing routines takes place before the end of the first year of life. Further evidence shows that the left hemisphere initially processes any language and gradually becomes specialized for the maternal language |
| 1993 |
| Abstract: | The development of a lexicon critically depends on the infant's ability to identify wordlike units in the auditory speech input. The present study investigated at what age infants become sensitive to language-specific phonotactic features that signal word boundaries and to what extent they are able to use this knowledge to segment speech input. Experiment 1 showed that infants at the age of 9 months were sensitive to the phonotactic structure of word boundaries when word-like units were presented in isolation. Experiments 2 to 5 demonstrated that this sensitivity was present even when critical items were presented in context, although only under certain conditions. Preferences for legal over illegal word boundary clusters were found when critical items were embedded in two identical syllables, keeping language processing requirements and attentional requirements low. Experiment 6 replicated the findings of Experiment 1. Experiment 7 was a low-pass-filtered version of Experiment 6 that left the prosody of the stimulus items intact while removing most of the distinctive phonotactic cues. As expected, no listening preference for legal over illegal word boundary clusters was found in this experiment. This clearly suggests that the preferential patterns observed can be attributed to the infants' sensitivity to phonotactic constraints on word boundaries in a given language and not to suprasegmental cues. |
| 1992 |
| 1991 |
| 1990 |
| 1989 |
| 1988 |
| 1987 |
| Abstract: | A model for formal neural networks that learn temporal sequences by selection is proposed on the basis of observations on the acquisition of song by birds, on sequence-detecting neurons, and on allosteric receptors. The model relies on hypothetical elementary devices made up of three neurons, the synaptic triads, which yield short-term modification of synaptic efficacy through heterosynaptic interactions, and on a local Hebbian learning rule. The functional units postulated are mutually inhibiting clusters of synergic neurons and bundles of synapses. Networks formalized on this basis display capacities for passive recognition and for production of temporal sequences that may include repetitions. Introduction of the learning rule leads to the differentiation of sequence-detecting neurons and to the stabilization of ongoing temporal sequences. A network architecture composed of three layers of neuronal clusters is shown to exhibit active recognition and learning of time sequences by selection: the network spontaneously produces prerepresentations that are selected according to their resonance with the input percepts. Predictions of the model are discussed |
| 1986 |
| Abstract: | 4-day-old infants from French-speaking families were examined for their ability to discriminate French from Russian sentences. The infants gave evidence of discrimination as well as preference for French |
| no year |
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