Rapporter 'j'ai vu le stimulus' est la marque de la vision consciente et implique deux caractéristiques fondamentales de l'expérience consciente, à savoir son aspect qualitatif et la subjectivité. L'aspect qualitatif correspond à la perception vive du stimulus, alors que la subjectivité correspond à la conscience implicite que j'ai fait l'expérience du stimulus. Afin de déterminer les bases neurales de la subjectivité, nous introduisons le concept de cadre neural subjectif ('neural subjective frame') qui correspond aux mécanismes biologiques définissant le sujet en tant qu'une entité biologique, un point d'ancrage à partir duquel des expressions relatives à l'expérience consciente à la première personne peuvent être formulées. De plus, je propose que la représentation neurale de l'information viscérale pourrait constituer ce cadre neural subjectif. Afin de tester expérimentalement cette hypothèse, à l'aide de la magnétoencéphalographie, nous avons enregistré les réponses neuronales aux battements cardiaques, pendant que les participants réalisaient une tâche de détection visuelle. Nous avons trouvé que les réponses neuronales aux battements cardiaques avant la présentation du stimulus, dans le cortex cingulaire ventral antérieur et le lobule postérieur intrapariétal droit, pouvaient prédire la détection d'un stimulus visuel de faible contraste. Les réponses neuronales aux battements cardiaques de plus forte amplitude s'accompagnaient d'un taux de détection et d'une sensibilité plus importants, sans changement du critère de décision. Ni les fluctuations des paramètres corporels ni l'excitabilité corticale générale ne semblent contribuer à ces résultats. Par ailleurs, le fait de voir consciemment le stimulus a ralenti le battement cardiaque après la réponse des participants et cet effet de décélération cardiaque pouvait être prédit par la réponse neuronale aux battements cardiaques du cortex cingulaire antérieur ventral, avant la présentation du stimulus. Ainsi, nos résultats sont en faveur de l’hypothèse selon laquelle la cartographie des afférents viscéraux façonne notre expérience subjective perceptive. Au-delà de la vision consciente, ces résultats suggèrent que les signaux internes du corps et leurs représentations neuronales peuvent être la source de fluctuations dans des aires corticales multi-fonctionnelles. / Reporting “I saw the stimulus” is the hallmark of conscious vision and implies two fundamental characteristics of conscious experience, namely qualitativeness and subjectivity. Qualitativeness refers to the vivid feeling of the stimulus, whereas subjectivity refers to the implicit awareness that the experience occurred for me. To account for the neural basis of subjectivity, we introduce a concept termed the neural subjective frame which corresponds to the basic biological mechanisms defining the subject as a biological entity, as an anchoring point from which the first-person statements of conscious experience can be expressed. I further propose that neural representation of visceral information could constitute the neural subjective frame. To experimentally test this proposal, using magnetoencephalography, we recorded neural events locked to heartbeats while participants conducted visual detection task. We found that neural responses to heartbeats before stimulus onset in ventral anterior cingulate and right posterior intraparietal lobule could predict the detection of faint visual stimulus. Larger amplitude of neural responses to heartbeats were accompanied by enhanced hit-rate and sensitivity, but without changes in decision criterion. Neither fluctuations in measured bodily parameters nor in overall cortical excitability could account for this finding. In addition, consciously seeing the stimulus decelerated heartbeat after participants responded and the heartbeat slowing effect could be predicted from the prestimulus neural responses to heartbeats in ventral anterior cingulate cortex. Our findings therefore support the hypothesis that neural mapping of visceral afferents shape perceptual subjective experience. Beyond conscious vision, our findings suggest that signals from internal body and their neural representations could be sources of fluctuations in multi-functional cortical areas.

La perception explicite du temps écoulé (la durée, l'ordre temporel¿) et les jugements implicites des dynamiques de notre environnement (percevoir le mouvement, la parole) nécessitent l'extraction des relations temporelles entre événements sensoriels. Alors que le temps physique est communément évalué en rapport à un référentiel externe (celui de l'horloge), le cerveau lui n'a pas accès à ce référentiel. Dans cette thèse, nous émettons l'hypothèse que le cerveau génère son propre référentiel temporel à partir des dynamiques neurales. Combinant la magnétoencéphalographie (MEG) aux données psychophysiques, les présents travaux suggèrent que les oscillations corticales sont impliquées dans l'encodage du temps perçu. Une première étude montre que la phase des oscillations corticales basse-fréquences peut encoder l'ordre temporel perçu entre événements sensoriels s'il y a entrainement neural, i.e. si l'activité cérébrale suit les régularités temporelles de la stimulation. L'implication des oscillations cérébrales en l'absence d'entrainement est testée dans une seconde expérience. Les résultats d'une troisième expérience suggèrent que l'entrainement neural n'a d'influence sur le traitement temporel des informations multisensorielles qu'à basse fréquence (1-2 Hz). Un dernier chapitre aborde le rôle de l'entrainement neural dans l'encodage des dynamiques du signal acoustique pour la perception de la parole. En conclusion, cette thèse suggère que le cerveau est capable de suivre la structure temporelle du monde extérieur, et que cet ajustement permet la construction d'un référentiel temporel interne pour la perception explicite et implicite du temps. / The timing of sensory events is a crucial perceptual feature, which affects both explicit judgments of time (e.g. duration, temporal order) and implicit temporal perception (e.g. movement, speech). Yet, while the relative external timing between events is commonly evaluated with a clock in physics, the brain does not have access to this external reference. In this dissertation, we tested the hypothesis that the brain should recover the temporal information of the environment from its own dynamics. Using magnetoencephalography (MEG) combined with psychophysics, the experimental work suggests the involvement of cortical oscillations in the encoding of timing for perception. In the first part of this dissertation, we established that the phase of low-frequency cortical oscillations could encode the explicit timing of events in the context of entrainment, i.e. if neural activity follows the temporal regularities of the stimulation. The implications of brain oscillations for the encoding of timing in the absence of external temporal regularities were investigated in a second experiment. Results from a third experiment suggest that entrainment does only influence audiovisual temporal processing when bound to low-frequency dynamics in the delta range (1-2 Hz). In the last part of the dissertation, we tested whether oscillations in sensory cortex could also ‘tag’ the timing of acoustical features for speech perception. Overall, this thesis provides evidence that the brain is able to tune its timing to match the temporal structure of the environment, and that such tuning may be crucial to build up internal temporal reference frames for explicit and implicit timing perception.

La cécité précoce induit des modifications majeures dans l’architecture fonctionnelle du cerveau. Les lobes occipitaux ne traitent plus l’information visuelle mais vont désormais traiter les signaux auditifs et tactiles et participer à des fonctions cognitives telles que le langage et la mémoire. Cette nouvelle organisation fonctionnelle nous permet de mieux comprendre l’influence de l’expérience sensorielle sur le développement cérébral. Nous avons étudié cette réorganisation et certains de ses possibles déterminants. Tout d’abord, nous avons utilisé l’imagerie par résonnance magnétique fonctionnelle (IRMf) pour identifier les régions du cortex visuel activées par différentes fonctions cognitives. Nous avons également montré que chacune de ces régions est fonctionnellement connectée au réseau cérébral qui sous-tend la fonction correspondante chez les sujets sains. Puis, nous avons réalisé une étude de la connectivité fonctionnelle chez des nouveau-nés, qui suggère que la connectivité innée du cortex visuel guide la réorganisation observée chez les aveugles. Ensuite, grâce à la magnétoencéphalographie (MEG), nous avons étudié la réorganisation de la représentation cérébrale du sens des mots. Chez les aveugles, l’accès au sens des mots a le même décours temporel que chez les sujets voyants, mais il recrute le cortex occipital en sus des régions habituelles. Enfin, nous avons observé une variabilité individuelle plus importante chez les aveugles dans l’organisation cérébrale du système sémantique. Nos résultats contribuent ainsi à la compréhension de la réorganisation cérébrale dans la cécité, et plus généralement du rôle de l’expérience perceptive dans le développement. / Blindness early in life leads to major changes in the functional architecture of the brain. The occipital lobes, no longer processing visual information, turn to processing auditory and tactile input and high-order cognitive functions such as language and memory. This functional reorganization offers a window into the influence of experience on brain development in humans. We studied the outcomes of this reorganization and its potential precursors. First, we used functional magnetic resonance imaging (fMRI) in order to delineate regions in the visual cortex according to their sensitivity to high-order cognitive functions. Then, using functional connectivity, we demonstrated distinct connections from those regions to the rest of the brain. Crucially, we found a functional correspondence between the visual regions and their connected brain networks. Then, using functional connectivity in neonates, we provided preliminary evidence in support of the proposition that innate connectivity biases underlie functional reorganization. Second, we focused on language, one of the reorganized functions in blindness, and used magnetoencephalography (MEG) to investigate verbal semantic processing. We found temporally equivalent but spatially different activation across the blind and the sighted. In the blind, the occipital cortex had a unique contribution to semantic category discrimination. However, the cerebral implementation of semantic categories was more variable in the blind than in the sighted. Our results advance the knowledge about brain.

This study investigated the neuroplastic changes that accompany speech identification training using magnetoencephalography (MEG). Participants completed three practice sessions over two consecutive days. In the morning group, practice occurred in the morning and evening of the first day, and in the morning of the next day; whereas, in the evening group, practice occured in the evening of the first day, and in the morning and evening of the second day. In both groups, behavioural improvement between the first session and last session was comparable. Neuromagnetic data showed practice-related changes in N1m amplitude between the first and last sessions. A time-of-day (TOD) of practice effect was found for P2m mean amplitude. In both groups, P2m-related changes with practice were greater when consecutive sessions occurred between days than within a day. The results are consistent with the proposal that task-related changes in the P2m wave are an index of perceptual learning.

This study investigated the neuroplastic changes that accompany speech identification training using magnetoencephalography (MEG). Participants completed three practice sessions over two consecutive days. In the morning group, practice occurred in the morning and evening of the first day, and in the morning of the next day; whereas, in the evening group, practice occured in the evening of the first day, and in the morning and evening of the second day. In both groups, behavioural improvement between the first session and last session was comparable. Neuromagnetic data showed practice-related changes in N1m amplitude between the first and last sessions. A time-of-day (TOD) of practice effect was found for P2m mean amplitude. In both groups, P2m-related changes with practice were greater when consecutive sessions occurred between days than within a day. The results are consistent with the proposal that task-related changes in the P2m wave are an index of perceptual learning.

In the human brain, electrical activities are created due to the body functions. These electrical activities create potentials and magnetic fields which can be monitored elec- trically (Electroencephalography - EEG) or magnetically (Magnetoencephalography - MEG). Electrical activities in human brain are usually modeled by electrical dipoles. The purpose of Electro-magnetic source imaging (EMSI) is to determine the position, orientation and strength of dipoles. The first stage of EMSI is to model the human head numerically. In this study, The Finite Element Method (FEM) is chosen to han- dle anisotropy in the brain. The second stage of EMSI is to solve the potentials and magnetic fields for an assumed dipole configuration (forward problem). Realistic con- ductivity distribution of human head is required for more accurate forward problem solutions. However, to our knowledge, conductivity distribution for an individual has not been computed yet. The aim of this thesis study is to investigate the feasibility of a new approach to update the initially assumed conductivity distribution by using the evoked potentials and fields acquired during EMSI studies. This will increase the success of source localization problem, since more realistic conductivity distribution of the head will be used in the forward problem. This new method can also be used as a new imaging modality, especially for inhomogeneities where the conductivity value deviates. In this thesis study, to investigate the sensitivity of measurements to conductivity perturbations, a FEM based sensitivity matrix approach is used. The performance of the proposed method is tested using three different head models - homogeneous spherical, 4 layer concentric sphere and realistic head model. For spherical head models rectangular grids are preferred in the middle and curved elements are used nearby the head boundary. For realistic cases, head models are developed using uniform grids. Tissue boundary information is obtained by applying segmentation algorithms to the Magnetic Resonance (MR) images. A paralel computer cluster is employed to assess the feasibility of this new approach. PETSc library is used for forward problem calculations and linear system solutions. The performance of this novel approach depends on many factors such as the head model, number of dipoles and sensors used in the calculation, noise in the measure- ments, etc. In this thesis study, a number of simulations are performed to investigate the effects of each of these parameters. Increase in the number of elements in the head model leads to the increase in the number of unknows for linear system solu- tions. Then, accuracy of the solution is improved with increased number of dipoles or sensors. The performance of the adopted approach is investigated using noise-free measurements as well as noisy measurements. For EEG, measurement noise decreases the accuracy of the approach. For MEG, the effect of measurement noise is more pronounced and may lead to a larger error in tissue conductivity calculation.

Nous avons étudié une population de patients Traumatisés Crâniens Sévères (TCS). Le premier objectif était de préciser les mécanismes cognitifs impliqués dans le déficit du partage attentionnel (ou Attention Divisée - AD) chez ces patients TCS, et en particulier d'étudier les relations avec la lenteur cognitive et la charge en Mémoire de Travail (MdT). Nos résultats suggèrent que les composantes MdT et AD seraient deux composantes distinctes et dissociables dans les fonctions exécutives, et donc que les déficits de l'AD et les limitations des ressources en MdT reposeraient sur des déficits cognitifs distincts.<br />Le deuxième objectif était d'évaluer les supports anatomiques et neurophysiologiques du déficit de MdT grâce à l'IRMf et la MEG. Deux résultats principaux se dégagent :<br />1. Le ralentissement cognitif des patients TCS semble toucher tous les processus du traitement de l'information, mais ceux impliquant le cortex préfrontal semblent plus sensibles, suggérant des dysfonctionnements exécutifs plus généraux, et non pas spécifiquement liés à la MdT<br />2. Le réseau pariéto-frontal bilatéral classiquement retrouvé était fortement perturbé chez les patients TCS avec : défaut de latéralisation du cortex préfrontal, hypoactivation de l'ensemble du réseau, présence de régions corticales supplémentaires (fronto-polaire).<br />Les données MEG et IRMf nous indiquent que les patients sont significativement ralentis par rapport aux sujets contrôles, que ce ralentissement est global mais qu'il s'accentue dans les régions préfrontales dont l'activation est déficitaire chez ces patients.

Brain decoding involves the determination of a subject’s cognitive state or an associated stimulus from functional neuroimaging data measuring brain activity. In this setting the cognitive state is typically characterized by an element of a finite set, and the neuroimaging data comprise voluminous amounts of spatiotemporal data measuring some aspect of the neural signal. The associated statistical problem is one of classification from high-dimensional data. We explore the use of functional principal component analysis, mutual information networks, and persistent homology for examining the data through exploratory analysis and for constructing features characterizing the neural signal for brain decoding. We review each approach from this perspective, and we incorporate the features into a classifier based on symmetric multinomial logistic regression with elastic net regularization. The approaches are illustrated in an application where the task is to infer from brain activity measured with magnetoencephalography (MEG) the type of video stimulus shown to a subject. / Graduate

Λαμβάνοντας υπ’ όψη τις διάφορες τεχνικές Blind Source Separation (BSS) που αναφέρονται στη βιβλιογραφία και που εφαρμόζονται σε πολλά πεδία ενδιαφέροντος, αποφασίσαμε να τις εφαρμόσουμε σε ανθρώπινο μαγνητοεγκεφαλογράφημα. Οι τεχνικές αυτές προσπαθούν να εξάγουν πηγές από σήμα το οποίο είναι γραμμικός συνδυασμός των πηγών. Απαραίτητη προϋπόθεση τα σήματα από τις πηγές να είναι στατιστικώς ανεξάρτητα. Πιο συγκεκριμένα, θεωρώντας τις πηγές στον εγκέφαλο ανεξάρτητες, η τεχνική που επιλέχθηκε είναι η Independent Component Analysis (ICA) και κάποιες παραλλαγές της που χρησιμοποιούν κυματίδια ώστε να εισάγουμε τη δυνατότητα περιορισμού στο χρόνο ή στο χώρο. Επιλέγοντας χωρικά ή χρονικά την περιοχή ενδιαφέροντος εστιάζουμε σε κάποια περιοχή του εγκεφάλου ή σε κάποια συγκεκριμένη χρονική στιγμή π.χ. μετά από κάποιο ερέθισμα. Θα χρησιμοποιήσουμε αυτές τις τεχνικές για να αναλύσουμε ανθρώπινο μαγνητοεγκεφαλογράφημα (MEG). Τα δεδομένα των καταγραφών είναι από το ινστιτούτο Riken στην Ιαπωνία. / Bearing in mind the Blind Source Separation Techniques mentioned in the literature and applied to many fields of interest, we decided to apply them on human MEG. These techniques extract sources from the initial signal, which is considered as linear combination of theses sources. It is required that the recorded signals are statistically independent. More specifically, considering the sources in the brain as independent, the method used is called Independent Component Analysis (ICA) and some versions which use wavelets in order to introduce spatial and temporal constraints. By selecting the region of interest either in space or in time, we focus on a region in the brain or on a specified latency e.g. after a stimulus. We use these methods to analyse MEG. The data are recorded in RIKEN institute in Japan.

A novel technology, the Magneto-articulography for the Assessment of Speech Kinematics (MASK) system, which measures brain activity and oromotor movement simultaneously, was validated for its speech tracking capabilities. MASK has not been systematically tested, so its movement tracking accuracy and practicality for research was still unknown. An error testing and mapping protocol is developed to validate MASK accuracy against established electromagnetic articulography (EMA) speech tracking systems. Data from human speech experiments are also compared. MASK exhibited higher positional error and fluctuation than EMA, and more inconsistent distribution of errors. Error mapping and potential error correction protocols were also developed. MASK spatial and temporal resolutions were found insufficient for precise tracking of small and quick articulatory movements. MASK requires much improvement to reach the capabilities of EMA. Further investigation into numerical instabilities of the position calculation algorithms is encouraged. This project provides the first assessment of MASK, which may advance speech research for future applications.