33 research outputs found
Nature of crossmodal plasticity in the blind brain and interplay with sight restoration
Get PDFThèse réalisée en cotutelle avec l'Université catholique de Louvain.Ce travail de thèse s’est intéressé à la plasticité cérébrale associée à la privation/restauration visuelle. A travers deux études transversales utilisant l’imagerie par résonance magnétique fonctionnelle auprès d’un groupe de participants présentant une cécité congénitale ou précoce (ainsi qu’auprès d’un groupe contrôle de participants voyants), nous avons tenté de caractériser la manière dont le cortex occipital - typiquement dédié au traitement de l’information visuelle - se réorganise afin de traiter différents stimuli auditifs. Nous démontrons qu’en cas de cécité précoce, différentes régions du cortex occipital présentent une préférence fonctionnelle pour certains types de stimuli non-visuels, avec une spécialisation fonctionnelle qui respecte celle de régions typiquement impliquées dans le traitement d’informations similaires en vision. Ces découvertes constituent une avancée conceptuelle concernant le rôle joué par les contraintes intrinsèques d’une part, et par l’expérience d’autre part, dans l’émergence de réponses sensorielles et fonctionnelles du cortex occipital. D’une part, l’observation de réponses occipitales à la stimulation auditive chez le non-voyant précoce (réorganisation transmodale) rend compte de la capacité du cortex occipital à réorienter sa modalité sensorielle préférentielle en fonction de l’expérience. D’autre part, l’existence de modules cognitifs spécialisés dans le cortex occipital du non-voyant précoce, semblables à ceux du cerveau voyant, démontre les contraintes intrinsèques imposées à une telle plasticité. Dans une étude de cas longitudinale, nous avons également exploré comment les changements plastiques associés à la cécité interagissent avec une récupération visuelle partielle à l’âge adulte. Nous avons réalisé des mesures pré et post-opératoires auprès d’un patient ayant récupéré la vision, en combinant les techniques comportementales ainsi que de neuroimagerie fonctionnelle et structurelle afin d’investiguer conjointement l’évolution de la réorganisation transmodale et de la récupération des fonctions visuelles à travers le temps. Nous démontrons que les changements structurels et fonctionnels caractérisant le cortex occipital du non-voyant sont partiellement réversibles suite à une récupération visuelle à l’âge adulte. De manière générale, ces recherches témoignent de l’importante adaptabilité du cortex occipital aux prises avec des changements drastiques dans l’expérience visuelle.The present Ph.D. work was dedicated to the study of experience-dependent brain plasticity associated with visual deprivation/restoration. In two cross-sectional studies involving the use of functional magnetic resonance imaging in a group of participants with congenital or early blindness (and in a control group of sighted participants), we attempted to characterize the way the occipital cortex - typically devoted to vision – reorganizes itself in order to process different auditory stimuli. We demonstrate that in case of early visual deprivation, distinct regions of the occipital cortex display a functional preference for specific non-visual attributes, maintaining a functional specialization similar to the one that characterizes the sighted brain. Such studies have shed new light on the role played by intrinsic constraints on the one side, and experience on the other, in shaping the modality- and functional tuning of the occipital cortex. On the one hand, the observation of occipital responses to auditory stimulation (crossmodal plasticity) highlights the ability of the occipital cortex to reorient its preferential tuning towards the preserved sensory modalities as a function of experience. On the other hand, the observation of specialized cognitive modules in the occipital cortex, similar to those observed in the sighted, highlights the intrinsic constraints imposed to such plasticity. In a longitudinal single-case study, we further explored how the neuroplastic changes associated with blindness may interact with the newly reacquired visual inputs following partial visual restoration in adulthood. We performed both pre- and post-surgery measurements in a sight-recovery patient combining behavioral, neurostructural and neurofunctional methods in order to jointly investigate the evolution of crossmodal reorganization and visual recovery across time. We demonstrate that functional and structural changes evidenced in the visually-deprived occipital cortex can only partially reverse following sight restoration in adulthood. Altogether, our findings demonstrate the striking adaptability of the occipital cortex facing drastic changes in visual experience
Nature of crossmodal plasticity in the blind brain and interplay with sight restoration
Get PDFThe present Ph.D. work was dedicated to the study of experience-dependent brain plasticity associated with visual deprivation/restoration. In two cross-sectional studies involving the use of functional magnetic resonance imaging in a group of participants with congenital or early blindness (and in a control group of sighted participants), we attempted to characterize the way the occipital cortex - typically devoted to vision – reorganizes in order to process different auditory stimuli. We demonstrate that in case of early visual deprivation, distinct regions of the occipital cortex display a functional preference for specific non-visual attributes, maintaining a functional specialization similar to the one that characterizes the sighted brain. Such studies have shed new light on the role played by intrinsic constraints on the one side, and experience on the other, in shaping the modality- and functional tuning of the occipital cortex. On the one hand, the observation of occipital responses to auditory stimulation (crossmodal plasticity) highlights the ability of the occipital cortex to reorient its preferential tuning towards the preserved sensory modalities as a function of experience. On the other hand, the observation of specialized cognitive modules in the occipital cortex, similar to those observed in the sighted, highlights the intrinsic constraints imposed to such plasticity. In a longitudinal single-case study, we further explored how the neuroplastic changes associated with blindness may interact with the newly reacquired visual inputs following partial visual restoration in adulthood. We performed both pre- and post-surgery measurements in a sight-recovery patient combining behavioral, neurostructural and neurofunctional methods in order to jointly investigate the evolution of crossmodal reorganization and visual recovery across time. We demonstrate that functional and structural changes evidenced in the visually-deprived occipital cortex can only partially reverse following sight restoration in adulthood. Altogether, our findings demonstrate the striking adaptability of the occipital cortex facing drastic changes in visual experience.(PSYE - Sciences psychologiques et de l) -- UCL, 201
A common right fronto-parietal network for numerosity and duration processing : an fMRI study
No full textNumerosity and duration processing have been modeled by a functional mechanism taking the form of an accumulator working under two different operative modes. Separate investigations of their cerebral substrates have revealed partly similar patterns of activation, mainly in parietal and frontal areas. However, the precise cerebral implementation of the accumulator model within these areas has not yet been directly assessed. In this study, we asked participants to categorize the numerosity of flashed dot sequences or the duration of single dot displays, and we used functional magnetic resonance imaging (fMRI) to examine the common neural correlates of these processes. The results reveal a large right-lateralized fronto-parietal network, including the intraparietal sulcus (IPS) and areas in the precentral, middle and superior frontal gyri, which is activated by both numerosity and duration processing. Complementary psychophysiological interaction (PPI) analyses show a functional connectivity between the right IPS and the frontal areas in both tasks, whereas the right IPS was functionally connected to the left IPS and the right precentral area in the numerosity categorization task only. We propose that the right IPS underlies a common magnitude processing system for both numerosity and duration, possibly corresponding to the encoding and accumulation stages of the accumulator model, whereas the frontal areas are involved in subsequent working-memory storage and decision-making processes. © 2011 Wiley Periodicals, Inc
Functional selectivity in sensory-deprived cortices
No full textIn a recent study, Lomber, Meredith, and Kral (2010) investigated crossmodal reorganization in congenitally deaf cats. They demonstrated that specific regions of the auditory cortex are responsible for distinct supranormal visual performances following early auditory deprivation. These exciting results are considered in light of recently increasing research suggesting that crossmodal plasticity associated with early sensory deprivation follows organizational principles that maintain the functional specialization of the colonized brain regions. © 2011 the American Physiological Society
Mode-dependent and mode-independent representations of numerosity in the right intraparietal sulcus
No full textIn humans, areas around the intraparietal sulcus (IPS) have been found to play a crucial role in coding nonsymbolic numerosities (i.e., number of elements in a collection). In the parietal cortex of monkeys, some populations of neurons were found to respond selectively to sequentially- or simultaneously-presented numerosities, whereas other populations showed similar activation in both modes of presentation. However, whether such mode-dependent and -independent representations of numerosity also exist in humans is still unknown. Here, we used fMRI to identify the areas involved in numerosity processing while participants classified linear arrays of dots (simultaneous stimuli) or flashed dot sequences (sequential stimuli). The processing of simultaneous numerosities induced activations bilaterally in several areas of the IPS, whereas activations during the processing of sequential numerosities were restricted to the right hemisphere. A conjunction analysis showed that only the right IPS and precentral gyrus showed overlapping activations during the judgement of sequential and simultaneous stimuli. Voxelwise correlations confirmed the highly similar pattern of activation found in these regions during both tasks. This pattern was weaker or absent in mode-dependent regions, like the right inferior frontal cortex and the lateral occipital complex. Finally, a close look at the right IPS revealed an anterior-to-posterior gradient of activation with selective activation for sequential and simultaneous stimuli in the anterior and posterior areas, respectively, and overlapping activations in-between. This study provides the first direct evidence that, in humans, the right IPS contains both mode-dependent and mode-independent representations of numerosity
Plasticity of the Dorsal “Spatial” Stream in Visually Deprived Individuals
Get PDFStudies on visually deprived individuals provide one of the most striking demonstrations that the brain is highly plastic and is able to rewire as a function of the sensory input it receives from the environment. In the current paper, we focus on spatial abilities that are typically related to the dorsal visual pathway (i.e., spatial/motion processing). Bringing together evidence from cataract-reversal individuals, early- and late-blind individuals and sight-recovery cases of long-standing blindness, we suggest that the dorsal “spatial” pathway is mostly plastic early in life and is then more resistant to subsequent experience once it is set, highlighting some limits of neuroplasticity
Zur Welt kommen : Politische Theorie mit Maurice Merleau-Ponty
Get PDFDer Phänomenologe Maurice Merleau-Ponty war ein hochpolitischer Autor, der in der Politischen Theorie heute aber wenig präsent ist. Der Beitrag will zeigen, dass er gleichwohl fruchtbare Anregungen für gegenwärtige Debatten liefern kann. Dazu wird erstens sein Leib- und Weltbegriff auf die Problematik eines gefährdeten Wirklichkeitssinns bezogen. Zweitens wird gezeigt, dass Merleau-Pontys Denken den Dualismus von Identität und Differenz unterläuft und sich einem resonanzorientierten Politikverständnis annähert. Drittens ist seine zugleich narrative und transformative Konzeption des Verhältnisses von Freiheit und Geschichte geeignet, einen Kontrapunkt zu einseitig anfangs- und ereignisorientierten Auffassungen von Freiheit zu setzen. Neben Claude Lefort und Jean-Paul Sartre wird Merleau-Ponty dabei insbesondere auch mit Hannah Arendt ins Gespräch gebracht.Philosopher Maurice Merleau-Ponty was a highly political author. However, he is widely ignored in contemporary debates. It is argued that Political Theory can draw valuable insights from his work. His theory of perception and world can help us understand the ways in which the ‘sensus communis’ establishes a shared political reality as well as the dangers it encounters. His notion of a relational political interworld provides an alternative to theories that operate primarily in terms of identity and difference. Finally, his concept of history and freedom offers a way to think political action not as a radical event, but as embedded in a narrative which it transforms at the same time. Throughout the article, Merleau-Ponty is contrasted not only with Claude Lefort (his student) and Jean-Paul Sartre (his friend), but also with Hannah Arendt
Attentional shifts due to irrelevant numerical cues: Behavioral investigation of a lateralized target discrimination paradigm
No full textpeer reviewedBehavioural evidence indicates the existence of a link between numerical representations and visuo-spatial processes. A striking demonstration of this link was provided by Fischer and colleagues (2003), who reported that participants detect a target more rapidly in the left hemifield, if it is preceded by a small number (e.g. 2 or 3) and more rapidly in the right hemifield if preceded by a large number (e.g. 8 or 9). This is strong evidence that numbers orient visuo-spatial attention to different visual hemifields (e.g., left and right) depending on their magnitude (e.g., small and large, respectively). Here, we sought to replicate number-related attentional shifts using a discrimination task. The participants (n=16) were presented 1 digit (1,2 vs. 8,9) at the centre of the screen for 400ms. After 500ms, 1000ms or 2000ms, a target was briefly flashed in either the right or left hemifield and participants had to report its colour (red or green). They were told that the central digit was irrelevant to the task. We hypothesized that the attentional shift induced by the centrally presented numbers should induce congruency effects for the target discrimination task, so that small (or large) numbers would facilitate the processing of left (or right) targets. Our results confirmed this prediction, but only for the shortest digit-target interval (500ms). This is supported by a significant interaction between number magnitude (small/large) and target hemifield (left/right). The link between numerical and spatial representations further predicts a positive relation between number magnitude and the difference in RT between left and right targets. Regression slopes were computed individually and a positive slope was obtained for short number-target interval. These findings indicate that the attentional shifts induced by irrelevant numerical material are independent of the exact nature of target processing (discrimination vs. detection)
Embodied numbers: The role of vision in the development of number-space interactions
No full textThe strong association between numbers and space is found in the well-documented SNARC effect (Spatial Numerical Association of Response Codes), where responses on small/large numbers are faster in the left/right side of space, respectively. However, little is known about the developmental process through which numbers are mapped onto external physical space. Here we show that early blind individuals, but not late blind or sighted, demonstrate a reversed SNARC effect when performing a numerical comparison task with hands crossed over the body midline. Importantly, this reversed SNARC effect was not observed in any group of participants in a control parity judgment task. The present study therefore demonstrates that early visual experience drives the development of an external coordinate system for the visuo-spatial representation of numbers and further supports the idea that different types of spatial information are engaged in specific numerical tasks
Building the brain in the dark: Functional and specific crossmodal reorganization in the occipital cortex of blind individuals
No full textn/
