11 research outputs found

    Progressive Thinning of Visual Motion Area in Lower Limb Amputees

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    Accumulating evidence has indicated that amputation or deafferentation of a limb induces functional or structural reorganization in the visual areas. However, the extent of the visual areas involved after lower limb amputation remains uncertain. In this investigation, we studied 48 adult patients with unilateral lower limb amputation and 48 matched healthy controls using T1-weighted magnetic resonance imaging. Template-based regions of interest analysis was implemented to detect the changes of cortical thickness in the specific visual areas. Compared with normal controls, amputees exhibited significantly lower thickness in the V5/middle temporal (V5/MT+) visual area, as well as a trend of cortical thinning in the V3d. There was no significant difference in the other visual areas between the two groups. In addition, no significant difference of cortical thickness was found between patients with amputation at different levels. Across all amputees, correlation analyses revealed that the cortical thickness of the V5/MT+ was negatively correlated to the time since amputation. In conclusion, our findings indicate that the amputation of unilateral lower limb could induce changes in the motor-related visual cortex, and provide an update on the plasticity of the human brain after limb injury

    Using diffusion MRI to discriminate areas of cortical grey matter

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    Cortical area parcellation is a challenging problem that is often approached by combining structural imaging (e.g., quantitative T1, diffusion-based connectivity) with functional imaging (e.g., task activations, topological mapping, resting state correlations). Diffusion MRI (dMRI) has been widely adopted to analyse white matter microstructure, but scarcely used to distinguish grey matter regions because of the reduced anisotropy there. Nevertheless, differences in the texture of the cortical 'fabric' have long been mapped by histologists to distinguish cortical areas. Reliable area-specific contrast in the dMRI signal has previously been demonstrated in selected occipital and sensorimotor areas. We expand upon these findings by testing several diffusion-based feature sets in a series of classification tasks. Using Human Connectome Project (HCP) 3T datasets and a supervised learning approach, we demonstrate that diffusion MRI is sensitive to architectonic differences between a large number of different cortical areas defined in the HCP parcellation. By employing a surface-based cortical imaging pipeline, which defines diffusion features relative to local cortical surface orientation, we show that we can differentiate areas from their neighbours with higher accuracy than when using only fractional anisotropy or mean diffusivity. The results suggest that grey matter diffusion may provide a new, independent source of information for dividing up the cortex

    Mapping human cortical areas in vivo based on myelin content as revealed by t1- and t2-weighted MRI

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    Non-invasively mapping the layout of cortical areas in humans is a continuing challenge for neuroscience. We present a new method of mapping cortical areas based on myelin content as revealed by T1-weighted (T1w) and T2-weighted (T2w) MRI. The method is generalizable across different 3T scanners and pulse sequences. We use the ratio of T1w/T2w image intensities to eliminate the MR-related image intensity bias and enhance the contrast to noise ratio for myelin. Data from each subject was mapped to the cortical surface and aligned across individuals using surface-based registration. The spatial gradient of the group average myelin map provides an observer-independent measure of sharp transitions in myelin content across the surface—i.e. putative cortical areal borders. We found excellent agreement between the gradients of the myelin maps and the gradients of published probabilistic cytoarchitectonically defined cortical areas that were registered to the same surface-based atlas. For other cortical regions, we used published anatomical and functional information to make putative identifications of dozens of cortical areas or candidate areas. In general, primary and early unimodal association cortices are heavily myelinated and higher, multi-modal, association cortices are more lightly myelinated, but there are notable exceptions in the literature that are confirmed by our results. The overall pattern in the myelin maps also has important correlations with the developmental onset of subcortical white matter myelination, evolutionary cortical areal expansion in humans compared to macaques, postnatal cortical expansion in humans, and maps of neuronal density in non-human primates

    Parcellation of the human cerebral cortex using diffusion MRI

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    Histological methods have long been used to segment the cerebral cortex into structurally distinct cortical areas that have served as a basis for research into brain structure and function and remain in use today. There is great interest in adapting and extending these methods to be able to use non-invasive imaging, so that tighter structure-function relationships can be measured in living subjects. Whilst diffusion neuroimaging methods have been widely applied to white matter, the reduced anisotropy in the thin, complexly folded grey matter of the cortex has so far limited its study. In vivo parcellation pipelines have instead focussed on T1 and T2 weighted MRI. Recent advances in imaging hardware have reignited interest in grey matter diffusion MRI as a viable candidate for characterising architectonic domains. This Thesis explores the capabilities of dMRI as a measure of cortical microstructure using in vivo datasets from healthy adult participants. A cortical parcellation pipeline was developed in which both unsupervised and supervised algorithms were explored. Results were presented at both the group level and single subject level across the entire cortical sheet. The diffusion-based feature space characterised the known variation in cellular composition and fibre density relative to the local cortical surface normal. Thus they remain invariant to the confounding orientation changes associated with cortical folding, which usually inhibit studies of cortical microstructure. The features were compared to the alternative T1w/T2w myelin mapping methods to demonstrate that the diffusion MRI signal provides a complementary mode of contrast. A series of classification experiments were used to determine the most effective methods for utilising diffusion in grey matter applications. Several additional methods from the dMRI literature were compared to highlight the benefit of higher-order tissue representations. Similarly, classification tasks were used to corroborate the benefits of sampling multiple b-values in cortical studies. The experimental chapters provide strong evidence in favour of the future use of diffusion MRI as a measure of the varying microstructure that defines cortical areas

    Visual Cortex

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    The neurosciences have experienced tremendous and wonderful progress in many areas, and the spectrum encompassing the neurosciences is expansive. Suffice it to mention a few classical fields: electrophysiology, genetics, physics, computer sciences, and more recently, social and marketing neurosciences. Of course, this large growth resulted in the production of many books. Perhaps the visual system and the visual cortex were in the vanguard because most animals do not produce their own light and offer thus the invaluable advantage of allowing investigators to conduct experiments in full control of the stimulus. In addition, the fascinating evolution of scientific techniques, the immense productivity of recent research, and the ensuing literature make it virtually impossible to publish in a single volume all worthwhile work accomplished throughout the scientific world. The days when a single individual, as Diderot, could undertake the production of an encyclopedia are gone forever. Indeed most approaches to studying the nervous system are valid and neuroscientists produce an almost astronomical number of interesting data accompanied by extremely worthy hypotheses which in turn generate new ventures in search of brain functions. Yet, it is fully justified to make an encore and to publish a book dedicated to visual cortex and beyond. Many reasons validate a book assembling chapters written by active researchers. Each has the opportunity to bind together data and explore original ideas whose fate will not fall into the hands of uncompromising reviewers of traditional journals. This book focuses on the cerebral cortex with a large emphasis on vision. Yet it offers the reader diverse approaches employed to investigate the brain, for instance, computer simulation, cellular responses, or rivalry between various targets and goal directed actions. This volume thus covers a large spectrum of research even though it is impossible to include all topics in the extremely diverse field of neurosciences

    Functional Brain Organization in Space and Time

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    The brain is a network functionally organized at many spatial and temporal scales. To understand how the brain processes information, controls behavior and dynamically adapts to an ever-changing environment, it is critical to have a comprehensive description of the constituent elements of this network and how relationships between these elements may change over time. Decades of lesion studies, anatomical tract-tracing, and electrophysiological recording have given insight into this functional organization. Recently, however, resting state functional magnetic resonance imaging (fMRI) has emerged as a powerful tool for whole-brain non-invasive measurement of spontaneous neural activity in humans, giving ready access to macroscopic scales of functional organization previously much more difficult to obtain. This thesis aims to harness the unique combination of spatial and temporal resolution provided by functional MRI to explore the spatial and temporal properties of the functional organization of the brain. First, we establish an approach for defining cortical areas using transitions in correlated patterns of spontaneous BOLD activity (Chapter 2). We then propose and apply measures of internal and external validity to evaluate the credibility of the areal parcellation generated by this technique (Chapter 3). In chapter 4, we extend the study of functional brain organization to a highly sampled individual. We describe the idiosyncratic areal and systems-level organization of the individual relative to a standard group-average description. Further, we develop a model describing the reliability of BOLD correlation estimates across days that accounts for relevant sources of variability. Finally, in Chapter 5, we examine whether BOLD correlations meaningfully vary over the course of single resting-state scans

    Applications of Gradient Representations in Resting-State fMRI

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    Classical models of brain organization have often considered the brain to be made up of a mosaic of patches that are demarcated by discrete boundaries, often defined histologically. In contrast, emerging views have pointed towards an alternative paradigm – referred to as gradients – by conceptualizing brain organization as sets of organizational axes that characterizes spatial variation of differing connectivity principles over the extent of a region. Such organizational axes provide a well-suited framework for elucidating underpinnings of brain connectivity and has garnered widespread attention across various domains of neuroimaging. This work seeks to explore various applications of gradient estimation techniques, in combination with resting-state functional connectivity data, across the fields of basic, comparative, and clinical neuroscience. First, gradient estimation was performed on resting-state functional connectivity (RSFC) patterns of the primary somatosensory cortex to unveil a secondary organizational axis that spans the region’s anterior-posterior axis, akin to circuitry fundamental to sensory cortical information processing. Second, gradient techniques were used in a cross-species comparison study to unify connectivity principles of humans and marmosets by mapping them simultaneously onto a set of organizational axes. In doing so, this provided a systematic framework to compare the functional architecture of both species, facilitating novel insight of a well-integrated default-mode network in humans, compared to marmosets. Third, connectivity gradients, along with a myriad of other resting-state fMRI features were used to explore the implications of focal lesion pathophysiology on functional organization of the thalamus in individuals with Multiple Sclerosis. A lack of focal changes to resting-state related features was observed suggesting the limited role of focal thalamic lesions to functional organization in MS. Together, these different avenues of research highlight the capacity for a gradient-centric view in neuroimaging to provide profound insights into brain organization, and its utility across the applications of basic, comparative, and clinical neuroscience

    Delineating extrastriate visual area MT(V5) using cortical myeloarchitecture

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    Visual area MT is a model of choice in primate neurophysiological and human imaging research of visual perception, due to its considerable sensitivity to moving stimuli and the strong direction selectivity of its neurons. While the location of MT(V5) in the non-human primate is easily identifiable based on the gross anatomy and appears consistent between animals, this is less the case in human subjects. Functional localisation of human MT+ with moving stimuli can identify a group of motion-sensitive regions, but defining MT proper has proved more challenging. In this review we consider approaches to studying the cyto- and myeloarchitecture of this cortical area that may, in the future, allow identification of human MT in vivo based on anatomy.</p

    Perception audiovisuelle de la parole chez le sourd postlingual implanté cochléaire et le sujet normo-entendant : étude longitudinale psychophysique et neurofonctionnelle

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    Nos travaux ont consisté à étudier les mécanismes perceptifs et neuronaux impliqués lors de la perception audiovisuelle de la parole chez des patients sourds postlinguaux implantés cochléaires et des sujets contrôles normo-entendants. Dans ce but nous avons testé les performances audiovisuelles des patients implantés au cours de suivis longitudinaux en comportemental et en tomographie par émission de positrons, ainsi qu'au travers d'un paradigme de parole audiovisuelle incongruente (McGurk). Pour comparaison nous avons systématiquement testé des sujets contrôles dans les mêmes conditions. Nous avons également testé ces derniers en condition de lecture labiale puis en utilisant des dégradations de la parole auditive et audiovisuelle par du bruit blanc et par une simulation acoustique d'implant cochléaire. L'ensemble de ces études concorde pour indiquer, à la suite de l'implantation cochléaire, une réorganisation rapide des réseaux corticaux multisensoriels impliqués dans l'intégration audiovisuelle de la parole, aboutissant à une combinaison optimale des indices perceptifs audiovisuelsOur present work consisted in studying perceptual and neuronal mechanisms involved during audiovisual speech perception in postlingually deaf cochlear-implant patients and normally-hearing controls. With this aim in view we tested audiovisual performance for implanted patients during longitudinal follow-ups using behavioral methods and positron emission tomography neuroimagery, as well as incongruent audiovisual speech (McGurk paradigm). In order to achieve appropriate comparisons, control subjects were tested in identical conditions. Moreover they were tested in a speechreading condition and with auditory and audiovisual speech degraded through white-noise masks and cochlear-implant computer simulations. Results from these studies agree to point out, following cochlear implantation, a fast reorganization of multisensory cortical networks involved in audiovisual speech integration, leading to an optimal combination of audiovisual perceptual cue
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