Microstructural differences in the thalamus and thalamic radiations in the congenitally deaf

There is evidence of both crossmodal and intermodal plasticity in the deaf brain. Here, we investigated whether sub-cortical plasticity, specifically of the thalamus, contributed to this reorganisation. We contrasted diffusion weighted magnetic resonance imaging data from 13 congenitally deaf and 13 hearing participants, all of whom had learnt British Sign Language after 10 years of age. Connectivity based segmentation of the thalamus revealed changes to mean and radial diffusivity in occipital and frontal regions, which may be linked to enhanced peripheral visual acuity, and differences in how visual attention is deployed in the deaf group. Using probabilistic tractography, tracts were traced between the thalamus and its cortical targets, and microstructural measurements were extracted from these tracts. Group differences were found in microstructural measurements of occipital, frontal, somatosensory, motor and parietal thalamo-cortical tracts. Our findings suggest there is sub-cortical plasticity in the deaf brain, and that white matter alterations can be found throughout the deaf brain, rather than being restricted to, or focussed in auditory cortex

Differential activity in Heschl's gyrus between deaf and hearing individuals is due to auditory deprivation rather than language modality

Sensory cortices undergo crossmodal reorganisation as a consequence of sensory deprivation. Congenital deafness in humans represents a particular case with respect to other types of sensory deprivation, because cortical reorganisation is not only a consequence of auditory deprivation, but also of language-driven mechanisms. Visual crossmodal plasticity has been found in secondary auditory cortices of deaf individuals, but it is still unclear if reorganisation also takes place in primary auditory areas, and how this relates to language modality and auditory deprivation.  Here, we dissociated the effects of language modality and auditory deprivation on crossmodal plasticity in Heschl's gyrus as a whole, and in cytoarchitectonic region Te1.0 (likely to contain the core auditory cortex). Using fMRI, we measured the BOLD response to viewing sign language in congenitally or early deaf individuals with and without sign language knowledge, and in hearing controls.  Results show that differences between hearing and deaf individuals are due to a reduction in activation caused by visual stimulation in the hearing group, which is more significant in Te1.0 than in Heschl's gyrus as a whole. Furthermore, differences between deaf and hearing groups are due to auditory deprivation, and there is no evidence that the modality of language used by deaf individuals contributes to crossmodal plasticity in Heschl's gyrus

Experience Dependent Plasticity over short and long timescales

The brain is constantly changing. Genetically specified developmental pathways interact with extrinsic factors including illness, injury and learning to shape the brain. This thesis presents two projects on experience dependent plasticity over different timescales. Exerting its effect across years, deafness provides a model of long term crossmodal plasticity. In the first part of this thesis I ask how deafness affects the thalamus. Diffusion weighted imaging was used to segment the thalamus and with probabilistic tractography, thalamo-cortical connections were traced. Microstructural properties of visual and frontal thalamic segmentations, thalamo-cortical tracts throughout the brain, apart from the temporal thalamo-cortical tract were altered. The neuroanatomical sequelae of deafness are evident throughout the brain. Deaf people have enhanced peripheral vision, facilitating a protective orienting mechanism when hearing cannot be relied upon. Widefield population receptive field (pRF) modeling with fMRI was completed to examine the functional and structural properties of primary visual cortex. Deaf participants had enlarged pRF profiles and thinner cortex in peripheral visual regions, again emphasizing plasticity across many years. In the second part I examine plasticity over the course of days. Visuomotor transformations translate visual input to motor actions, and its neural instantiation might change with training. We used a pattern component model on fMRI data to reveal a gradient of visual to motor information from occipital to parietal to motor cortex. Strikingly, we observed motor coding in visual cortex and visual coding in motor cortex. More tentatively, our results suggest that during sensorimotor skill learning there is decreased dependence on visual cortex as motor cortex learns the novel visuomotor mapping. In summary, I show crossmodal processing and plasticity in regions previously considered not to exhibit these properties, both in long- and short-term plasticity. This work emphasizes the contribution that computational neuroimaging can provide to the field of experience dependent plasticity

Crossmodal reorganisation in deafness: mechanisms for functional preservation and functional change

The study of deafness and blindness has contributed unique knowledge to our understanding of the brain, showing that environmental experience critically shapes neural structure and function. Nevertheless, the most prevalent theories of crossmodal plasticity propose opposing views about the function of reorganised cortical regions. Some theories agree on functional preservation, where in the absence of early sensory stimulation, cortical regions respond to a different sensory modality, but perform the same function. Others propose that the absence of sensory stimulation from birth results in cortical regions changing their “typical” sensory processing function to higher-order cognition. Both deafness and blindness have provided vast evidence in support of each of these theories. Here we use examples from the study of deafness to explore organisational mechanisms that would allow functional preservation and functional change to co-exist either in the same or adjacent regions. We provide a set of predictions and testable hypotheses that support each of these accounts, and lay out some steps that could move us towards more specific theories of cortical reorganisation

Action control in uncertain environments

A long-standing dichotomy in neuroscience pits automatic or reflexive drivers of behaviour against deliberate or reflective processes. In this thesis I explore how this concept applies to two stages of action control: decision-making and response inhibition. The first part of this thesis examines the decision-making process itself during which actions need to be selected that maximise rewards. Decisions arise through influences from model-free stimulus-response associations as well as model-based, goal-directed thought. Using a task that quantifies their respective contributions, I describe three studies that manipulate the balance of control between these two systems. I find that a pharmacological manipulation with levodopa increases model-based control without affecting model-free function; disruption of dorsolateral prefrontal cortex via magnetic stimulation disrupts model-based control; and direct current stimulation to the same prefrontal region has no effect on decision-making. I then examine how the intricate anatomy of frontostriatal circuits subserves reinforcement learning using functional, structural and diffusion magnetic resonance imaging (MRI). A second stage of action control discussed in this thesis is post-decision monitoring and adjustment of action. Specifically, I develop a response inhibition task that dissociates reactive, bottom-up inhibitory control from proactive, top-down forms of inhibition. Using functional MRI I show that, unlike the strong neural segregation in decision-making systems, neural mechanisms of reactive and proactive response inhibition overlap to a great extent in their frontostriatal circuitry. This leads to the hypothesis that neural decline, for 4 example in the context of ageing, might affect reactive and proactive control similarly. I test this in a large population study administered through a smartphone app. This shows that, against my prediction, reactive control reliably declines with age but proactive control shows no such decline. Furthermore, in line with data on gender differences in age-related neural degradation, reactive control in men declines faster with age than that of women

Réorganisation cérébrale chez l’adulte sourd : de la privation à la restauration auditive

On estime que 5 % de la population dans le monde souffre d’une perte auditive handicapante, dont 34 millions d’enfants. Ce déficit perceptif, lorsqu’il survient dès la naissance ou lors des premières années de vie, a de multiples répercussions sur le développement cérébral et neurocognitif. La réorganisation cérébrale ayant cours dans le cerveau des individus privés de l’audition précocement constitue un sujet d’étude très prisé par la communauté scientifique, mais pour laquelle de nombreuses questions restent en suspens. Ainsi, les articles qui composent cette thèse ont pour objectif principal d’améliorer nos connaissances portant sur les mécanismes de réorganisation cérébrale, tant au niveau fonctionnel que structurel afin de mieux comprendre leur implication comportementale chez les individus sourds. Pour ce faire, nous avons souhaité investiguer, par le biais de l’imagerie par résonance magnétique fonctionnelle, quel était le lien entre les activations cérébrales et les performances comportementales lors d’une tâche portant sur les mouvements biologiques chez des adultes sourds congénitaux, en comparaison à des pairs neurotypiques. L’article 1 révèle que les individus sourds présentent une sensibilité accrue à la reconnaissance du mouvement biologique, et notamment des emblèmes, en comparaison à des individus neurotypique. De plus, cette spécificité comportementale observée uniquement chez les individus sourds, s’accompagne d’un recrutement extensif des régions comprises dans le gyrus temporal supérieur, et tout particulièrement le cortex auditif primaire ainsi que le planum temporale. Nos résultats supportent la présence d’une réorganisation intermodale qui s’exprime par le recrutement cérébral des régions auditives lors de stimulations visuelles complexes, entraînant une amélioration de la reconnaissance des mouvements biologiques chez les adultes sourds. Par la suite, nous avons souhaité préciser les mécanismes de réorganisation cérébrale de type structurel. En raison de l’hétérogénéité des résultats rapportés précédemment dans la littérature à propos des changements de matière grise et de matière blanche chez les enfants, les adolescents et les adultes sourds privés de l’audition précocement, la réalisation d’une revue systématique a permis de répertorier l’ensemble des changements structurels obtenus par le biais de diverses techniques d’analyse en imagerie par résonance magnétique. L’article 2 de la présente thèse offre une généralisation des altérations structurelles et intègre une visée clinique à la compréhension de ces changements anatomiques et notamment leur impact sur le développement langagier et neurocognitif. Mis ensemble, ces résultats contribuent à une meilleure appréciation des changements cérébraux à la suite d’une privation précoce de l’audition. En outre, ils offrent une perspective développementale à ces changements par la description de comportements adaptatifs à la situation de handicap auditif, ainsi que du profil neurocognitif de ces individus, dans le but d’apporter de nouvelles pistes aux stratégies de restauration de l’audition et du langage.It is estimated that 5% of the world’s population suffers from a disabling hearing loss, including 34 million children. This sensory deficit, when it occurs at birth or in the first years of life, has multiple repercussions on the brain and neurocognitive development. The brain reorganization taking place in the brain of early-deaf individuals is an area of research highly valued by the scientific community but for which many questions remain unanswered. Thus, the main objective of the articles in this thesis is to improve our knowledge of brain reorganization mechanisms, both at the functional and structural levels, in deaf individuals. This will allow a better understanding of their impact on the behavioural adaptations of deaf individuals. To do this, we investigated, through functional magnetic resonance imaging, the relationship between brain activation and behavioural performance in a task involving biological motions in early-deaf adults, compared to hearing peers. Article 1 reveals that deaf individuals are more sensitive to the recognition of biological motion, including emblems, than hearing individuals. In addition, this behavioural specificity, observed only in deaf individuals, is accompanied by extensive recruitment of the regions included in the superior temporal gyrus, such as the primary auditory cortex but more particularly, the planum temporale. Our results support the presence of intermodal reorganization, which is expressed by brain recruitment of auditory regions during complex visual stimuli, leading to improved recognition of the biological motion in early deaf adults. On the other hand, we wanted to specify the mechanisms of structural brain reorganization. Due to the heterogeneity of the results previously reported in the literature on changes in grey matter and white matter in early-deaf children, adolescents, and adults, the completion of a systematic review identified all the structural changes obtained through various magnetic resonance imaging analysis techniques. The second article of this thesis offers a generalization of structural alterations. It also integrates a clinical frame to the understanding of these anatomical changes to optimize the language and neurocognitive development of these individuals. Together, these results contribute to a better appreciation of brain changes following an early hearing loss at both the functional and structural levels. Besides, they offer a developmental perspective to these changes by describing adaptive behaviours and the neurocognitive profile of these individuals, with providing new insights into hearing and language restoration strategies

Of One Mind: Proposal for a Non-Cartesian Cognitive Architecture

Intellectually, we may reject Cartesian Dualism, but dualism often dominates our everyday thinking: we talk of “mental” illness as though it were non-physical; we tend to blame people for the symptoms of brain malfunctions in a way that differs from how we treat other illnesses. An examination of current theories of mind will reveal that some form of dualism is not always limited to the non-scientific realm. While very few, if any, cognitive scientists support mind-body dualism, those who support the view of the mind as a symbol-manipulator are often constrained to postulate more than one cognitive system in response to the failure of the symbol-system model to account for all aspects of human cognition. In this dissertation, I argue for an empiricist, rather than a realist, theory of perception, for an internalist semantics, and for a model of cognitive architecture which combines a connectionist approach with highly-specialized, symbolic, computational component which includes functions that provide input to a a causally-inert conscious mind. I reject the symbol-system hypothesis and propose a cognitive architecture which, I contend, is biologically-plausible and more consistent with the results of recent neuroscientific studies. This hybrid model can accommodate the processes commonly discussed by dual-process theorists and can also accommodate the processes which have proved to be so problematic for models based on the symbol-system hypothesis

Using Statistics, Computational Modelling and Artificial Intelligence Methods to Study and Strengthen the Link between Kinematic Impacts and mTBIs

Mild traumatic brain injuries (mTBIs) are frequently occurring, yet poorly understood, injuries in sports (e.g., ice hockey) and other physical recreation activities where head impacts occur. Helmets are essential pieces of equipment used to protect participants’ heads from mTBIs. Evaluating the performance of helmets to prevent mTBIs using simulations on anatomically accurate computational head finite element models is critically important for advancing the development of safer helmets. Advancing the level of detail in, and access to, such models, and their continued validation through state-of-the-art brain imaging methods and traditional head injury assessment procedures, is also essential to improve safety. The significant research contributions in this thesis involve evaluating the decrease in blunt impact-induced brain axon fiber tract strains that various helmets provide by studying outputs of existing finite element brain models and implementing open-source artificial intelligence technology to create a novel pipeline for predicting such strains