Intégration et topographie fonctionnelles : l'influence de la cécité précoce

The brain’s functional organization shows a remarkable similarity across individuals suggesting that its functional structure is innate. However, this observation is not unequivocal for the brain is also plastic; its organization is subject to changes following important early life experiences. Of such event, early or congenital vision-loss features among the most outstanding. Following blindness, cerebral regions usually involved with visual analysis (e.g. occipital lobes) are reorganized to process auditory, tactile, and olfactory stimuli, as well as higher order cognitive tasks such as memory and language.  Recent studies on blind individuals have focused on the relationship between the reorganization of the occipital lobes and the fundamental organizational principles of the brain: functional localization and integration. The first indicates that specific functions take place within circumscribed cerebral regions, whereas the second indicates that these regions communicate together. However, the extent to which blindness influences the brain’s organization is still unknown. First, the topography of modules possessing specific functions has yet to be the object of a direct, in depth, and holistic investigation in blind people. This is true even though module topography is a key characteristic of spatially localized functions. Moreover, studies investigating functional integration in blind individuals appear to contradict those which study integration indirectly. The dependence of results upon the method employed demands further investigation of the elements that might influence integration so that appropriate conclusions can be drawn.    Thus, even though blindness-induced brain plasticity has been the object of multiple studies, its influence upon cerebral organization is partly subject to debate, and must be further defined. Accordingly, the main objective of this thesis was to study the influence of early blindness upon the topography of functional modules and upon functional integration. In order to do so, three distinct experiments were carried.  The first experiment was conceived so as to compare the spatial topographies of functional modules across blind and sighted participants. To this end, a holistic and bias-free method, which made use of resting-state functional magnetic resonance imagery, was developed. Results indicate that only blind people’s occipital poles possess modules which were topographically different from those found in sighted individuals whereas higher level “visual” areas do not show this effect. This suggests that the formation of functional modules within the “visual” cortices depends on both sensory inputs as well as the hierarchical level of a region.   The second experiment of this thesis examined factors which might cause the contradictions observed by studies on blind people’s functional integration. Data obtained from a method devised to measure functional integration, resting-state functional magnetic resonance imagery, were compared to those obtained while participant focused on auditory stimuli. Results are twofold. First, results showed that group differences in functional integration vary as a function of the participant’s cognitive states, suggesting that differences in functional integration observed using resting-state fMRI should be interpreted with caution. Second, our results reveal novel findings pertaining to the occipital poles of blind people; these regions showed strikingly different characteristic following early visual loss which are suggestive of their role in a multitude of networks.   The third experiment of this thesis was designed following the first two. Specifically, it validates a method to examine the functions of blind people’s occipital poles, a region which had been shown to possess reorganized functional characteristics by the first two experiments of this thesis. A new methodology was designed to control the difficulty of a voice discrimination task, showing its validity. Thus, it will be possible to use it in future studies to investigate the function of the occipital lobes in blind people. Furthermore, results from the experiment revealed multiple mechanisms which might underlie voice perception in humans.   Overall, the experiments forming this thesis evidence a reorganisation of the occipital poles’ functional characteristics in early-blind individuals. Additionally, they provide methods needed to investigate the functions of this understudied region. Most importantly, the observation that blind people’s occipital poles show modified functional characteristics paves the way to new hypotheses and studies which will allow to further investigate and deepen our understanding of the human brain and its plasticity.

Keyphrases: Early blindness, cerebral plasticity, functional connectivity, functional topography, functional imaging