A hemispherectomy involves the surgical disconnection or removal of a cerebral hemisphere in the service of treating certain intractable conditions (e.g. epilepsy). Despite the extreme nature of this procedure, the visual deficits introduced by hemispherectomy can undergo impressive improvement over time, especially when the procedure is conducted at an early age. The basis of this recovery lies in the ability of the intact hemisphere to 'take over'the functions normally performed by its counterpart in a remarkable display of neural plasticity. Surprisingly, rather little systematic exploration has been directed at the mechanisms supporting this plasticity. The goal of our project is to characterize the remapping of visual processing in the intact hemisphere of these unique individuals by means of behavioral and neuroimaging (fMRI) investigations. First, at the level of the early visual cortex, we aim to characterize the (re)organization of retinotopic maps. One intriguing hypothesis that motivates this investigation concerns the ability of the intact occipital lobe to develop sensitivity over time to the affected ipsilateral hemifield. Second, at the level of higher-level visual cortex, we aim to characterize the cortical topography involved in the recognition of prominent object categories such as faces and visual word forms. The ability of a single hemisphere to cope with multiple such categories is indicative of important adjustments in the architecture of high-level object recognition. For instance, a more compact integration of visual representations underlying different categories could mediate successful recognition processes at the neural level and we aim to clarify the neural underpinnings of this functional integration. Third, we propose to characterize the extent and the profile of the recovery as a function of a number of key variables pertaining to the specifics of the procedure. In particular, the side on which the procedure was performed (i.e. right or left hemispherectomy) could determine the differential recovery / development of different sets of abilities. This hypothesis is particularly relevant for certain visual categories commonly displaying hemispheric dominance / lateralization (e.g. faces in the right ventral cortex and visual word forms in the left). In sum, by virtue of our access to this rare population, our investigation provides an unprecedented opportunity to explore a radical case of cortical remapping and the consequences of this remapping for visual perception. The degree and the manner by which visual functions are regained under a drastic reduction in the availability of neural resources can shed light on the boundary conditions of neural plasticity. Critically, the differential remapping and improvement of different visual abilities speak to the key neural mechanisms underlying recovery and plasticity.
The research proposed here adopts cutting-edge neuroimaging procedures to examine the cortical reorganization and recovery of visual functions following hemispherectomy (i.e. surgical disconnection or removal of one cerebral hemisphere). Specific emphasis is placed on exploring the plasticity of the visual system, at both earlier and higher levels, as well as the variables that determine the nature of the recovery. Importantly, thi research offers novel ground for developing general models of neural plasticity able to inform and support the design of new rehabilitation methods targeting an entire range of visual deficits.
