Visual rehabilitation and reorganization: case studies of cortical plasticity in patients with age-related macular degeneration
Main, Keith Leonard
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The extent to which cortical maps may reorganize in adult humans is a significant and topical debate in visual neuroscience. Though there are conflicting findings, evidence from humans and animals indicates that the topography of the visual cortex may change after retinal deafferentation. Remarkably, this reorganization seems to be possible in adults, whose brains are less amenable to plastic change. If adult visual reorganization is legitimate, an understanding of its causes and consequences could be profound considering the millions suffering from age-related visual disorders. This dissertation explores whether visual training may yield a reorganization of sensory maps in the adult visual cortex. It describes research in which patients, diagnosed with age-related macular degeneration (AMD), underwent visual rehabilitation therapy. Functional brain scans and behavioral tests were conducted pre and post training. These interventions generated valuable knowledge regarding whether "reorganized" activity is a true rewiring of feed forward cortical processes or an artifact of attentional feedback. The rehabilitation training produced demonstrable differences in activation patterns along the primary visual cortex (V1), but sparse improvement in the behavioral tests. In contrast, there was significant improvement in fixation tests which assessed oculomotor control. These results suggest that the nature of reorganized activity has more to do with attentional mechanisms than feed forward reorganization. Future investigations could benefit from examining the brain sites that govern visual attention in the frontal and parietal cortices. These areas may have more to do with visual adaptation in AMD patients than V1.