Brain-based vision deficits: Characterization and simulation for presurgical planning

By using functional magnetic resonance imaging (fMRI) as part of a presurgical planning process it may be possible to minimize the post-surgical visual impairment suffered by patients undergoing brain surgery in or around the occipital lobe. The deficit estimation becomes more straightforward if the impairments experienced by patients are caused primarily by the loss of visual input as opposed to higher order brain damage. This thesis investigates the feasibility of using fMRI to predict treatment-induced scotomata and also characterizes the functional effects of common brain-based scotomata. The first aim of this thesis was to build, test and validate a system to accurately simulate the functional effects of brain-based scotomata. This was achieved using a high speed eye tracking system, a sophisticated visual display system, and custom software; together this system comprises the Visual Defect Simulator (VDS). In aim two the VDS was used to simulate scotomata for subjects while they performed a visual search task, a reading task, and a heading estimation task. Individuals with actual scotomata also performed the same three tasks. In all three of the experiments the visual difficulties exhibited by patients with actual scotomata could also be elicited in healthy subjects using simulated scotoma. From this we conclude that the loss of sensory input, not damage to higher-order brain areas, is the primary cause of visual difficulty in patients with brain-based scotomata. The third aim of this thesis was to assess the accuracy of predicting scotomata using fMRI maps of an individual subject\u27s visual cortex and a surgical scenario. Our results indicate that presurgical fMRI mapping can be used to estimate post-surgical scotomata. Overall this thesis provides an in depth assessment of the functional effects of various scotomata. Because the visual impairments experienced by patients appear to be caused primarily by the loss of sensory input, our data indicate that presurgical planning using fMRI visual field mapping could be used to minimize post-surgical vision impairments in patients undergoing cortically-invasive treatments