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Blindsight in Action: New Evidence from Patients with Lesions of Primary Visual Cortex (talk recording available)
Dr. Mel Goodale
Tuesday, November 08, 2011, 01:00pm - 02:00pm
The University of Western Ontario, The Centre for Brain and Mind
Human beings are capable of reaching out and grasping objects with great accuracy and precision - and vision plays a critical role in this ability. The visual guidance of skilled movements, however, requires transformations of incoming visual information that are quite different from those required for visual perception. For us to grasp an object successfully, our brain must compute the actual (absolute) size of the goal object, and its orientation and position with respect to our hand and fingers - and must ignore the relative size or distance of the object with respect to other elements in the visual array. These differences in the required computations have led to the emergence of dedicated visuomotor modules in the dorsal visual stream that are quite separate from the networks in the ventral visual stream that mediate our conscious perception of the world. Although the selection of goal objects and an appropriate course of action depends on the perceptual machinery of the ventral stream and associated cognitive modules, the programm ing and control of goal-directed actions are mediated by dedicated on-line control systems in the dorsal stream and associated motor areas. Although the dorsal stream receives prominent inputs from primary visual cortex, visual information also reaches this region of the brain via subcortical structures outside the geniculo-striate pathway. I will present evidence showing that individuals with large lesions of primary visual cortex can perform a broad range of skilled visually guided movements including scaling the hand in flight for the size of goal objects and avoiding obstacles in the workspace of the hand. In addition, I will present fMRI data showing that such "blindsight" patients continue to show task-related activation in dorsal-stream areas that have been implicated in the visual control of reaching and grasping. Taken together, these results suggest that extra-geniculostriate projections to the dorsal stream are capable of mediating the processing of object features such as size, shape, and orientation for the control of visually guided grasping and other skilled actions.