Speculations on the neural basis of islands of blindsight

Blindsight, residual visual function in the absence of conscious awareness, can sometimes be found within the scotomas of patients with lesions to primary visual cortex. However, cases in which blindsight is distributed across large regions of a scotoma are extremely rare. In contrast, blindsight is relatively frequent within small islands of residual visual function. We review the evidence for the existence of these islands. We argue that blindsight is likely to depend on vestiges of geniculostriate function, and that in humans the secondary retinotectal pathway has little functional utility in the absence of geniculostriate support. To account for the frequency of blindsight within residual islands of function, we speculate that patients may be unaware of such islands precisely because they are islands, which are isolated from the integrated network of neural activity that represents visual space. The relationship of blindsight to the hemispatial neglect is considered in this context.