Walsh & Hoyt: Blindsight and Residual Vision

Patients with visual loss due to lesions of the striate cortex or optic radiations may have some remnant visual function in their blind field. Some of these patients, including the often-studied patient G.Y., retain some awareness of visual stimuli and hence have residual vision, implying a severe field defect that is relative, not absolute. Others deny awareness of stimuli, even though they perform better than chance when asked to indicate or guess at some property of the stimulus: these are said to possess blindsight. Whether residual vision differs in pathophysiology from blindsight is unclear. For one, perceptual awareness varies along a spectrum: patients may retain a vague awareness of the presence of a stimulus, particularly of rapid onset or offset, but not of its particular features, which they can nonetheless discriminate or act upon. Also, stimulus parameters can be manipulated so that a patient shows residual vision under some conditions and blindsight under others. Similar stimulus manipulations can degrade awareness and generate blindsight-like performance in normal subjects.