RESIDUAL VISION IN MULTIPLE RETINAL LOCATIONS WITHIN A SCOTOMA - IMPLICATIONS FOR BLINDSIGHT

There is an important new proposal that ''blindsight''-the ability to detect and identify visual stimuli by forced-choice guessing and in th e absence of conscious awareness when they fall in blind regions of th e visual field-is a function of residual ''islands'' of undamaged visu al cortex. This stands in contrast to the widely accepted view that bl indsight is exclusively a function of secondary visual pathways. Accor ding to the new view residual vision in blindsight should be patchy. T hus, when apparently wide areas of residual vision in blindsight are f ound, these may be due to eye-movements that allow stimuli to pass ove r retinal locations corresponding to islands of sparing. We tested thi s hypothesis by examining the distribution of residual vision in blind sight when the effects of eye movements on the retinal location of sti muli were minimized. We report a series of experiments that examined t wo-alternate forced-choice discrimination in the blind held of the sub ject GY. Using a dual-Purkinje image eye-tracker we applied three meth ods of minimizing the effects of retinal slippage due to eye-movements on discrimination performance: fixation stability dependent trials, s oftware image stabilization, and post hoc rejection of trials in which saccadic eye-movements were detected. In the first experiment, GY's d iscrimination performance was significantly above chance in 8 of 15 lo cations tested. In the subsequent experiments the subject knew the loc ation of the target in each block of trials, and this resulted in impr ovements to performance in a further three locations. Increasing the l uminance of the stimulus display (while maintaining 95% target contras t), and increasing the temporal discriminability of the forced choice produced performance above chance in all but two of the locations test ed. The consistent chance performance observed in two locations in the lower visual field nevertheless implies that GY's blindsight does not extend over the whole of his scotoma. Nevertheless, abolishing, or mi nimizing, the effects of eye-movements did not result in a loss of det ection in all the widely separated regions tested, and we thus conclud e that GY's blindsight cannot adequately be explained in terms of isla nds of spared vision. Islands may account for residual vision in scoto mata in some patients, but cannot be a universal account of the phenom enon of blindsight.