Visual detection in monkeys with blindsight

Abstract

Monkeys with unilateral striate cortical removal show residual visual abilities in their affected hemifield. To learn whether the monkeys, like patients with blindsight, lose the phenomenal representation of the visual stimuli they nevertheless respond to, we first studied their ability to localize a briefly presented target in either hemifield. By varying the luminance of the stimuli we determined their visual sensitivity, which was reduced by 0.3-1.5 log units in the impaired hemifield; suprathreshold stimuli yielded almost perfect performance. We then presented two tests designed to show whether the monkeys categorized visual stimuli in the impaired field in the same manner as they categorize them in the normal field. In the first test, they had to respond differently according to whether one or two lights were presented, with the relative position of the two stimuli in a pair being varied. Whenever one of the paired stimuli lay in the impaired hemifield, two of the three monkeys consistently ignored it, and responded as if it had been a single stimulus in the good field. In the second test, trials consisting of a single stimulus light were interleaved with blank trials. The monkey touched the position of the light or made a different response, indicating that no light had appeared. All three monkeys responded to a light of supra-threshold luminance presented in the impaired field as if it were a blank trial. These results suggest that monkeys with striate cortical destruction, like neurological patients with similar lesions, have blindsight rather than phenomenal vision when they have to detect brief static visual targets.