VISUAL DETECTION IN MONKEYS WITH BLINDSIGHT

Monkeys with unilateral striate cortical removal show residual visual abilities in their affected hemifields. To learn whether the monkeys, like patients with blindsight, lose the phenomenal representation of t he visual stimuli they nevertheless respond to, we first studied their ability to localize a briefly presented target in either hemifield. B y varying the luminance of the stimuli we determined their visual sens itivity, which was reduced by 0.3-1.5 log units in the impaired hemifi eld; suprathreshold stimuli yielded almost perfect performance. We the n presented two tests designed to show whether the monkeys categorized visual stimuli in the impaired held in the same manner as they catego rize them in the normal field. In the fi;st test, they had to respond differently according to whether one or two lights were presented, wit h the relative position of the two stimuli in a pair being varied. Whe never 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 c onsisting of a single stimulus light were interleaved with blank trial s. The monkey touched the position of the light or made a different re sponse, indicating that no light had appeared. All three monkeys respo nded 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 si milar lesions, have blindsight rather than phenomenal vision when they have to detect brief static visual targets. (C) 1997 Elsevier Science Ltd.