Walsh & Hoyt: Blindsight and Hemidecortication

Patients lacking a cerebral hemisphere are of obvious interest in the debate over the role of extrastriate cortex in blindsight functions. Visual localization studies in blindsight were motivated by the hypothesis that the superior colliculus could mediate this function in the absence of striate cortex. Indeed, some have found that hemidecorticate infants will look toward their blind field when a target is presented there, and adults with such lesions have some residual manual localization of blind field targets, as well as a surprising awareness of these targets. Beyond localization, discrimination of motion and speed, but not direction, was reported in three patients, but the results could be explained by response bias. Some elementary form perception was reported in 3 of 10 patients. A ""spatial summation"" effect, in which normal subjects respond faster to two simultaneous flashes of light than to one, was found in two of four patients, even when one of the twin flashes was in the blind hemifield. A functional MR imaging (fMRI) study of these same patients suggested that blindsight may not be mediated solely by the superior colliculus, but also by activity in the ipsilateral V5 area. Similar ipsilateral extrastriate activity was found in a positron emission tomography (PET) study of another patient.