Blindsight in hemispherectomized patients as revealed by spatial summation across the vertical meridian.

The present study provides a demonstration of blindsight in two hemispherectomy patients who showed a visual spatial summation effect across the vertical meridian despite their lack of visual awareness in one hemifield. Such an effect cannot be related to light diffusion onto the sighted hemifield because it was not present when one of the stimuli fell into the retinal blind spot of control subjects. We conclude that blindsight phenomena of the simple type described in the present study can be subserved by sub-cortical mechanisms and do not necessarily require cortical processing.