Blindsight, Residual Vision

Abstract

A lesion within the geniculostriatal projection of the visual system produces a scotoma i.e., an area of blindness in a corresponding region of the visual field. Until recently, such blindness, determined by visual field measurement or perimetry, has been considered absolute. However, observations with nonhuman primates involving experimental ablation of striate cortex or parts of it have shown that visual information can still be processed in regions of the visual field that are blind. Such animals are observed to look toward targets that, because of their brain lesion, they are not supposed to see. On the basis of such evidence, the visual capacities of human subjects have been tested using nonverbal techniques. Most of the human subjects, i.e., patients that have suffered a lesion within the geniculostriatal projection system, can demonstrate some residual vision. They are able to look toward unseen targets within their scotoma.