BILATERAL EFFECTS OF UNILATERAL VISUAL-CORTEX LESIONS IN HUMAN

We studied the vision of 12 patients with unilateral lesions of the vi sual cortex. All had a VI-type scotoma located in the contralateral vi sual fields, as expected, and visual acuity of 20/30 or better. Our ai m was to test the hypothesis that they also had a visual deficit in th eir ipsilesional or 'good' visual fields. The first experiment tested the subjects' ability to respond to transient signals presented at unp redictable temporal intervals and spatial locations amongst many spati ally random and identical distracter elements. The results showed that , compared with controls, the lesion group had a significantly reduced sensitivity to signal and increased response times affecting both hem ifields. In a second experiment, we tested the useful field of view (U FOV) in two of the patients under conditions of differing attention de mand. Both showed bilateral constriction, compatible with the results of the first experiment. One possible explanation for the bilateral ef fects of unilateral occipital lobe lesions is damage to interhemispher ic connections along their presplenial course, affecting the synthesis of visual information from both hemifields (i.e. the interhemispheric diaschisis effects put forth by von Monakow). The trouble is task dep endent and can be construed as a global reduction in visual attention capacity. It is subtle in comparison with the contralesioned V-1-type scotoma that Holmes measured, yet may account for unexplained complain ts of reduced performance in some patients, particularly in tasks with high visual information processing demands, such as reading and autom obile driving.