Bilateral effects of unilateral visual cortex lesions in human.

We studied the vision of 12 patients with unilateral lesions of the visual cortex. All had a VI-type scotoma located in the contralateral visual fields, as expected, and visual acuity of 20/30 or better. Our aim was to test the hypothesis that they also had a visual deficit in their ipsilesional or 'good' visual fields. The first experiment tested the subjects' ability to respond to transient signals presented at unpredictable temporal intervals and spatial locations amongst many spatially 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 hemifields. In a second experiment, we tested the useful field of view (UFOV) in two of the patients under conditions of differing attention demand. Both showed bilateral constriction, compatible with the results of the first experiment. One possible explanation for the bilateral effects of unilateral occipital lobe lesions is damage to interhemispheric 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 dependent and can be construed as a global reduction in visual attention capacity. It is subtle in comparison with the contralesional V1-type scotoma that Holmes measured, yet may account for unexplained complaints of reduced performance in some patients, particularly in tasks with high visual information processing demands, such as reading and automobile driving.