VISUAL EFFECTS OF LESIONS OF CORTICAL AREA V2 IN MACAQUES

Ibotenic acid lesions were placed in two monkeys in a portion of corti cal area V2 that corresponds to a lower quadrant of the visual field e xtending approximately 3-7-degrees-from the fovea. For purposes of com parison, another lesion was placed in area V1 in one animal. A wide ra nge of visual capacities were then measured, using a discrimination be tween vertical and horizontal orientation, in and near the affected re gions of the visual field. Visual acuity declined sharply as the test stimulus approached the visual field location corresponding to the V1 lesion, and no threshold could be measured at its center. In contrast, lesions of area V2 caused no measurable decrease in acuity, nor was t here any substantial effect on several measures of contrast sensitivit y. Subsequently, two types of more complex visual discriminations were measured (also using a vertical-horizontal discrimination), and these discriminations were severely disrupted by V2 lesions. The first disc rimination was of the orientation of two parallel lines of five coline ar dots each. We measured the number of background dots that would bri ng the discrimination to threshold, and this number of dots was greatl y decreased by a V2 lesion. The second discrimination was of the orien tation of a group of three distinctive texture elements embedded in a six by six element texture. This task could not be done in the visual field region affected by the V2 lesion when the distinctive elements d iffered in orientation from the others. Control experiments showed tha t the discrimination could be done when the three distinctive elements differed in size or color. These results suggest that cortical area V 2 is not needed for some low-level discriminations, but may be essenti al for tasks involving complex spatial discriminations.