Visual effects of lesions of cortical area V2 in macaques

Ibotenic acid lesions were placed in two monkeys in a portion of cortical area V2 that corresponds to a lower quadrant of the visual field extending approximately 3-7 degrees from the fovea. For purposes of comparison, another lesion was placed in area V1 in one animal. A wide range of visual capacities were then measured, using a discrimination between vertical and horizontal orientation, in and near the affected regions 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 there any substantial effect on several measures of contrast sensitivity. 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 discrimination was of the orientation of two parallel lines of five colinear dots each. We measured the number of background dots that would bring the discrimination to threshold, and this number of dots was greatly decreased by a V2 lesion. The second discrimination was of the orientation 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 differed in orientation from the others. Control experiments showed that the discrimination could be done when the three distinctive elements differed in size or color. These results suggest that cortical area V2 is not needed for some low-level discriminations, but may be essential for tasks involving complex spatial discriminations.