Journal of Neuroscience, Vol 13, 3180-3191, Copyright © 1993 by Society for Neuroscience
Visual effects of lesions of cortical area V2 in macaques
WH Merigan, TA Nealey and JH Maunsell
Center for Visual Science, University of Rochester, New York 14642.
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.
