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.