Contour discrimination in the rat following removal of posterior neocortex

Long-Evans hooded rats were trained on one of three two-choice visual discrimination problems. The stimuli, black equilateral triangles on white Plexiglas, were equated for total luminous flux and total black-white area, but differed in amount of contour, defined as total amount of black-white edge. The ratios of contour varied for each of the three problems, being respectively 3:1, 4:1 or 5:1. Following acquisition of the problems, posterior neocortical ablations were sustained by at least 30 animals in each group, the remainder serving as normal controls. When retested, normal animals retained the problem, while operated animals were unable to relearn the 3:1 or 4:1 ratio problems. Operated animals did relearn the 5:1 ratio contour problem, although they required more than twice as many trials than during original learning. These results suggest that rats deprived of visual cortex can make visual discriminations based upon differences other than total luminous flux. Density distributions of light or stimulus differences in amounts of darklight boundaries appear to be the likely parameters which, when large enough, permit discrimination of such problems.