Essential role of the perirhinal cortex in complex tactual discrimination tasks in rats.

We designed a battery of tactual discrimination tasks to study whether rats with perirhinal cortex (Prh) lesions had any deficit in resolving complex/ambiguous tactual tasks in the dark. Animals had to discriminate among 3 stimuli simultaneously exposed in 3 arms of a 4-arm plus-shaped maze. Rats with Prh lesions showed a profound impairment in a texture discrimination learning task when the stimuli had a high or intermediate degree of feature ambiguity (experiments 1a and 1b), but not when they had a low degree of feature ambiguity (experiment. 1c). Hippocampal lesions, however, did not cause any impairment in task acquisition even when the stimuli had a high degree of feature ambiguity (experiment 2). Experiments 3a, 3b, and 4 showed that perirhinal and control rats performed the task similarly when the animals had to discriminate on the basis of simple/individual, nonoverlapping features of the stimuli (size) with different levels of difficulty. Finally, to isolate the task's memory functions from its perceptual functions, a reversal learning task revealed a profound deficit in the initial learning phase, but unimpaired learning in the reversal phase with identical stimuli (experiment 5). The findings suggest that the Prh plays an essential role in somatosensory perceptual functions.