Neurotoxic lesions of the rat perirhinal and postrhinal cortices and their impact on biconditional visual discrimination tasks.

It has been argued that damage to the perirhinal cortex should impair visual discriminations when the stimuli have overlapping features. In Experiment 1, rats with perirhinal cortex lesions were trained on a series of visual discriminations in a water tank, culminating in a biconditional discrimination. No evidence was found of a perirhinal lesion deficit, although the same rats showed an object recognition deficit. In Experiment 2 the lesions were extended to involve both the perirhinal and postrhinal cortices in a new group of rats. An impairment was now found on acquisition of the biconditional task, but this was not specific as impairments were also found on two elemental visual discriminations. Taken together, the study failed to find evidence that the rat perirhinal cortex is necessary for configural visual discriminations and so revealed that some ambiguous visual discriminations can be learnt when this area is removed. Furthermore, there was no evidence that the parahippocampal region is selectively dedicated to configural learning, even though the loss of this area can impair the acquisition of some configural tasks.