The role of the perirhinal cortex in visual learning in the rat

Abstract

The aim of this thesis was to investigate the role of the perirhinal cortex in visual discrimination learning in the rat. Experiment one measured postoperative reacquisition of two sets of concurrent visual discriminations acquired at different time intervals prior to surgery. Perirhinal ablation did not result in a temporally graded retention deficit, but a deficit limited to immediate postoperative performance. Experiment two measured postoperative acquisition of a new set of concurrent visual discriminations, with a stimuli set of 15 pairs, thereby increasing demands on stimuli identification. It was found that perirhinal ablation did not affect postoperative acquisition. Experiment 3a measured postoperative acquisition of a two choice visual discrimination and generalisations to it. Perirhinal ablation led to an impairment in performing the visual discrimination when presented in the generalisation task. Experiment 3b measured the effects of perirhinal ablation on postoperative performance in generalisation to a visual discrimination learnt prior to surgery. It was found that perirhinal ablation led to a deficit in generalising to transformations in stimuli form, but not to transformations in stimuli size. Experiment four measured acquisition of a titrating visual generalization task that increased demands on stimuli identification. Perirhinal ablation impaired acquisition of this task. Experiment five was designed to measure the effects of perirhinal ablation on acquisition of a generalization task with complex visual stimuli. However, neither the sham or perirhinal animals succeeded in learning the task. The final experiment measured postoperative acquisition of a simple visual discrimination and its partial reversal and acquisition of a biconditional visual discrimination task. Perirhinal ablation impaired acquisition of the biconditional discrimination, whereas acquisition of the simple discrimination and its reversal remained intact. Therefore, perirhinal ablation in the rat leads to selective impairments in the acquisition and retention of visual discrimination learning. These findings suggest that the perirhinal cortex may contribute to discrimination learning tasks that require the identification of complex visual stimuli.