Perirhinal cortex and visual discrimination learning in the rat

Sixteen Dark Agouti rats learned two sets of concurrent visual discriminati ons in a computer-operated apparatus at different time intervals (approxima tely 8 and 3 weeks) prior to perirhinal ablation or control surgery. Follow ing surgery, the perirhinal animals demonstrated a deficit in reacquisition of the discriminations learned preoperatively. There was no evidence that this deficit was temporally graded, but it was limited to initial postopera tive performance. Analysis of the speed to approach the stimulus or to retr ieve the reward revealed no differences between the groups, suggesting that non-mnemonic factors were not playing a significant role in the deficit. I n order to test postoperative acquisition of visual associative information , all animals learned anew and larger set of concurrent visual discriminati ons, thereby increasing demands on the processes of object identification. The perirhinal lesioned animals were not impaired in acquiring the new set of discriminations. These results suggest that the deficit may be transient .