Retrograde and anterograde amnesia for spatial discrimination in rats: Role of hippocampus, entorhinal cortex, and parietal cortex

Abstract

The present study was conducted to examine whether lesions of hippocampus (HIP), entorhinal cortex (EC), or parietal cortex (PC) in rats produce retrograde amnesia for spatial two-choice discriminations in an eight-arm radial maze. Rats were trained successively on two single-pair discriminations, and lesions were performed approximately 24 h after the rat had acquired the second discrimination. After recovery from surgery, the animals were tested for retention of the two preoperatively acquired pairs at least for 2 days, and training continued until the animals reattained criterion performance for each of the pairs. Results indicated that when the two discrimination pairs were tested concurrently (Experiment 1), the HIP- and EC-lesioned rats were impaired in retention of the two presurgically acquired discriminations, whereas the PC lesioned rats displayed retention levels similar to the sham lesioned controls (SH). However, the EC- and PC-lesioned groups of animals were normal in reacquiring postoperatively the two pairs concurrently and in acquiring a new one-pair discrimination, whereas the HIP-lesioned rats displayed retarded relearning. Furthermore, under conditions in which the two pairs were tested successively, one at a daily session (Experiment 2), the two lesioned groups (HIP and EC) were still impaired in retention as well as in relearning of the discriminations in comparison with the control group. The HIP-lesioned rats, however, exhibited a significant improvement of performance in comparison with that of the HIP-lesioned rats in Experiment 1. The EC-lesioned rats displayed higher retention scores for the first (remote) than for the second (recent) pair acquired prior to surgery (temporal gradient). Furthermore, both the HIP- and the EC-lesioned rats were slower in acquisition of a new two-pair concurrent discrimination. These results were discussed in the context of current theories of learning and memory.