RETROGRADE-AMNESIA AND MEMORY REACTIVATION IN RATS WITH IBOTENATE LESIONS TO THE HIPPOCAMPUS OR SUBICULUM

The retrograde effects of hippocampal lesions on spatial memory were s tudied. Rats were given a series of 48 place-navigation trials in an o pen-field water-maze followed, either 3 days or 14 weeks later, by ibo tenic acid lesions of the hippocampus (HPC) or subiculum (SUB), or by sham-surgery (SHAM). Two weeks after surgery they were given a retenti on test without a hidden escape platform. There was a significant decl ine in performance with time in the SHAM group, but with the 14-week S HAM group performing significantly better than chance levels, whereas both lesioned groups performed at chance at both retention intervals. All Tats were then retrained for 24 trials. SHAM rats escaped rapidly within 2 trials, suggesting a reactivation of memory rather than relea rning. The HPC groups were severely impaired during retraining, with a developing trend towards better performance in the 3-day group. After 24 trials of training with the escape platform placed in the opposite quadrant of the pool, this new location was learned successfully by S HAm and SUB rats, but not by Hpc rats. These results indicate that sel ective hippocampal formation lesions can cause deficits in retrieval b ut do not reveal a time-dependent gradient of memory consolidation.