Memory deficits following lesions of hippocampus or amygdala in rat: Assessment by an object-memory test battery

Abstract

We tested rats with bilateral lesions of the hippocampus or the amygdala on a battery of five object-memory tasks, which resemble those that have been used in the study of amnesia in humans and monkeys: (1) object discrimination, (2) discrimination reversal, (3) eight-pair concurrent object discrimination, (4) nonrecurring-items delayed nonmatching-to-sample (DNMS) with retention delays of 4, 15, 30, 60, and 120 sec and with lists of three, five, and seven samples, and (5) order discrimination. All testing was postsurgery. Relative to control rats, the rats with hippocampal lesions required more trials to master the object discrimination and the concurrent object discrimination. Rats with hippocampal lesions required about as many trials as did control rats to master DNMS at the 4-sec delay and displayed only mild deficits at the longest (120-sec) delay. Rats with amygdalar lesions required more trials to master the concurrent object discriminations than did controls, but significantly fewer than did rats with hippocampal lesions. They required more trials than did controls to master DNMS with a 4-sec delay; however, after they had done so, they continued to perform normally as the delay was increased. Neither lesion produced deficits on discrimination reversal, DNMS with lists, or order discrimination. While these findings demonstrate that the mnemonic effects of hippocampal and amygdalar lesions can be dissociated using a battery of object-memory tasks, they also suggest a limited role for these two structures in many object-memory abilities.