Impaired recognition memory in rats after damage to the hippocampus

Abstract

Rats with radio-frequency or ibotenic acid lesions of the hippocampus and rats with radio-frequency lesions of the fornix were tested on the visual paired comparison task (VPC), a test of recognition memory. Memory was assessed at five different delay intervals ranging from 10 sec to 24 hr. All operated groups performed normally at the shorter delays (10 sec and 1 min). Across longer delays, the two groups with hippocampal damage were impaired. Rats with fornix lesions performed well on the VPC task but were impaired on a spatial task (spontaneous alternation). The results show that the hippocampus is essential for normal recognition memory. Moreover, fornix lesions need not mimic the effects of direct damage to hippocampal tissue. The findings are discussed in the context of the contribution of the hippocampus to recognition memory.

Fig. 1.

Reconstructions of coronal sections of the largest (striped) and smallest (black) lesions in the rats with radio-frequency lesions of the hippocampus (H-RF), ibotenic acid lesions of the hippocampus (H-IBO), ibotenic acid lesions of the cortex dorsal to the hippocampus (CTX), and radio-frequency lesions of the fornix (FX). Each series of sections progresses (left to right) from anterior to posterior levels. Numbers represent the distance in millimeters posterior to bregma.

Fig. 2.

Percent preference for the novel object across 30 sec of cumulative object exploration (CON group; n= 16). The data are from the test phase (all delays combined). Preference for the novel object first increased and then gradually decreased during the 30 sec test period. Error bars indicate SEM.

Fig. 3.

Mean time that the CON group spent at the novel and familiar objects on each visit (all delays combined). Visits to the novel object lasted longer than visits to the familiar object. Error bars indicate SEM.

Fig. 4.

Percent preference (30 sec time bin) for the novel object across five delays for two control groups (CONand CTX) and two groups with radio-frequency or ibotenate lesions of the hippocampus (H-RF andH-IBO).

Fig. 5.

Percent preference (30 sec time bin) for the novel object across five delays for two control groups (CON+CTX, combined) and the group with fornix lesions (FX).