Combined lesions of hippocampus and subiculum Do not produce deficits in a nonspatial social olfactory memory task

Abstract

Rats transmit information to each other about which foods are safe to eat. If a rat smells a food odor on the breath of another rat, it is subsequently more likely to eat that food than an alternative. Work by Galef et al. (1988) has shown that the observer rat forms an association between two olfactory stimuli on the breath of the demonstrator rat that has eaten the food, the food odor and carbon disulphide, which is normally present in the rat breath. Bunsey and Eichenbaum (1995) claimed that the hippocampus/subicular region is required for the long-term retention of this nonspatial form of associative memory on the basis that combined lesions of the hippocampus and subiculum produced a deficit, but lesions of either structure alone did not. We report here a failure to repeat this finding. Rats with either combined lesions of the hippocampus and subiculum or with amygdala lesions were tested on their ability to remember this association either immediately (testing short-term memory) or after a 24 hr delay (testing long-term memory). Neither lesion group exhibited significant memory deficits on this nonspatial associative task at either test interval. In contrast, a deficit was observed on a spatial memory task (forced-choice alternation t-maze) for animals with combined lesions of the hippocampus and subiculum. These results contradict the findings of Bunsey and Eichenbaum (1995) and support the idea that the hippocampus/subicular region is not required for this nonspatial associative memory.

Fig. 1.

Average preference of 12 normal control animals for each flavor within a pair.

Fig. 2.

Average percentage of the target food eaten by the lesion group and the control animals either immediately (left) or after a 24 hr delay period (right) in experiment 2.

Fig. 3.

Average percentage of the target food eaten by the three groups either immediately (left) or after a 24 hr delay period (right) in experiment 3.

Fig. 4.

Average percentage of the target food eaten either immediately (left) or after a 24 hr delay period (right) in experiment 4.

Fig. 5.

Average total amount of food eaten in experiments 3 and 4.

Fig. 6.

Mean correct scores of animals with hippocampus/subiculum lesions, amygdala lesions, and controls on the forced-choice spatial alternation in experiment 5.

Fig. 7.

A reconstruction of the largest (gray) and smallest (black) hippocampus/subiculum lesions. Lesions of the left and right hemisphere are shown on the right. The drawings of horizontal sections are adapted from the atlas of Paxinos and Watson (1986).

Fig. 8.

Representative lesion from the hippocampus/subiculum group in experiments 3–5 (C, D) with a control for comparison (A, B).

Fig. 9.

A reconstruction of the largest (gray) and smallest (black) amygdala lesions. Lesions of the left and right hemisphere are shown on the right. The drawings are adapted from the atlas ofPaxinos and Watson (1997)

Fig. 10.

A lesion from amygdala group in experiments 3–5 (B) with a control for comparison (A).