VARIANTS OF OLFACTORY MEMORY AND THEIR DEPENDENCIES ON THE HIPPOCAMPAL-FORMATION

Olfactory memory in control rats and in animals with entorhinal cortex lesions was tested in four paradigms: (I)a known correct odor was pre sent in a group of familiar but nonrewarded odors, (2) six known corre ct odors were simultaneously present in a maze, (3) correct responses required the learning of associations between odors and objects, and ( 4) six odors, each associated with a choice between two objects, were presented simultaneously. Control rats had no difficulty with the firs t problem and avoided repeating selections in the second; this latter behavior resembles that reported for spatial mazes but, in the present experiments, was not dependent upon memory for the configuration of p ertinent cues. Control animals varied considerably in their acquisitio n of odor-object associations with only a subgroup learning every set of pairings. These latter animals also performed well in the fourth ta sk and, as indicated by post hoc analyses, developed complex strategie s in dealing with the problem of serial odor-object pairs. Lesioned an imals had no difficulty in selecting correct odors learned prior to su rgery (problem one) but repeated their choices in problem two. This la tter result suggests that hippocampus contributes to the transient mem ory of prior choices for odors as it does for prior choices in spatial mazes. Entorhinal rats were able to form odor-object associations (pr oblem three), and a subgroup of the animals periodically succeeded in doing a long series of such choices (problem four), though with less f requency than controls. These results indicate that rats use both long -term memory and transient memory in dealing with olfactory problems a nd suggest that the second of these is dependent upon a hippocampal pr ocess that encodes a type of information other than the relationship b etween cues.