Post-training excitotoxic lesions of the dorsal hippocampus attenuate generalization in auditory delay fear conditioning.

An abundance of evidence indicates a role for the dorsal hippocampus (DH) in learning and memory. Pavlovian fear conditioning provides a useful model system in which to investigate DH function because conditioning to polymodal contextual cues, but generally not to discrete unimodal cues, depends upon the integrity of the DH. There is some suggestion that the hippocampus may be involved in generalization to discrete auditory stimuli following conditioning, but the available literature offers conflicting results regarding the nature of hippocampus involvement. The present experiments were designed to address a role for the DH in auditory generalization following delay fear conditioning. Rats were trained with two or 16 trials of delay fear conditioning and subsequently given a neurotoxic lesion of the DH or sham surgery. Upon recovery, they were tested for fear conditioned responding to the auditory stimulus they were trained with, as well as generalized responding to a novel auditory stimulus. Sham animals showed substantial generalization to the novel stimulus when trained with two or 16 trials. However, lesion animals showed much less generalization (better discriminative performance) to the novel stimulus following 16 conditioning trials while still showing substantial fear conditioned freezing to the trained stimulus. A second experiment showed that this effect was not the result of a non-associative response to the novel stimulus. We conclude that, with extended training, animals become capable of discriminating between trained and novel stimuli but another hippocampus-dependent process maintains generalized responding.