Dorsal hippocampal contributions to unimodal contextual conditioning.

Although there is general consensus that the hippocampus is not critically involved in the acquisition of fear conditioned to an explicit conditioned stimulus (CS), the extent to which the hippocampus participates in contextual fear conditioning remains unclear. To further characterize the potential role of the hippocampus in contextual fear conditioning, the present experiments examined the effect of excitotoxic lesions of dorsal hippocampus on the acquisition of a novel contextual fear conditioning paradigm in which a unimodal (olfactory) cue served to disambiguate discrete "contexts" within a single behavioral training chamber. Selective lesions of dorsal hippocampus severely attenuated olfactory contextual conditioning without affecting conditioning to an explicit auditory or olfactory CS. Additional experiments indicate that these contextual conditioning deficits cannot be attributed to a lesion-induced decrement in olfactory perception, a preferential impairment of "weak" forms of conditioning, or hyperactivity. Thus, the hippocampus appears to contribute importantly to the acquisition of fear conditioned to explicitly nonspatial, unimodal, temporally, and spatially diffuse contextual stimuli.