Hippocampal lesions prevent trace eyeblink conditioning in the freely moving rat.

The effect of hippocampal aspiration lesions on trace eyeblink conditioning was examined in young, freely-moving F1 hybrid rats (Fisher 344 x Brown Norway). Rats which received either bilateral neocortical or bilateral hippocampal aspiration lesions were compared with each other or with sham lesioned control rats. The rats were trained with a 250 ms tone conditioning stimulus (CS), a 250 ms stimulus free trace interval and a 100 ms corneal airpuff unconditioned stimulus (US). Rats with lesions of the hippocampus were significantly impaired relative to the neocortical and sham lesioned control rats. Analyses of different behavioral parameters (e.g. percent conditioned responses, amplitude, and area of response) indicated that all of the measures for the conditioned response were significantly impaired by the hippocampal lesion. The unconditioned response was not significantly affected by the lesion, and there was no significant difference among the groups after 2 days of subsequent conditioning with the delay paradigm (zero trace interval). We conclude that the hippocampus is required for rats to learn the association between a tone CS and an airpuff US when a 250 ms trace interval is interposed between the two stimuli.