Effect of the interstimulus (CS–UCS) interval on hippocampal unit activity during classical conditioning of the nictitating membrane response of the rabbit (Oryctolagus cuniculus).

Abstract

Neuronal activity (multiple unit) was recorded from the dorsal hippocampus during classical conditioning of the nictitating membrane response in the rabbit. Groups of subjects were trained with conditioned stimulus/unconditioned stimulus (CS-UCS) interstimulus intervals of 250 ms, 150 ms or 50 ms, and an additional group received unpaired presentations of the CS and UCS, with a tone CS and a corneal air puff UCS. Increased hippocampal unit activity and nictitating membrane conditioned responses (CR) occurred only in the 250 ms and 150 ms groups. Both the temporal distribution of hippocampal unit activity and the averaged CR topographies were strongly and similarly influenced by the CS-UCS interval, with both neuronal and behavioral response peaks occurring near the time of UCS onset. The peak of hippocampal unit activity preceded the peak of the nictitating membrane CR by 30-60 ms. When the 250 ms group was shifted to a CS-UCS interval of 500 ms, the peaks of both the hippocampal unit response and the behavioral CR topography shifted in the same direction. The shift appeared to occur sooner in the hippocampal unit response. The hippocampus may function to provide a temporal map for learned behaviors.