Variations in CS associability and multiple unit hippocampal activity in the rabbit.

Hippocampal multiple unit activity was recorded in rabbits during each of four preacquisition treatments and during subsequent classical conditioning of the nictitating membrane response. Three preexposure conditions were employed: CS alone presentations, presentations of the CS paired with a second, neutral stimulus, or unpaired presentations of the CS and second stimulus. It was predicted that (a) CS alone preexposures would produce a decrease in hippocampal activity and a retarded rate of subsequent conditioned response (CR) acquisition and (b) the magnitude of both effects would be attenuated by preexposures of the CS paired with a second stimulus. The results partially supported both predictions. Hippocampal activity was inhibited during CS alone preexposures and that inhibition was attenuated by pairing the CS with a second, neutral stimulus. Behaviorally, all of the preexposure groups showed equivalent, retarded rates of acquisition compared to a nonpreexposed control group. Hippocampal activity throughout acquisition was significantly greater in the nonpreexposed group compared to the group preexposed to the CS alone. Hippocampal activity of the other two groups was intermediate between the nonpreexposed and the CS alone groups. It is suggested that alterations in the magnitude of hippocampal activity may provide a reliable, neuronal correlate of CS associability changes.