Dorsal hippocampal involvement in appetitive trace conditioning and interval timing.
Involvement of the dorsal hippocampus (DHPC) in acquisition of Pavlovian trace conditioning and interval timing was examined in an appetitive preparation in which presentations of one conditioned stimulus (CS) were immediately followed by food (delay conditioning), and presentations of another CS were followed by food 15 seconds after its termination (trace conditioning). DHPC lesions did not disrupt acquisition of trace conditioning, but they selectively affected the distribution of conditioned responding over the course of the trace CS in the early stage of acquisition. In addition, lesions disrupted accuracy of timing the conditioned response (CR) for both delay and trace CSs: The control subjects showed maximum CR at the time of food delivery, but in the DHPC-lesioned subjects, the maximum CR was at an earlier time point. This timing deficit did not seem to be due to impulsive responding or deficits in response inhibition because, when the early portion of the delay CS was interrupted shortly by an empty interval, the difference in the time of maximum responding between the lesioned and control subjects was eliminated. Thus, although the involvement of the DHPC in appetitive trace conditioning was not found when a gross measure of conditioning was employed, it was revealed when the temporal distribution of conditioned responding was examined on a moment-by-moment basis as in eyeblink trace conditioning studies.