Hippocampal function is required for feedback control of an internal clock's criterion.

A discrete-trial peak-interval procedure was used to evaluate the effects of hippocampal damage on the control of an internal clock's criterion. Rats first received either lesions of the fimbria-fornix or sham operations. Following surgery rats were trained on a 20-s peak-interval procedure and later were transferred to a 10-s peak-interval procedure. Rats with sham operations were maximally responsive about the time that reinforcement was sometimes made available (10 or 20 s) and showed an oscillation of successive peak-time values similar to biological feedback control systems. In contrast, rats with fimbria-fornix lesions were maximally responsive at a time about 20% earlier than the time that reinforcement was made available (8 or 16 s) and showed no control of successive peak-time values. Taken together, these results demonstrate that a fimbria-fornix lesion reduces the remembered time of reinforcement stored in reference memory, interferes with the internal control of temporal criteria stored in working memory, and has no effect on the animal's sensitivity to stimulus duration or the acquisition of a new temporal criterion.