Abstract
Electrolytic destruction of the dorsal hippocampus prevented the acquisition of a conditioned reflex to time regardless of the volume of brain damage. A similar effect was obtained in rats with combined lesions consisting of damage to the suprachiasmatic nuclei of the hypothalamus and hippocampectomy (in the same animal). Denucleation (without hippocampectomy), conversely, accelerated the acquisition of this type of time-related behavior. Lesions to the hippocampus had no effect on the recovery of the conditioned reflex to time when this had been fixed before surgery in denucleated rats. It is proposed that the suprachiasmatic nuclei and hippocampus have simultaneous roles in organizing the endogenous counting of short time periods.