Hippocampus, time, and memory

Five experiments determined the effects of hippocampal damage on timing and the memory for temporal events. Norway rats ( N = 20) were trained to discriminate between auditory signals that differed in both duration (2 or 8 sec) and rate (2 or 16 cycles/sec). After Ss acquired the discrimination, signals with intermediate durations and rates were presented. Ss then received either fimbria-fornix lesions or control operations. Postoperatively, the accuracy of duration and rate discriminations as measured by the difference limen (DL) was unaffected by the lesion, but the point of subjective equality was shifted to a shorter duration and a slower rate by the lesion. Both Ss with lesions and Ss with control operations showed cross-modal transfer of duration and rate from the auditory signals used in training to visual signals. When a 5-sec delay was imposed between the end of a signal and the opportunity to respond, lesioned Ss were selectively impaired by the addition of the delay as measured by an increase in the DL. When a peak procedure was employed, the maximum response rate of controls was approximately at the time of scheduled reinforcement (20 sec), but the maximum response rate of lesioned Ss was earlier than the time of reinforcement. When a 5-sec gap was imposed in the signal, controls summed the signal durations before and after the gap, whereas lesioned Ss showed no retention of the signal duration prior to the gap. Lesions impaired spatial working memory in an 8-arm radial maze. (51 ref)