Separation of hippocampal and amygdaloid involvement in temporal memory dysfunctions

The role of the hippocampus and the amygdala in timing and in the memory of previously timed events was investigated in rats. Two testing procedures used the peak time (the time at which the maximum response rate occurred) to identify the time at which the rat expected reinforcement. Amygdala (AMG) lesions had no effect on the remembered time of reinforcement or on the ability to remember the duration of a previous stimulus. Fimbria-fornix (FF) lesions had two effects: these rats remembered the time of reinforcement as occurring earlier than it really did, and could not remember the duration of a previous stimulus even after a gap of only 0.5 s. This behavior pattern endured throughout testing in spite of reinforcement contingencies designed to eliminate it. Atropine, 0.45 mg/kg, caused control rats to forget the duration of a previous stimulus, while haloperidol, 0.15 mg/kg, did not. Taken together, these data indicate that the hippocampus, but not the amygdala, has an important role in the memory for time. They suggest that alterations in temporal processes may be intimately involved in the amnesic syndrome seen following damage to temporal lobe structures.