AN INVESTIGATION OF THE ROLE OF THE HIPPOCAMPUS AND THE AMYGDALA IN THE ENCODING OF A SERIAL PATTERN - EFFECTS OF A LONG INTER-ELEMENT INTERVAL

Previous research has provided convincing evidence that rats can learn to anticipate each element of a stimulus series consisting of differi ng reinforcement quantities. The present experiment examined the effec ts of electrolytic lesions of the hippocampus, amygdala, or combined h ippocampal and amygdala lesions on the acquisition of a 14-7-3-0 monot onic pattern with a 5 min inter-element interval. Anticipation was ope rationally defined as faster running on 14- than on 7-pellet elements, 7- than on 3-pellet elements, and 3- than 0-pellet elements. Male Spr ague-Dawley rats (sham and lesioned) received two trials a day for 30 days on the 14-7-3-0 pattern of food pellets. Anticipatory tracking de veloped rapidly in the control animals and not until the end of traini ng in the hippocampus + amygdala-lesioned rats, while the rate of acqu isition in the single lesion groups was intermediate between the two. The results are discussed in terms of temporal and working memory proc esses. Mediation of the temporal and affective attributes of memory ar e considered for each structure. The present experiment provided addit ional evidence for a hippocampal role in maintenance of a temporal rec ord of the elements of the series, thus providing the rat with the abi lity to track which stimulus elements have been presented and to antic ipate future elements. Finally, via a role in processing the affective and reinforcing properties of the task, the amygdala appears to play a role in mnemonic processes.