Encoding of a nonmonotonic serial pattern: role of the dorsal hippocampus and amygdala

The present experiment examined the effects of lesions of the hippocampus, amygdala, or combined hippocampal + amygdala lesions on the acquisition of a presumably difficult serial pattern. Lesioned rats, sham-surgical and nonsurgical control rats were trained in a runway to track one of a four-element series of Noyes food pellets consisting of 14-0-3-7 pellets, respectively. Control rats were capable of tracking the elements of the series. Tracking developed in the amygdala-lesion group by the middle stages of training, but tracking did not develop in the hippocampal-lesion group until the end of training and failed to develop in the hippocampus + amygdala-lesion group. The results are discussed in terms of hippocampus and amygdala involvement in working memory processes. The role of each structure in processing the temporal and affective attributes of the task is considered. Presumably, the hippocampus is necessary for tasks requiring the sequential coding of information. The hippocampus appears to maintain a temporal record of the elements of the series providing the rat with the ability to track which stimulus elements have been presented and which have not. In addition, by processing the affective attributes of the task, the amygdala appears to mediate mnemonic processes.