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This study presents a double dissociation between the dentate gyrus (DG) and CA1. Rats with either DG or CA1 lesions were tested on tasks requiring either spatial or spatial temporal order pattern separation. To assess spatial pattern separation, rats were trained to displace an object which covered a baited food-well. The rats were then allowed to choose between two identical objects: one covered the same well as the sample phase object (correct choice), and a second object covered a different unbaited well (incorrect choice). Spatial separations of 15-105 cm were used to separate the correct object from the incorrect object. To assess spatial temporal order pattern separation, rats were allowed to visit each arm of a radial eight-arm maze once in a randomly determined sequence. The rats were then presented with two arms and were required to choose the arm which occurred earliest in the sequence. The choice arms varied according to temporal separation (0, 2, 4, or 6) or the number of arms that occurred between the two choice arms in the sample phase sequence. On each task, once a preoperative criterion was reached, each rat was given either a DG, CA1, or control lesion and then retested. The results demonstrated that DG lesions resulted in a deficit on the spatial task but not the temporal task. In contrast, CA1 lesions resulted in a deficit on the temporal task but not the spatial task. Results suggest that the DG supports spatial pattern separation, whereas CA1 supports temporal pattern separation.

The present experiments investigated the role of the prelimbic-infralimbic areas in behavioral flexibility using a place-response learning paradigm. All rats received a bilateral cannula implant aimed at the prelimbic-infralimbic areas. To examine the role of the prelimbic-infralimbic areas in shifting strategies, rats were tested on a place and a response discrimination in a cross-maze. Some rats were tested on the place version first followed by the response version. The procedure for the other rats was reversed. Infusions of 2% tetracaine into the prelimbic-infralimbic areas did not impair acquisition of the place or response discriminations. Prelimbic-infralimbic inactivation did impair learning when rats were switched from one discrimination to the other (cross-modal shift). To investigate the role of the prelimbic-infralimbic areas in intramodal shifts (reversal learning), one group of rats was tested on a place reversal and another group tested on a response reversal. Prelimbic-infralimbic inactivation did not impair place or response intramodal shifts. Some rats that completed testing on a particular version in the cross-modal and intramodal experiments were tested on the same version in a new room for 3 d. The transfer tests revealed that rats use a spatial strategy on the place version and an egocentric response strategy on the response version. Overall, these results suggest that the prelimbic-infralimbic areas are important for behavioral flexibility involving cross-modal but not intramodal shifts.

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The purpose of this review is to determine whether specific subregions (dentate gyrus [DG], CA3, and CA1) of the hippocampus provide unique contributions to specific processes associated with intrinsic information processing exemplified by novelty detection, encoding, pattern separation, pattern association, pattern completion, retrieval, short-term memory and intermediate-term memory. Based on anatomical neural network organization, electrophysiology of cellular activity, lesions, early gene activation, and computational modeling, it can be shown that there exists extensive cooperation among the three subregions of the hippocampus, but there also exists reliable specificity of function for each of the subregions of the hippocampus. The primary process supported by the DG subregion of the hippocampus can be characterized by orthogonalization of sensory inputs to create a metric spatial representation. Furthermore the DG participates in conjunction with CA3 in supporting spatial pattern separation. The CA3 subregion of the hippocampus supports processes associated with spatial pattern association, spatial pattern completion, novelty detection, and short-term memory. The CA1 subregion of the hippocampus supports processes associated with temporal pattern association, temporal pattern completion, and intermediate-term memory. Furthermore, the CA3 in conjunction with CA1 supports temporal pattern separation. All the above-mentioned processes are assumed to reflect intrinsic processing of information within the hippocampus. The diversity of functions associated with the different subregions of the hippocampus suggests that one should not treat the hippocampus as a single entity, but rather that one should concentrate on elucidating further the functions of both dorsal and ventral subregions of the hippocampus and pathways that directly connect each of the subregions as well as their connections with the entorhinal cortex.