No abstract

Previous literature suggests that the hippocampus subserves processes associated with the encoding of novel information. To investigate the role of different subregions of the hippocampus, the authors made neurotoxic lesions in different subregions of the dorsal hippocampus (i.e., CA1, dentate gyrus [DG], or CA3) of rats, followed by tests using a spontaneous object exploration paradigm. All lesion groups explored normally an object newly introduced in a familiar location. However, when some of the familiar objects were moved to novel locations, both DG and CA3 lesion groups were severely impaired in reexploring the displaced objects, whereas the CA1 lesion group was only mildly impaired in reexploration. The results suggest that the DG-CA3 network is essential in detecting novelty for spatial, but not for individual object, information.

It has been suggested that the dentate gyrus (DG) and CA3 cooperate to efficiently process spatial information. The DG has been proposed to be important for fine spatial discrimination, and the CA3 has been proposed to mediate larger scale spatial information processing. To evaluate the roles of the DG and CA3a,b for spatial processing, we developed a task that measures responses to either overall environmental novelty or a response to more subtle changes within the environment. Animals with lesions to the DG showed impaired novelty detection for both environment as well as smaller changes in the environment, whereas animals with lesions to CA3a,b showed no such deficits. A closer look at the lesions suggested that the CA3 lesions included only CA3a and CA3b, but spared CA3c. To test the role of the spared CA3c region, animals with selective lesions to CA3c that spared CA3a,b were run on the same task and showed an intermediate pattern of deficits. These results suggest that the DG is critical for spatial information processing. These data also suggest that CA3 is a heterogeneous structure, with CA3c lesioned animals showing greater spatial processing deficits than CA3a,b lesioned animals. These findings extend our knowledge of hippocampal function and need to be accounted for in future computational models.

Several studies have demonstrated the significance of a spatial cognitive map and its role for guided and accurate navigation through the environment. Learning and recalling spatial knowledge depends upon proper topological and metric spatial information processing. The present objectives are to better characterize the role of the hippocampus for processing topological and metric spatial information. Rats with dorsal hippocampal subregional lesions (dDG, dCA3, dCA1) were tested on a previously established metric task and topological task. The results of the present study suggest that dCA1, but not dDG or dCA3, mediates topological memory. Furthermore, dDG, dCA3, and dCA1 mediate metric memory. Dorsal DG is required for spatial information processing via pattern separation or orthogonalization of sensory inputs to generate metric representations. Dorsal CA3 and dCA1 then receive these metric representations transmitted from dDG along the trisynaptic loop. The present data add to a growing body of literature suggesting a diversity of function among the hippocampal subregions.