Retrosplenial cortex contributes to spatial memory, navigation and hippocampal representations of space

There is an emerging consensus that, although navigation critically depends on the integrity of the hippocampus, multiple brain areas contribute to this complex behavior. However, how different areas of the brain contribute to hippocampal processing of spatial information has received little attention. The experiments described in this dissertation are designed to address the functional contribution of retrosplenial cortex to spatial learning, memory, and hippocampal representations of space. The first experiment demonstrated that the retrosplenial cortex is necessary for accurate navigation when visual cues are obscured. The second experiment expanded on this initial result and showed that retrosplenial cortex is critical for spatial memory, as well as navigation in darkness. In the third experiment, the contribution of retrosplenial cortex to hippocampal processing of space was examined. It was found that when the retrosplenial cortex was temporarily inactive, by infusion of a local anesthetic, there were changes in hippocampal processing of space. Changes in spatial coding were observed in both light and dark testing conditions. Behavioral performance was only impaired when visual cues were removed. Thus, single unit activity in hippocampus may not always correlate with behavioral performance. In the fourth experiment, the retrosplenial cortex was permanently damaged, and hippocampal place coding was compared between lesion and sham control groups. Single cells recorded in the lesion group were more dependent on local maze cues and were less likely to follow visual cues when they were rotated from their standard position. The final chapter provides a summary and a model, which suggests that memory-guided navigation is controlled by in an interactive neural system that includes the retrosplenial cortex and the hippocampus. It is proposed that the unique contribution of the retrosplenial cortex is to integrate mnemonic spatial information (visual and nonvisual cues) with movement-generated cues. Such information is proposed to be essential for normal spatial learning and for navigation when visual cues are removed. This information is sent to the hippocampal system, and may be used to guide and direct navigation.