Does the dorsal hippocampus process navigational routes or behavioral context? A single-unit analysis.

In humans the hippocampus plays a role in both episodic memory and spatial navigation. Similar findings have been shown in other animals including monkeys and rats. The relationship between the processing of episodic and spatial related inputs within the hippocampus remains a puzzle. One approach to understanding how the hippocampus processes information is to examine how hippocampal cell activity corresponds to environmental experience. Hippocampal pyramidal cells can alter their spatial tuning (re-map) in response to changes in task demands. The degree to which this re-mapping is related to contextual/episodic information or to changes in spatial navigation/trajectories is unclear. The current study was designed to examine cell activity under two conditions that differed in contextual information without alterations in the goal-directed trajectories taken by the animals. Adult and aged rats were trained to do an alternation task on a fixed pathway [J. A. Oler et al. (2005)Neuroscience, 131, 1-12]. The animals ran this pathway during either 'safe' or 'unsafe' (a tone indicating a shock region) trials, with hesitation during 'unsafe' trials providing a clear behavioral measure of discrimination between these two conditions. Relatively few place cells displayed re-mapping between the two conditions. We propose that the principle source of re-mapping in the dorsal hippocampus is changes in the animal's trajectories rather than behavioral context. Possible reasons why so few cells responded to the change in context are discussed.