Dynamic coding of dorsal hippocampal neurons between tasks that differ in structure and memory demand

Hippocampal place fields show remapping between environments that contain sufficiently different contextual features, a phenomenon that may reflect a mechanism for episodic memory formation. Previous studies have shown that place fields remap to changes in the configuration of visual landmarks in an environment. Other experiments have demonstrated that remapping can occur with experience, even when the visual features of an environment remain stable. A special case of remapping may be trajectory coding, the tendency for hippocampal neurons to exhibit different firing rates depending upon recently visited or upcoming spatial locations. To further delineate the conditions under which different task features elicit remapping, we recorded from place cells in dorsal CA1 of hippocampus while rats switched between tasks that differed in memory demand and task structure; continuous spatial alternation (CA), delayed spatial alternation (DA), and tactile-visual conditional discrimination (CD). Individual hippocampal neurons and populations of simultaneously recorded neurons showed coherent remapping between CA and CD. However, task remapping was rarely seen between DA and CD. Analysis of individual units revealed that even though the population retained a coherent representation of task structure across the DA and CD tasks, the majority of individual neurons consistently remapped at some point during recording sessions. In contrast with previous studies, trajectory coding on the stem of the T-maze was virtually absent during all of the tasks, suggesting that experience with multiple tasks in the same environment reduces the likelihood that hippocampal neurons will represent distinct trajectories. Trajectory coding was, however, observed during the delay period of DA. Whether place fields change in response to task or trial type or remain stable within the same environment may depend on which aspects of the context are most salient or relevant to behavior.