Spatial representations of hippocampal CA1 neurons are modulated by behavioral context in a hippocampus-dependent memory task.

Although it is well known that hippocampal neurons code spatial information, it is less clear how these spatial representations are influenced by memory demands, especially in hippocampus-dependent tasks. Recently, our laboratory has demonstrated that hippocampal spatial representations are influenced by mnemonic factors in a T-maze continuous alternation task. Another unique experimental approach that might reveal the ways in which task-related factors impact hippocampal spatial representations is to compare firing patterns between events that require distinct episodic memory processes. Therefore, we recorded from CA1 single neurons during a discrete trial delayed-nonmatch-to-place task that allowed within-trial comparison between an encoding (sample) phase and a retrieval (choice) phase. A large subset of neurons that fired on the central stem of the maze showed dramatic selectivity for either the sample or choice phase of the trial. However, surprisingly, there were fewer neurons that showed differential firing rates between left- and right-bound trajectories. Our results suggest that trial-phase-selective coding is common in tasks that require rapid alternation between encoding and retrieval processes.