Recognition memory correlates of hippocampal theta cells

Investigations of hippocampal theta cell activity have typically involved behavioral tasks with modest cognitive demands. Recordings in rats locomoting through space or engaged in simple stimulus discrimination or conditioning have revealed some place specificity and S(+)/S(-) selectivity in addition to the hippocampal EEG theta-related behavioral/motor correlates. However, little data exist regarding theta cell activity during performance of more cognitively demanding, hippocampal-dependent recognition memory tasks. Here, we examined the cognitive firing correlates of theta cells in rats that were performing an olfactory recognition memory task with distinct sample and test phases. Discriminant analysis revealed odor and match/nonmatch memory correlates in theta cell activity comparable in relative magnitude to that of the principal cells. Odor-specific theta cell responses in the sample phase were restricted primarily to CA1 and linked to task performance. In the test recognition phase, match/nonmatch theta cells were found primarily in the CA3 and CA1 fields, most of which exhibited greater activity on correct nonmatch trials in which recognition occurred than on error match trials in which recognition failed. Odor selectivity of the match/nonmatch signaling was greatest in the dentate gyrus (DG) and CA3 and least in CA1. This inverted pattern of stimulus specificity in the sample versus test phase was similar to that observed in principal cells but with a greater contrast between the CA1 and DG/CA3 fields. Together, these findings suggest that theta cells actively participate in hippocampal recognition memory processing and play a specific role in shaping the cognitive firing properties of the hippocampal principal cells.