The first study to demonstrate theta phase-locking between rat hippocampus and mPFC reported that approximately 40% of mPFC neurons were significantly phase-locked to CA1 theta rhythms [20]. mPFC spikes were maximally phase-locked to CA1 theta rhythms that occurred ~50 ms earlier, suggesting that CA1 theta entrains mPFC firing within the time scale of a single theta cycle. In this seminal study, recordings were collected during a variety of behaviors, leaving open the question of whether effects were behaviorally relevant. Shortly afterward, hippocampal theta phase-locking of mPFC neurons was reported to be associated with task-specific changes in mPFC firing rates in spatial navigation tasks [21]. Consistent with earlier findings [26], the firing rates of cells in mPFC correlated with particular aspects of behavior, such as running direction on a linear track. The majority of cells that exhibited theta phase-locking in only one running direction also had unidirectional behavioral correlates, and all cells that were theta phase-locked in both running directions also responded in both directions to particular behaviors. A subsequent study investigated the influence of hippocampal theta phase-locking of mPFC neurons
