In the hippocampus, separate slow and fast gamma oscillations have been observed to preferentially occur on significantly different phases of theta [31••], raising the question of whether hippocampal theta-modulated gamma in mPFC is slow or fast. One study reported hippocampal theta modulation of slow (33–55 Hz) gamma oscillations in mPFC [30]; however, the original study reported hippocampal theta modulation of primarily fast (>100 Hz) gamma in mPFC [22]. It remains unclear why frequency differences were observed between the two studies, but it is possible that hippocampal theta modulates slow and fast gamma in mPFC at different times, reminiscent of how hippocampal theta modulates slow and fast gamma oscillations at different times in CA1 [31••]. In line with this idea, separate slow and fast gamma peaks are apparent in coherence spectra for paired mPFC–ventral hippocampus recordings from mice exploring a familiar arena ([18••], Figure 2). Moreover, a comparison of mPFC–hippocampus coherence spectra from wild-type mice reveals a trend toward increased hippocampal–mPFC coherence in the slow, but not fast, gamma frequency range during the choice phase of a delayed non-match-to-sample spatial memory
