In addition to theta phase-gamma amplitude coupling, phase-phase coupling has also been reported between theta and gamma [30]. Coupling with theta phase was seen for both ~30–50 Hz (defined here as ‘slow’) and ~60–90 Hz (defined here as ‘fast’ but defined in [30] as ‘midfrequency’) gamma phase in CA1. This phase-phase coupling suggests that gamma-generating interneurons become active at a particular theta phase, emitting trains of gamma frequency spikes that produce gamma rhythmic inhibitory postsynaptic potentials in pyramidal cells. Other recent results provide support for this mechanism. Whole cell recordings from dentate gyrus granule cells in vivo revealed theta rhythmic excitatory postsynaptic currents (EPSCs), which originated from the entorhinal cortex, and gamma rhythmic inhibitory postsynaptic currents (IPSCs) within each theta cycle [31]. Additionally, Pastoll and colleagues [32] found that theta frequency stimulation of MEC layer II in vitro triggered theta rhythms and ~85 Hz fast gamma oscillations that coupled to a specific part of the theta cycle. This fast gamma activity corresponded to rhythmic IPSCs in stellate cells. These results suggest that theta-mediated excitation activates gamma-generating interneurons at a particular part of the theta cycle.
