Phase synchronization has been suggested to play a number of different roles in brain function. Cross-location, same-frequency phase coupling between different brain areas has been studied extensively because of its potential role in regulating inter-area communication [7–9, 24, 25]. Similarly, same-location, cross-frequency phase coupling may serve as a potential mechanism to regulate communication between different spatiotemporal scales [6, 26–28]. Phase-phase CFC provides a plausible physiological mechanism for linking activity that occurs at significantly different rates. For example, patterns of firing-rate correlations observed during learning repeat during NREM sleep, but at a 6–7 times speedup [29, 30]. Similar temporal compression of spike sequences has been reported in the hippocampus [31]. Mechanisms to explain such pattern compression remain unknown, but the dependence of spiking upon LFP phases at different frequencies suggests phase-phase CFC as an explanatory hypothesis deserving further investigation.
