(Dzirasa et al. 2010; Tort et al. 2008), and 2 and 4 Hz sharp narrow-band coherence with frontal cortical EEG was reported as far as in brainstem sympathetic circuits (Kenney et al. 1990). In the PFC-HC network, 4-Hz oscillation may play a complementary role with theta oscillations, such that the 4-Hz signal can phase-couple with theta by skipping every other cycle to coordinate oscillations at various frequencies across the brain (Fujisawa and Buzsaki 2011; Deshmukh et al. 2010). In our study, we found that the amplitude of theta and 4-Hz signal negatively correlated. With increasing RPO stimulation, hippocampal theta power increased, whereas prefrontal 4-Hz power decreased. This finding may be explained by bidirectional flow between the HC and PFC with one signal, either theta or 4-Hz, dominating at a given time. With increasing RPO stimulation and thus increased hippocampal theta power generation, HC-to-PFC circuit may be dominant, suppressing PFC-to-HC flow.
