Gamma Oscillations and Their Cross-frequency Coupling in the Primate Hippocampus during Sleep

Study Objectives: The mechanism by which sleep consolidates memory is unclear. Based on the two-stage model of memory consolidation, different functions for slow wave sleep (SWS) and rapid eye movement (REM) sleep have been proposed; thus, state-dependent changes of neural oscillations in the hippocampus might clarify this fundamental question. Methods: We recorded hippocampal local field potentials from freely behaving monkeys via telemetry and analyzed their nonstationary oscillations using Hilbert-Huang transform. Results: By applying a recently developed empirical mode decomposition analysis, we found strong cross-frequency coupling between high-frequency and slow wave oscillations during SWS and a prominent increase of gamma band activity in short bursts during REM sleep in unanesthetized primates' hippocampus. Conclusion: Spatiotemporal integration through coupled oscillations during slow wave sleep might be a physiological basis of system consolidation, whereas gamma bursts during rapid eye movement sleep might be related to synaptic consolidation in the local hippocampal neural circuit.