Phase amplitude coupling (PAC) between the phase of theta oscillations and amplitude of gamma band oscillations is thought to represent a mechanism by which the hippocampus organizes information during memory formation and retrieval.

     PAC has been identified in humans, but previous efforts have not shown that changes in PAC are associated with successful memory encoding or retrieval.

     Participants performed the Free Recall task, a standard test of episodic memory. We analyzed data from 271 hippocampal electrodes in 39 neurosurgical patients undergiong phase II monitoring at 5 different institutions.

     We extracted phase and power for each frequency from 2 to 128 Hz using Morlet wavelets and the Hilbert transform. We identified significant PAC using a binned-ANOVA method as well as circular regression. Counts of electrodes exhibiting PAC in each frequency band were compiled. We compared the magnitude of PAC during successful versus unsuccessful memory encoding and during memory retrieval.

     PAC occurs between slow-theta oscillations (2.5--4.5 Hz) and both low (35-70 Hz) and high (70-120 Hz) gamma band oscillations (43 and 41 out of 271 electrodes, p = 0.001, binomial test). PAC is stronger during successful memory encoding (p<.0001, ANOVA).The preferred slow theta phase is both pi radians and pi/3 radians for PAC. An increase in PAC during memory encoding has not been demonstrated in humans. The coupling at two different phases of the theta oscillation was also unexpected.

     This is a retrospective study.

     PAC represents an important mechanism of information processing in the human hippocampus.

     However the frequency of the theta oscillation and the phase of coupling may be slightly different in humans as compared to rodents.