Phase-based coordination of hippocampal and neocortical oscillations during human sleep

During sleep, new memories undergo a gradual transfer from the hippocampus (HPC) to the neocortex (NC). Precisely timed neural oscillations interacting within and between these brain structures are thought to mediate this sleep-dependent memory consolidation, but exactly which sleep oscillations instantiate the HPC-NC dialog, and via what mechanisms, remains elusive. Employing invasive electroencephalography in ten neurosurgical patients across a full night of sleep, we identified three broad classes of phase-based HPC-NC communication. First, we observed interregional phase synchrony for non-rapid eye movement (NREM) spindles, N2 and rapid eye movement (REM) theta, and N3 beta activity. Second, and most intriguingly, we found asymmetrical N3 cross-frequency phase-amplitude coupling between HPC SOs and NC activity spanning the delta to high-gamma/ripple bands, but not in the opposite direction. Lastly, N2 theta and NREM spindle synchrony were themselves modulated by HPC SOs. These novel forms of phase-based interregional communication emphasize the role of HPC SOs in the HPC-NC dialog, and may offer a physiological basis for the sleep-dependent reorganization of mnemonic content.