Traveling waves enhance hippocampal-parahippocampal couplings in human episodic and working memory

Multiple brain regions in the human medial temporal lobe (MTL) are coordinated in memory processing. Traveling waves is a potential mechanism to coordinate information transfer through organizing the timing or spatiotemporal patterns of wave propagation. Based on direct human intracranial EEG recordings, we detected bidirectional hippocampal and parahippocampal traveling waves (4-10 Hz) along the posterior-anterior axis during a verbal memory task. Hippocampal traveling waves enhanced hippocampal-parahippocampal and intra-hippocampal couplings in both amplitude and phase as well as hippocampal theta phase-gamma amplitude coupling, suggesting a facilitatory role of TWs. Granger causality analysis showed asymmetric information flow, with greater predictability in the parahippocampal-to-hippocampal direction and dominant peak at the beta band (20-30 Hz). Hippocampal power and bidirectional hippocampal-parahippocampal information flow at the gamma band (35-50 Hz) showed reductions during successful memory encoding trials. These results support functional significance of frequency-specific parahippocampal-hippocampal and intra-hippocampal communications during memory encoding and retrieval.