Hippocampal synchrony and neocortical desynchrony cooperate to encode and retrieve episodic memories

Episodic memories hinge upon our ability to process a wide range of multisensory information and bind this information into a coherent, memorable representation. On a neural level, these two processes are thought to be supported by neocortical alpha/beta desynchronisation and hippocampal theta/gamma synchronisation, respectively. Intuitively, these two processes should interact to successfully build and retrieve episodic memories, yet this hypothesis has not been tested empirically. Here, we address this question by analysing human intracranial EEG data recorded during an associative memory task that involved the pairing of life-like, dynamic stimuli to verbal stimuli. Our findings indicate that neocortical alpha/beta (8-20Hz) desynchronisation reliably precedes and predicts hippocampal 9fast9 gamma (60-80Hz) synchronisation during episodic memory formation; during episodic memory retrieval however, hippocampal 9slow9 gamma (40-50Hz) synchronisation reliably precedes and predicts later neocortical alpha/beta desynchronisation. We speculate that this cooperation reflects the flow of information from neocortex to hippocampus during memory formation, and hippocampal pattern completion inducing information reinstatement in the neocortex during memory retrieval.