Human hippocampal theta oscillations and the formation of episodic memories

The importance of the hippocampal theta oscillation (4-8 Hz) to memory formation has been well-established through studies in animals, prompting researchers to propose comprehensive theories of memory and learning that rely on theta oscillations for integrating information in the hippocampus and neocortex. Yet, empirical evidence for the importance of 4-8 Hz hippocampal theta oscillations to memory formation in humans is equivocal at best. To clarify this apparent interspecies discrepancy, we recorded intracranial EEG (iEEG) data from 237 hippocampal electrodes in 33 neurosurgical patients as they performed an episodic memory task. We identified two distinct patterns of hippocampal oscillations, at ∼3 and ∼8 Hz, which are at the edges of the traditional 4-8 Hz human theta band. The 3 Hz "slow-theta" oscillation exhibited higher power during successful memory encoding and was functionally linked to gamma oscillations, but similar patterns were not present for the 8 Hz "fast-theta" oscillation. For episodic memory, slow-theta oscillations in the human hippocampus appear to be analogous to the memory-related theta oscillations observed in animals. Both fast-theta and slow-theta oscillations exhibit evidence of phase synchrony with oscillations in the temporal cortex. We discuss our findings in the context of recent research on the electrophysiology of human memory and spatial navigation, and explore the implications of this result for theories of cortico-hippocampal communication.