Spatiotemporal context provides a unique tag for each event we experience, and the similarities among these tags organize the contents of episodic memory. Our results provide an important step toward a unified theory of MTL function encompassing its role in spatial navigation and episodic memory. We thereby pinpoint a role for theta oscillations in accessing the “cognitive map” during episodic retrieval and further highlight the dynamic interplay of hippocampus and extrahippocampal MTL in representing retrieved spatial context. SIGNIFICANCE STATEMENT By recording from the human medial temporal lobe (MTL) while subjects recall items experienced in a virtual environment, we establish a direct relation between the strength of theta activity during memory search and the extent to which memories are organized by their spatial locations. Finally, we demonstrate that hippocampal theta phase modulates parahippocampal gamma amplitude during retrieval of spatial context, suggesting a role for cross-frequency coupling in coding and transmitting retrieved spatial information. Closer to recall, it decreases in the HC and increases in the parahippocampal gyrus. We further show that our index of retrieved spatial context is high in the hippocampus (HC) in an early time window preceding recall.
Our results implicate theta oscillations across the MTL as a common neurophysiological substrate for spatial coding in navigation and episodic recall. Here, we use intracranial electroencephalography and a hybrid spatial-episodic memory task (29 subjects, 15 female) to determine how spatial information is dynamically reactivated in subregions of the human MTL and how this reactivation guides recall of episodic information.
The medial temporal lobe (MTL) is known as the locus of spatial coding and episodic memory, but the interaction between these cognitive domains as well as the extent to which they rely on common neurophysiological mechanisms is poorly understood.