Nora A. Herweg, Ashwini D. Sharan, Michael R. Sperling, Armin Brandt, Andreas Schulze-Bonhage and Michael J. Kahana. Reactivated spatial context guides episodic recall. Journal of Neuroscience 23 January 2020, 1640-19; DOI: https://doi.org/10.1523/JNEUROSCI.1640-19.2019

Abstract
“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. Here, we use intracranial EEG and a hybrid spatial-episodic memory task (29 subjects, 15 female) to determine how spatial information is dynamically reactivated in sub-regions of the human MTL and how this reactivation guides recall of episodic information. Our results implicate theta oscillations across the MTL as a common neurophysiological substrate for spatial coding in navigation and episodic recall. We further show that our index of retrieved spatial context is high in the hippocampus (HC) in an early time window preceding recall. Closer to recall, it decreases in the HC and increases in the parahippocampal gyrus (PHG). 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.”

SIGNIFICANCE STATEMENT

“By recording from the human 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. 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 extra-hippocampal MTL in representing retrieved spatial context. Our results provide an important step towards a unified theory of MTL function encompassing its role in spatial navigation and episodic memory.”

Nora A. Herweg, Ashwini D. Sharan, Michael R. Sperling, Armin Brandt, Andreas Schulze-Bonhage and Michael J. Kahana. Reactivated spatial context guides episodic recall. Journal of Neuroscience 23 January 2020, 1640-19; DOI: https://doi.org/10.1523/JNEUROSCI.1640-19.2019