Gamma oscillations during episodic memory processing provide evidence for functional specialization in the longitudinal axis of the human hippocampus

Abstract

The question of whether the anterior and posterior hippocampus serve different or complementary functional roles during episodic memory processing has been motivated by noteworthy findings in rodent experiments and from noninvasive studies in humans. Researchers have synthesized these data to postulate several models of functional specialization, However, the issue has not been explored in detail using direct brain recordings. We recently published evidence that theta power increases during episodic memory encoding occur in the posterior hippocampus in humans. In our current investigation we analyzed an expanded data set of 32 epilepsy patients undergoing stereo EEG seizure mapping surgery with electrodes precisely targeted to the anterior and posterior hippocampus simultaneously who performed an episodic memory task. Using a repeated measures design, we looked for an interaction between encoding versus retrieval differences in gamma oscillatory power and anterior versus posterior hippocampal location. Our findings are consistent with a recently articulated model (the HERNET model) favoring posterior hippocampal activation during retrieval related processing. We also tested for encoding versus retrieval differences in the preferred gamma frequency band (high versus low gamma oscillations) motivated by published rodent data.

Keywords: episodic memory; gamma oscillations; hippocampus.

Figure 1:. A. Difference between encoding and retrieval oscillatory power.

Bar plot of difference in oscillatory power between encoding and retrieval for anterior (red) and posterior (blue) electrodes *** denotes significant interaction effect between encoding/retrieval and anterior/posterior hippocampal location across both gamma frequency bands. * denotes significant encoding versus retrieval gamma power difference across subjects with each band/hippocampal location (t–test, Bonferroni corrected). B. Example of encoding and retrieval high gamma power from an individual subject. Normalized power during encoding and retrieval from anterior (red) and posterior (blue) electrodes from a single subject. Normalized power smoothed with 50 msec moving average. C. Aggregate plot of electrode locations. Electrodes targeted based upon location of the uncal apex, MNI Y coordinate ~ −21. D. Encoding subsequent memory effect. * denotes significantly greater power following presentation of successfully remembered items.