Dynamic Theta Networks in the Human Medial Temporal Lobe Support Episodic Memory

Abstract

Highlights d We assessed intra-MTL connectivity during a memory task in 131 epilepsy patients d Successful encoding and retrieval were correlated with elevated theta connectivity d Left entorhinal cortex emerged as a hub of intra-MTL connectivity d We found broad increases in high-frequency power but decreases in theta power

Tonic resting-state hubness supports high-frequency activity defined verbal-memory encoding network in epilepsy

“…Kucewicz and colleagues demonstrated that memory related broadband gamma activity centroids were in the range of 30-100Hz (Kucewicz MT, et al, 2017). Two recent studies on human memory network analysis with a pooled cohort of patients similar to that included in this study, have shown that successful memory encoding is associated with asynchronous organization of the memory network in gamma activity less than 100Hz and synchronous theta activity, with widespread high gamma activity considered to be the hallmark of memory encoding (Solomon EA, et al, 2017;Solomon EA, et al, 2019). In this project, we were interested in investigating the intrinsic BOLD correlations of a high frequency network already known to play a major role in human memory, all toward determining if such networks possess distinctive whole-brain organization properties.…”

“…Successful episodic verbal memory encoding, termed the 'Subsequent Memory Effect' (SME) is associated with high-frequency gamma activity (ranging from 30-100Hz) captured using power spectral and synchronization studies during invasive electroencephalography (IEEG) in association with neuronal firing Kucewicz MT et al, 2017;Lega BC et al, 2012;Long NM et al, 2014;Sederberg PB et al, 2006;Serruya MD et al, 2014;Solomon EA et al, 2017;Solomon EA et al, 2019). Despite these well-established linkages, relatively little is known about the whole-brain organization of HFA regions and the degree to which these regions possess either distinct network features relative to other brain areas, or rely on abnormal organizational properties due to epilepsy.…”

Electrophysiological foundations of the human default-mode network revealed by brain-wide intracranial-EEG recordings during resting-state and cognition

“…The first goal of our study was to characterize the intrinsic spectro-temporal organization of the DMN. Analysis of the spectral properties of the human DMN has been necessarily constrained by limited placement of iEEG electrodes, consequently previous work has mainly focused on activation and deactivation in isolated nodes of the network (Ossandon T et al 2011;Lopour BA et al 2013;Daitch AL and J Parvizi 2018;Solomon EA et al 2019). It is currently impossible to acquire iEEG data across the entire human brain with resolutions matching those of fMRI.…”

“…To address this, we first examined frequency-specific iEEG phase synchronization during a verbal episodic memory task in which participants had to subsequently recall a list of visuallypresented words (Solomon EA et al 2019). Our analysis procedures were identical to those used in the case of resting-state iEEG data.…”

“…A systematic analysis using depth iEEG recordings spanning distributed nodes of the DMN, including the PCC, MTL, angular gyrus, and anterior temporal cortex, in parallel with other large-scale brain networks is needed to resolve these discrepancies and address fundamental questions regarding the integrity of this critical brain network . To address this challenge, we used iEEG data from the UPENN-RAM study (Solomon EA et al 2019) that included brain-wide depth recordings from 102 participants and 12,780 electrodes, from which we selected 36 participants with 879 electrodes spanning seven fMRI-derived brain networks . This allowed us to probe intra-DMN connectivity and contrast it with DMN interactions with other brain networks in a common large-scale network framework consistent with prior whole-brain non-invasive imaging studies.…”