Hippocampal and Neocortical Gamma Oscillations Predict Memory Formation in Humans

Abstract

Functional magnetic resonance imaging (fMRI) of the human brain has shown that the hippocampus and the left temporal and frontal cortices play a key role in the formation of new verbal memories. We recorded electrical activity from 2349 surgically implanted intracranial electrodes in epilepsy patients while they studied and later recalled lists of common words. Using these recordings, we demonstrate that gamma oscillations (44-64 Hz) in the hippocampus and the left temporal and frontal cortices predict successful encoding of new verbal memories. This increase in gamma oscillations was not seen in other frequency bands, whose activity generally decreased during successful memory formation. These findings identify a role for gamma oscillations in verbal memory formation with the hippocampus and the left temporal and frontal cortices, the same regions implicated using noninvasive fMRI recording methods.

Slow-theta power decreases during item-place encoding predict spatial accuracy of subsequent context recall

“…The study demonstrates that theta power decreases co occurred and were co localized with BOLD signal increases in MTL, a region clearly driven by task's pro cesses. The same reasoning could be applied to decreases in slow theta (~2 5 Hz) activity found in our study, taking also into account the high overlap in frequency with broadband negative SMEs (Burke et al, 2013;Greenberg et al, 2015;Long et al, 2014;Sederberg et al, 2007). In concordance with these two perspectives, decreased slow theta activity could mean that higher degree of visual and spatial in formation was processed and encoded into memory, enabling a more accurate retrieval of the spatial location during the test phase.…”

“…However, other studies have reported negative SMEs in the broad theta band (Burke et al, 2013;Guderian et al, 2009;Long et al, 2014;Sederberg et al, 2007). These theta power decreases are seen during the successful encoding of single items and inter item associations (Greenberg et al, 2015) and are widely detected with surface sensors or iEEG contacts targeting medial temporal lobe (MTL) and fronto temporal structures (Greenberg et al, 2015;Long et al, 2014).…”

Neural correlates of spatial and nonspatial attention determined using intracranial electroencephalographic signals in humans

“…To show the task‐related differences across multiple participants in any region of interest, we adapted the individual electrode analysis method [Burke et al, 2013; Sederberg et al, 2007; van Vugt et al, 2010]. Briefly, this method shows the fraction of electrodes with a significant decrease (ERD) or increase (ERS) in oscillatory power in the same brain region for each task.…”

Techniques and devices to restore cognition

“…The same pattern of gamma activity over hippocampus and cortex is recapitulated just prior to successful retrieval of previously learned items [132]. Training people to increase gamma activity (recorded from the scalp or subdurally) in these regions, while decreasing alpha and beta activity over widespread sites [131], could potentially result in improved learning and recall. In addition to operant conditioning to consciously increase the magnitude or precise frequency of gamma activity, participants could be engaged in closedloop systems in which stimuli to be recalled were repeatedly presented until the system detected gamma activity signatures that predict later successful recall.…”

“…Possibilities for future research include the investigation of whether training people to increase oscillatory activity within the neural networks supporting memory function can significantly enhance learning or recall. In particular, recent studies have shown that oscillatory activity in the gamma frequency band, especially in hippocampus and temporal cortex, increases during successful encoding of study items (as measured by whether those items are subsequently recalled) [131]. The same pattern of gamma activity over hippocampus and cortex is recapitulated just prior to successful retrieval of previously learned items [132].…”