Ordering Information in Working Memory and Modulation of Gamma by Theta Oscillations in Humans

Abstract

Ordering information is a critical process underlying several cognitive functions, especially working memory. Theta phase-gamma amplitude coupling is regarded as a neurophysiological representation of ordering information during working memory performance. However, direct evidence has been lacking in humans. Seventy healthy subjects performed the N-back task, a working memory task that tests ordering information at 3 different levels of difficulties and with 3 different types of trials. Using electroencephalography (EEG) during N-back performance, theta-gamma coupling was assessed during response trials. Multivariate general linear model (GLM) and discriminant analysis were used to assess coupling and theta and gamma power across the N-back conditions and the trial types. During the N-back trials that required ordering of information, N-back condition had independent effects on coupling and on theta and gamma power, with equal contributions among these 3 variables. Theta-gamma coupling contribution declined significantly on the trials that did not require ordering and was intermediate on trials that favored but not necessarily required ordering. Our findings demonstrate for the first time the role of theta-gamma coupling as a mechanism that supports ordering information. They also highlight the potential of using theta-gamma coupling as a neurophysiological marker of brain function in health or disease states.

On cross-frequency phase-phase coupling between theta and gamma oscillations in the hippocampus

“…Both phase-amplitude and phase-phase coupling between theta and gamma have been considered experimental evidence for such coding scheme (Lisman and Buzsáki, 2008; Sauseng et al, 2009; Axmacher et al, 2010; Belluscio et al, 2012; Lisman and Jensen, 2013; Hyafil et al, 2015; Rajji et al, 2016). In the case of phase-amplitude coupling, the modulation of gamma amplitude within theta cycles would instruct a reader network when the string of items represented in different gamma cycles starts and terminates.…”

Serial representation of items during working memory maintenance at letter-selective cortical sites

“…Although it is now clear that the theta-gamma code has a critical role in hippocampal function, there is only limited information about the involvement of this coding scheme in neocortex (Canolty et al, 2006;Rajji et al, 2016;Sauseng et al, 2009), the site of most WM processes (Milner, 2005). Computational models based on this coding scheme have been developed that fit the RT distributions in the Sternberg paradigm (Lisman & Jensen, 2013) and provide a plausible physiological account of the ordered serial representation of memory items in WM.…”

Transcranial Magnetic and Electrical Stimulation in Alzheimer's Disease and Mild Cognitive Impairment: A Review of Randomized Controlled Trials

“…The study found that subgenual cingulate excitability and connectivity were increased in major depression and that changes in these measures were specific to receiving active rTMS vs. sham rTMS. Another study assessed in older adults the impact of modulating theta oscillations using transcranial alternating current stimulation (tACS) at a theta frequency on the coupling between theta oscillations phase and gamma oscillations amplitude (theta–gamma coupling (TGC)) and working memory, the cognitive function that TGC supports neurophysiologically . The study found that tACS enhanced TGC and working memory, and there was an association between changes in TGC and changes in working memory .…”

“…Another study 29 assessed in older adults the impact of modulating theta oscillations using transcranial alternating current stimulation (tACS) at a theta frequency on the coupling between theta oscillations phase and gamma oscillations amplitude (theta-gamma coupling (TGC)) and working memory, the cognitive function that TGC supports neurophysiologically. [30][31][32] The study found that tACS enhanced TGC and working memory, and there was an association between changes in TGC and changes in working memory. 29 Moreover, this physiologically informed tACS restored TGC and working memory in older adults to the levels of younger adults compared with sham tACS.…”