Is Cortical Theta-Gamma Phase-Amplitude Coupling Memory-Specific?

Abstract

One of the proposed neural mechanisms involved in working memory is coupling between the theta phase and gamma amplitude. For example, evidence from intracranial recordings shows that coupling between hippocampal theta and cortical gamma oscillations increases selectively during working memory tasks. Theta-gamma phase-amplitude coupling can also be measured non-invasively through scalp EEG; however, EEG can only assess coupling within cortical areas, and it is not yet clear if this cortical-only coupling is truly memory-specific, or a more general phenomenon. We tested this directly by measuring cortical coupling during three different conditions: a working memory task, an attention task, and a passive perception condition. We find similar levels of theta-gamma coupling in all three conditions, suggesting that cortical theta-gamma phase-amplitude coupling is not a memory-specific signal, but instead reflects some other attentional or perceptual processes. Implications for understanding the brain dynamics of visual working memory are discussed.

Keywords: EEG; phase-amplitude coupling; theta-gamma phase-amplitude coupling; working memory.

Figure 1

Schematic illustration of theta-gamma phase-amplitude coupling (A), with the gamma amplitude being modulated by the theta phase. (B) shows the average gamma amplitude (i.e., the average envelope amplitude) at different theta phases. Note that the peak gam.

Figure 2

Trial sequence for the three tasks: (A) Active WM+, (B) Active WM-, and (C) Passive WM-.

Figure 3

Average normalized gamma amplitude at each theta phase across our two clusters of interest, averaged across all 60 subjects of the primary sample.

Figure 4

Average PLV per task across the different clusters and frequency bands. The shaded areas on each bar represent the PLV of the null permutations and mark the PLV one would expect by chance. When marking significance, *** denotes p < 0.001.

Figure 5

A scatterplot of individual delay period PLV and K, a measure of working memory capacity for the main sample (left), and the replication sample (right). The correlation for and p-value for each sample are noted on the graph, as well as the line of best fit.