FIG. 6.

Strength of entrainment power moderates movement-related cross-frequency coupling. (Left): Conceptual figure of the multilevel moderation analyses conducted using single-trial data. We hypothesized that the level of entrainment power that is modulated by CES (i.e., increased spontaneous power at 15 Hz with CES) would moderate the theta–gamma power to power coupling during movement onset in the left M1. (Right): Conditional effects of theta oscillatory power on gamma oscillatory power in the left M1 as a function of increasing 15 Hz entrainment power. The x-axis denotes log-transformed 15 Hz entrainment power during the baseline period in the left M1, whereas the y-axis denotes the conditional coefficients of theta power on gamma power in the left M1. Ninety-five percent confidence intervals are displayed in light and dark blue for stimulation and no-stimulation conditions, respectively. Below each regression line is a histogram displaying the distribution of the moderator (15 Hz entrainment power) for each condition. During 15 Hz CES, the conditional effect of theta–gamma power to power coupling significantly decreased with increasing levels of 15 Hz spontaneous neural activity during the baseline period. In contrast, in the absence of stimulation, there was no significant change in theta–gamma cross-frequency coupling as a function of increasing 15 Hz entrainment power. Color images are available online.