After-Effects of Intermittent Theta-Burst Stimulation Are Differentially and Phase-Dependently Suppressed by α- and β-Frequency Transcranial Alternating Current Stimulation

Intermittent theta-burst stimulation (iTBS) using transcranial magnetic stimulation (TMS) is known to produce excitatory after-effects over the primary motor cortex (M1). Recently, transcranial alternating current stimulation (tACS) at 10 Hz (alpha) and 20 Hz (beta) have been shown to modulate M1 excitability in a phase-dependent manner. Therefore, we hypothesized that tACS would modulate the after-effects of iTBS depending on the stimulation frequency and phase. To test our hypothesis, we examined the effects of alpha- and beta-tACS on iTBS using motor evoked potentials (MEPs). Eighteen and thirteen healthy participants were recruited for alpha and beta tACS conditions, respectively. tACS electrodes were attached over the left M1 and Pz. iTBS over left M1 was performed concurrently with tACS. The first pulse of the triple-pulse burst of iTBS was controlled to match the peak (90 degrees) or trough (270 degrees) phase of the tACS. A sham tACS condition was used as a control in which iTBS was administered without tACS. Thus, each participant was tested in three conditions: the peak and trough of the tACS phases and sham tACS. As a result, MEPs were enhanced after iTBS without tACS (sham condition), as observed in previous studies. alpha-tACS suppressed iTBS effects at the peak phase but not at the trough phase, while beta-tACS suppressed the effects at both phases. Thus, although both types of tACS inhibited the facilitatory effects of iTBS, only alpha-tACS did so in a phase-dependent manner. Phase-dependent inhibition by alpha-tACS is analogous to our previous finding in which alpha-tACS inhibited MEPs online at the peak condition. Conversely, beta-tACS reduced the effects of iTBS irrespective of its phase. The coupling of brain oscillations and tACS rhythms is considered important in the generation of spike-timing-dependent plasticity. Additionally, the coupling of theta and gamma oscillations is assumed to be important for iTBS induction through long-term potentiation (LTP). Therefore, excessive coupling between beta oscillations induced by tACS and gamma or theta oscillations induced by iTBS might disturb the coupling of theta and gamma oscillations during iTBS. To conclude, the action of iTBS is differentially modulated by neuronal oscillations depending on whether alpha- or beta-tACS is applied.