Abstract
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 (α) and 20 Hz (β) 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 α- and β-tACS on iTBS using motor evoked potentials (MEPs). Eighteen and thirteen healthy participants were recruited for α and β 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°) or trough (270°) 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. α-tACS suppressed iTBS effects at the peak phase but not at the trough phase, while β-tACS suppressed the effects at both phases. Thus, although both types of tACS inhibited the facilitatory effects of iTBS, only α-tACS did so in a phase-dependent manner. Phase-dependent inhibition by α-tACS is analogous to our previous finding in which α-tACS inhibited MEPs online at the peak condition. Conversely, β-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 θ and γ oscillations is assumed to be important for iTBS induction through long-term potentiation (LTP). Therefore, excessive coupling between β oscillations induced by tACS and γ or θ oscillations induced by iTBS might disturb the coupling of θ and γ oscillations during iTBS. To conclude, the action of iTBS is differentially modulated by neuronal oscillations depending on whether α- or β-tACS is applied.
Highlights
Transcranial alternating current stimulation is a noninvasive brain stimulation (NIBS) method that uses alternating current over the scalp, typically without a direct current shift
Mean motor evoked potentials (MEPs) amplitudes for the sham and trough conditions were larger after α-Transcranial alternating current stimulation (tACS)/Intermittent theta-burst stimulation (iTBS)
We found significant differences between the peak and sham conditions (F(1,232) = 10.4, p = 0.004, 95% confidence interval (CI) = 0.029 – 0.118) and between the peak and trough conditions (F(1,232) = 5.15, p = 0.048, 95% CI = 0.006 – 0.103), but not between the trough and sham conditions (F(1,232) = 0.57, p = 0.45). These results indicate that synchronized tACS in the peak condition inhibited the effects of iTBS relative to the sham and trough conditions
Summary
Transcranial alternating current stimulation (tACS) is a noninvasive brain stimulation (NIBS) method that uses alternating current over the scalp, typically without a direct current shift. The effects of tACS over the primary motor cortex (M1) have been investigated and their dependency on tACS frequency has been reported (Feurra et al, 2011). TACS at 20 Hz but not 5, 10, or 40 Hz with 1 mA was effective during stimulation These studies indicate that tACS effects depend on the stimulation intensity as well as duration and frequency. The underlying mechanisms that produce this frequency dependency have not been established, the entrainment of cortical oscillations may be involved (Herrmann et al, 2013). The effects have been suggested to depend on the phase (Guerra et al, 2016; Nakazono et al, 2016; Raco et al, 2016), which is in line with the entrainment of oscillations
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