Abstract
Transcranial alternating current stimulation (tACS) at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phase-dependent manner. Repetitive paired-pulse TMS (rPPS) with I-wave periodicity (1.5 ms) induced short-lived facilitation of MEPs. We hypothesized that tACS would modulate the facilitatory effects of rPPS in a frequency- and phase-dependent manner. To test our hypothesis, we investigated the effects of combined tACS and rPPS. We applied rPPS in combination with peak or trough phase tACS at 10 Hz (α) or β, or sham tACS (rPPS alone). The facilitatory effects of rPPS in the sham condition were temporary and variable among participants. In the β tACS peak condition, significant increases in single-pulse MEPs persisted for over 30 min after the stimulation, and this effect was stable across participants. In contrast, β tACS in the trough condition did not modulate MEPs. Further, α tACS parameters did not affect single-pulse MEPs after the intervention. These results suggest that a rPPS-induced increase in trans-synaptic efficacy could be strengthened depending on the β tACS phase, and that this technique could produce long-lasting plasticity with respect to cortical excitability.
Highlights
Transcranial alternating current stimulation at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phasedependent manner
Several research groups proposed tuning Non-invasive brain stimulation (NIBS) to the amplitude and/or phase of this ongoing oscillatory activity, with the goal of enhancing the efficacy of the stimulation[11]. This idea was validated by Zrenner et al.[12], who applied repetitive transcranial magnetic stimulation that was synchronized with the specific phase of ongoing sensorimotor α oscillations (i.e., Mu-rhythm) as a closed-loop stimulation
We performed Repetitive paired-pulse TMS (rPPS) that was adjusted to the β Transcranial alternating current stimulation (tACS) phase (Fig. 1A), and evaluated the after-effects by comparing MEPs obtained by single-pulse TMS in Experiment 1 (Fig. 1B)
Summary
Transcranial alternating current stimulation (tACS) at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phasedependent manner. Several research groups proposed tuning NIBS to the amplitude and/or phase of this ongoing oscillatory activity, with the goal of enhancing the efficacy of the stimulation[11] This idea was validated by Zrenner et al.[12], who applied repetitive transcranial magnetic stimulation (rTMS) that was synchronized with the specific phase of ongoing sensorimotor α oscillations (i.e., Mu-rhythm) as a closed-loop stimulation. Goldsworthy et al.[15] showed that the LTD-like neuroplastic response to continuous TBS (cTBS) was enhanced when the cTBS burst stimulations were delivered with the trough phase of 10 Hz (α) tACS They inferred that the neuroplastic responses to the combined stimulation were the result of entrainment of endogenous α oscillations in sensorimotor areas by α tACS. We investigated the effects of α tACS on M1 to ascertain the frequency- and phase-dependent effects of tACS in Experiment 2
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