Graphical AbstractThirty healthy participants received 60 trials of intermittent SO (0.75 Hz) tACS (1 trial = 16 s on + 16 s off) at an intensity of 2 mA. Motor cortical excitability was assessed using TMS-induced MEPs (blue waveforms) acquired across different oscillatory phases during (i.e., online; red arrows) and outlasting (i.e., offline; green arrows) tACS, as well as at the start and end of the stimulation session (blue arrows). Mean MEP amplitude increased by ∼41% from pre- to post-tACS (P = 0.013); however, MEP amplitudes were not modulated with respect to the tACS phase.

Converging evidence suggests that transcranial alternating current stimulation (tACS) may entrain endogenous neural oscillations to match the frequency and phase of the exogenously applied current and this entrainment may outlast the stimulation (although only for a few oscillatory cycles following the cessation of stimulation). However, observing entrainment in the electroencephalograph (EEG) during stimulation is extremely difficult due to the presence of complex tACS artefacts. The present study assessed entrainment to slow oscillatory (SO) tACS by measuring motor cortical excitability across different oscillatory phases during (i.e., online) and outlasting (i.e., offline) stimulation. 30 healthy participants received 60 trials of intermittent SO tACS (0.75 Hz; 16s on / off interleaved) at an intensity of 2mA peak-to-peak. Motor cortical excitability was assessed using transcranial magnetic stimulation (TMS) of the hand region of the primary motor cortex (M1HAND) to induce motor evoked potentials (MEPs) in the contralateral thumb. MEPs were acquired at four time-points within each trial - early online, late online, early offline, and late offline - as well as at the start and end of the overall stimulation period (to probe longer-lasting aftereffects of tACS). A significant increase in MEP amplitude was observed from pre- to post-tACS (P = 0.013) and from the first to the last tACS block (P = 0.008). However, no phase-dependent modulation of excitability was observed. Therefore, although SO tACS had a facilitatory effect on motor cortical excitability that outlasted stimulation, there was no evidence supporting entrainment of endogenous oscillations as the underlying mechanism.

Transcranial alternating current stimulation (tACS) has been proposed to modulate neural activity through two primary mechanisms: entrainment and neuroplasticity. The current study aimed to probe both of these mechanisms in the context of the sensorimotor µ-rhythm using transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to assess entrainment of corticospinal excitability (CSE) during stimulation (i.e., online) and immediately following stimulation, as well as neuroplastic aftereffects on CSE and µ EEG power. Thirteen participants received 3 sessions of stimulation. Each session consisted of 90 trials of µ-tACS tailored to each participant’s individual µ frequency (IMF), with each trial consisting of 16 seconds of tACS followed by 8 seconds of rest (for a total of 24 minutes of tACS and 12 minutes of rest per session). Motor evoked potentials (MEPs) were acquired at the start and end of the session (n = 41) and additional MEPs were acquired across the different phases of tACS at 3 epochs within each tACS trial (n = 90 for each epoch): early online, late online, and offline echo. Resting EEG activity was recorded at the start, end, and throughout the tACS session. The data were then pooled across the three sessions for each participant to maximise the MEP sample size per participant. We present preliminary evidence of CSE entrainment persisting immediately beyond tACS and have also replicated the plastic CSE facilitation observed in previous µ-tACS studies, thus supporting both entrainment and neuroplasticity as mechanisms by which tACS can modulate neural activity.Graphical AbstractThirteen participants underwent 3 sessions of stimulation where they received 90 trials of mu-tACS (270 trials across the 3 sessions), with each trial consisting of 16 seconds of tACS (2mA at the participants individual mu frequency) followed by 8 seconds of rest. Motor evoked potentials (MEPs) were acquired at the start and end of the session (n = 41) and additional MEPs were acquired across the different phases of tACS at 3 epochs within each tACS trial (n = 90 for each epoch): early online, late online, and offline echo. We present preliminary evidence supporting entrainment of MEP amplitudes to tACS phase online to and immediately following stimulation and have also replicated the neuroplastic CSE facilitation observed in previous µ-tACS studies.

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