Effects of Slow Oscillatory Transcranial Alternating Current Stimulation on Motor Cortical Excitability Assessed by Transcranial Magnetic Stimulation

Abstract

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.

Mu-Transcranial Alternating Current Stimulation Induces Phasic Entrainment and Plastic Facilitation of Corticospinal Excitability

“…It is of interest to note that these facilitatory effects are of a similar magnitude to the facilitatory effects observed in our most recent slow oscillatory tACS study (45.05% increase; Geffen et al, 2021) despite the differences in stimulation frequency, suggesting that these plastic aftereffects may not necessarily be frequency-dependent. When compared to β-tACS (15-25 Hz), which has been previously shown to facilitate resting CSE across multiple studies (Cancelli et al, 2015a;Cancelli et al, 2015b;Feurra et al, 2011;Feurra et al, 2013;Feurra et al, 2019;Heise et al, 2016), a recent meta-analysis by Wischnewski et al (2019) reported that MEP amplitudes were only increased by β-tACS when using electrode montages with a more posterior location for the return electrode (e.g., M1-Pz or M1-Oz) but not when using a conventional M1-supraorbital region montage such as was used in the present study.…”

“…Although we did not observe entrainment of endogenous slow (0.75 Hz) oscillations in our previous experiment (Geffen et al, 2021), we attributed this null result to a lack of resonance dynamics between the eigenfrequency and stimulation frequency (Ali et al, 2013) as well as the relatively small MEP sample size per participant compared to simulation studies (Zoefel et al, 2019). The current study aimed to address both these limitations by using a tailored stimulation frequency that matches the eigenfrequency of the participants' sensorimotor cortex during wake, and by performing multiple experiment sessions for each participant then pooling the data across sessions.…”

“…Furthermore, a computational study by Lefebvre et al (2017) has suggested that decreases in endogenous oscillatory power from certain behavioural tasks may enhance tACS-induced entrainment by increasing the range of the Arnold tongue compared to rest, allowing entrainment across a wider range of stimulation frequencies and increasing peak power when stimulating close to the resonant frequency. This inverse relationship between endogenous power and tACS effects has also been demonstrated for the plastic aftereffects of tACS, with human studies reporting significant tACS aftereffects when endogenous power is low (Geffen et al, 2021;Neuling et al, 2013) but not when endogenous power is high (Neuling et al, 2013).…”

“…All participants completed a safety screening questionnaire (Keel et al, 2001) and provided a written statement of informed consent prior to commencing the experiment. The exclusion criteria from this safety screen were identical to our most recent experiment (Geffen et al, 2021) and included: "personal or family history of epilepsy/seizures, medication that could affect seizure threshold, history of brain injury/condition (e.g., stroke, concussion, etc. ), implanted devices or metal in the head, frequent or severe headaches, or current pregnancy".…”

“…The MEP exclusion criteria were identical to those used in our previous experiment (Geffen et al, 2021) and included: "the first MEP for each data set (as well as the first MEP after each of the break periods) which may be larger (Brasil-Neto et al, 1994) and more variable (Schmidt et al, 2009) than subsequent MEPs, as well as any MEPs with voluntary EMG activity detected in the 500 ms prior to TMS delivery (~1-2% of MEPs excluded). "…”