Abstract
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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
a.geffen{at}uq.net.au, n.bland{at}uq.edu.au, m.sale{at}uq.edu.au
Funding: This work was supported by the US Office of Naval Research Global [grant number N62909-17-1-2139] awarded to Martin V Sale. The funding body had no involvement in the study design; the collection, analysis, and interpretation of data; the writing of the report; or the decision to submit the article for publication.
Conflicts of interest: The authors have no relevant financial or non-financial interests to disclose.
Data Availability Statement: The data that support the findings of this study are freely available in the Open Science Framework at https://doi.org/10.17605/OSF.IO/DAV4T, reference “SO tACS 2019.”
Code Availability Statement: The MATLAB scripts used to automatically trigger tACS and TMS as well as the scripts used for statistical analysis are available from the authors upon request.
Ethics Approval Statement: Ethics approval was granted by the University of Queensland Human Research Ethics Committee in accordance with the National Health and Medical Research Council’s guidelines.
Consent to Participate: All participants completed a safety screening questionnaire and provided a written statement of informed consent prior to commencing the experiment.
Consent for Publication: N/A
Author Approvals: All authors have seen and approved the manuscript for submission. The manuscript has not been accepted or published elsewhere.
CRediT Author Statement: Asher Geffen: Conceptualization, Methodology, Software, Formal Analysis, Investigation, Data Curation, Writing – Original Draft, Visualisation
Nicholas Bland: Conceptualization, Methodology, Software, Data Curation, Writing – Review & Editing, Visualisation, Supervision
Martin V Sale: Conceptualization, Methodology, Validation, Resources, Writing – Review & Editing, Project Administration, Funding Acquisition, Supervision
Abbreviations
- tES
- Transcranial Electrical Stimulation
- tACS
- Transcranial Alternating Current Stimulation
- tDCS
- Transcranial Direct Current Stimulation
- SO
- Slow Oscillatory
- SWS
- Slow-Wave Sleep
- TMS
- Transcranial Magnetic Stimulation
- EEG
- Electroencephalography
- EMG
- Electromyography
- MEP
- Motor Evoked Potential
- M1HAND
- Hand Area of the Primary Motor Cortex
- APB
- abductor pollicis brevis
- STDP
- Spike-timing dependent plasticity