Abstract
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 Abstract Thirteen 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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
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/8QY64, reference “Mu tACS 2021.”
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
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
- tACS
- Transcranial Alternating Current Stimulation
- TMS
- Transcranial Magnetic Stimulation
- CSE
- Corticospinal Excitability
- IMF
- Individual µ (Mu) Frequency
- EEG
- Electroencephalography
- MEG
- Magnetoencephalography
- EMG
- Electromyography
- MEP
- Motor Evoked Potential
- M1HAND
- Hand Area of the Primary Motor Cortex
- S1
- Primary Somatosensory Cortex
- FDI
- first dorsal interosseous
- ADM
- abductor digiti minimi
- fMRI
- Functional Magnetic Resonance Imaging
- BOLD
- Blood Oxygenation Level Dependent
