Online modulation of neural drive to lower motor neurons with high-definition transcranial alternating current
Michael Houston1, Hao Meng2,3, Nicholas Dias4
1Department of Biomedical Engineering, University of Miami, 1251 Memorial Drive, Coral Gables, FL, 33146, USA.
Background:
Transcranial alternating current stimulation (tACS) is a form of non-invasive brain stimulation that entrains cortical neuronal activity through exogenously delivered sinusoidal currents. However, it remains unclear whether tACS can effectively modulate the common input to lower motor neurons. This study aims to address this gap by applying high-definition tACS (HD-tACS) at varying stimulation frequencies (11 Hz, 21 Hz, 40 Hz, and Sham) and intensities (1 mA and 2 mA).
Materials And Methods:
Fifteen healthy, young adults produced 10% maximal voluntary contraction (MVC) isometric forces for 100 s with their flexor digitorum superficialis (FDS) while HD-tACS was applied to the motor hotspot of FDS during the initial 60 s. High-definition surface electromyography (HD-sEMG) was decomposed into motor unit (MU) spike trains. The number of identified MUs, amplitude, firing rate, and MU connectivity (MUC) were assessed during the 60-s HD-tACS period. Cortico-muscular connectivity (CMC) of the FDS motor hotspot and FDS was quantified during the 40 s post-HD-tACS period.
Results:
HD-tACS delivered at 2 mA versus 1 mA elicited significantly greater delta-band (δ: 1-4 Hz) MUC (p = 0.033). Additionally, when HD-tACS was applied at 21 Hz and 2 mA, both δ MUC (p = 0.003) and α MUC (p = 0.015) were greater in magnitude than during Sham. However, no significant differences to CMC after HD-tACS were identified (p = 0.135).
Conclusion:
HD-tACS delivered at 21 Hz and 2 mA can produce online modulation of the corticospinal inputs to lower motor neurons. However, 60 s of online HD-tACS may not be ample time for corticospinal entrainment to persist post-stimulation.
