The effect of 10 Hz transcranial alternating current stimulation (tACS) on corticomuscular coherence

Abstract

Synchronous oscillatory activity at alpha (8-12 Hz), beta (13-30 Hz), and gamma (30-90 Hz) frequencies is assumed to play a key role for motor control. Corticomuscular coherence (CMC) represents an established measure of the pyramidal system's integrity. Transcranial alternating current stimulation (tACS) offers the possibility to modulate ongoing oscillatory activity. Behaviorally, 20 Hz tACS in healthy subjects has been shown to result in movement slowing. However, the neurophysiological changes underlying these effects are not entirely understood yet. The present study aimed at ascertaining the effects of tACS at 10 and 20 Hz in healthy subjects on CMC and local power of the primary sensorimotor cortex. Neuromagnetic activity was recorded during isometric contraction before and at two time points (2-10 min and 30-38 min) after tACS of the left primary motor cortex (M1), using a 306 channel whole head magnetoencephalography (MEG) system. Additionally, electromyography (EMG) of the right extensor digitorum communis (EDC) muscle was measured. TACS was applied at 10 and 20 Hz, respectively, for 10 min at 1 mA. Sham stimulation served as control condition. The data suggest that 10 Hz tACS significantly reduced low gamma band CMC during isometric contraction. This implies that tACS does not necessarily cause effects at stimulation frequency. Rather, the findings suggest cross-frequency interplay between alpha and low gamma band activity modulating functional interaction between motor cortex and muscle.

Keywords: corticomuscular coherence (CMC); magnetoencephalography (MEG); motor control; primary motor cortex (M1); transcranial alternating current stimulation (tACS).

Figure 1

CMC sensor-plot averaged across all subjects prior to tACS. The insert indicates the sensor with the largest amplitude. The gray shaded area represents the sensors selected for the analysis. Data were averaged across this selection.

Figure 2

Contralateral CMC amplitude during isometric contraction and rest prior to tACS. A significant increase of CMC amplitude during isometric contraction compared to rest was only found in the beta (13–30 Hz) and low gamma (30–45 Hz) band. Error bars indicate standard error of mean. Asterisks denote significant p-values < 0.001.

Figure 3

Averaged sensor-plot for CMC within the low gamma band (30–45 Hz) prior to and post 10 Hz tACS. The insert indicates the sensor with the largest amplitude change following tACS.

Figure 4

Effects of tACS at 10 Hz, 20 Hz and sham stimulation on CMC at low gamma frequency at two time points after tACS cessation. Shown are relative changes. Error bars indicate standard error of mean. Please note that a significant main effect of factor stimulation was found on low gamma band CMC, only suggesting more pronounced effects of 10 Hz tACS on low gamma CMC as compared to sham and 20 Hz stimulation.

Figure 5

Main effect of factor stimulation on low gamma CMC. Shown are relative changes of contralateral CMC amplitude in the low gamma band during isometric contraction after 10 Hz, 20 Hz and sham tACS collapsed across the two post tACS measurements. After 10 Hz tACS CMC amplitude was significantly lower as compared to 20 Hz and sham stimulation. Asterisk denotes significant p-values < 0.05. Error bars indicate standard error of mean.

Figure 6

Mean relative changes of power at the four frequency bands of interest following 10 Hz, 20 Hz and sham tACS. Error bars indicate standard error of mean. Please note that a trend toward a significant main effect of factor stimulation was found on low gamma band power, only. Post-hoc analysis revealed that low gamma band power following 20 Hz tACS was significantly reduced as compared to sham stimulation and 10 Hz tACS.