Off-line effects of alpha-frequency transcranial alternating current stimulation on a visuomotor learning task

Abstract

Introduction: It has been suggested that transcranial alternating current stimulation (tACS) at both alpha and beta frequencies promotes motor function as well as motor learning. However, limited information exists on the aftereffects of tACS on motor learning and neurophysiological profiles such as entrainment and neural plasticity in parallel. Therefore, in the present study, we examined the effect of tACS on motor learning and neurophysiological profiles using an off-line tACS condition.

Methods: Thirty-three healthy participants were randomly assigned to 10 Hz, 20 Hz, or the sham group. Participants performed visuomotor learning tasks consisting of a baseline task (preadaptation task) and training task (adaptation task) to reach a target with a lever-type controller. Electroencephalography was recorded from eight locations during the learning tasks. tACS was performed between the preadaptation task and adaptation task over the left primary motor cortex for 10 min at 1 mA.

Results: As a result, 10 Hz tACS was shown to be effective for initial angular error correction in the visuomotor learning tasks. However, there were no significant differences in neural oscillatory activities among the three groups.

Conclusion: These results suggest that initial motor learning can be facilitated even when 10 Hz tACS is applied under off-line conditions. However, neurophysiological aftereffects were recently demonstrated to be induced by tACS at individual alpha frequencies rather than fixed alpha tACS, which suggests that the neurophysiological aftereffects by fixed frequency stimulation in the present study may have been insufficient to generate changes in oscillatory neural activity.

Keywords: EEG; aftereffect; alpha; off‐line condition; tACS; visuomotor learning task.

Figure 1

Experimental setting and paradigm. (a) A circular target was displayed randomly at any one of five locations that uniformly spanned a circle around the central starting point. Participants were instructed to control a lever‐type controller to reach the target with the cursor. (b) In the preadaptation task, the direction of the cursor movement was the same as that of the hand movement. In the adaptation task, the direction of the cursor movement was rotated 30° clockwise or counterclockwise from the direction of the hand movement. The dot line indicates the cursor path, while the solid line indicates the hand path. (c) Representative data in the preadaptation task and adaptation task. In the adaptation task, the hand path was gradually corrected as participants learned the directional error. (d) The angle between the direct line from the start position to target position (dashed line) and the line representing the direction movement at the peak outward velocity (dotted line) was calculated and defined as the directional error

Figure 2

(a) The time course of the visuomotor learning task and the EEG measurement is shown. Directional errors were compared following three patterns among the three groups; † (Δ PB8‐PB1); ‡ (ΔAB1‐PB8); and § (Δ AB8‐AB1). TACS was applied between the preadaptation task and the adaptation task. (b) Grand averaged time–frequency plot in the adaptation tasks for all participants. Representative C3 electrode data are shown. The average values of the event‐related neural oscillatory activities in the alpha (white box) and beta bans (gray box), based on the 250‐ to 1,000‐ms poststimulus window, were compared before and after tACS. The black dashed line represents the onset cue of the movement

Figure 3

(a) Δ directional error in each block. Error bars indicate standard deviations (SD). A significant difference was obtained between AB1 and BP8 among the three groups (p < .01). Multiple comparison with Bonferroni correction showed the significant differences between 10 and 20 Hz (p < .05) and between 10Hz and sham (p < .05). (b) Differences in peak velocity each block showed no significant differences among the three groups. Error bars indicate SD