Modulation of mu rhythm desynchronization during motor imagery by transcranial direct current stimulation

Abstract

Background: The mu event-related desynchronization (ERD) is supposed to reflect motor preparation and appear during motor imagery. The aim of this study is to examine the modulation of ERD with transcranial direct current stimulation (tDCS).

Methods: Six healthy subjects were asked to imagine their right hand grasping something after receiving a visual cue. Electroencephalograms (EEGs) were recorded near the left M1. ERD of the mu rhythm (mu ERD) by right hand motor imagery was measured. tDCS (10 min, 1 mA) was used to modulate the cortical excitability of M1. Anodal, cathodal, and sham tDCS were tested in each subject with a randomized sequence on different days. Each condition was separated from the preceding one by more than 1 week in the same subject. Before and after tDCS, mu ERD was assessed. The motor thresholds (MT) of the left M1 were also measured with transcranial magnetic stimulation.

Results: Mu ERD significantly increased after anodal stimulation, whereas it significantly decreased after cathodal stimulation. There was a significant correlation between mu ERD and MT.

Conclusions: Opposing effects on mu ERD based on the orientation of the stimulation suggest that mu ERD is affected by cortical excitability.

Figure 1

The time course of a single trial consisted of three states: relaxed state, cue state, and motor imagery state. The directions were displayed on a monitor in front of the subjects. A trial started with an 8-s period of a relaxed state during which the word "Rest" was shown at the center of the monitor. After that, a 2-s period during which a word "Ready" was shown began. Then, the word "Start" was presented for 5 s, and subjects were asked to imagine themselves grasping something with their right hand

Figure 2

Changes of mu ERD during the motor imagery of the right hand grasping something after the three types of tDCS (anodal, cathodal, and sham). a) mu ERD of each subject before and after the tDCS. Each symbol shows one subject. b) Changes of mu ERD before and after tDCS. The circles and vertical lines show the mean and standard deviation of the changes of mu ERD for each stimulation condition. *Post hoc LSD analysis showed a statistically significant difference (p < 0.05). ** Post hoc LSD analysis showed a statistically significant difference (p < 0.01).

Figure 3

Correlations between mu ERD and the MTs (RMT and AMT). Each symbol shows one subject. The x-axis shows the RMT (left) and AMT (right). The y-axis shows the mu ERD before the tDCS. Mu ERD showed a significant correlation with the RMT (r = 0.94, p < 0.05), whereas it did not show a significant correlation with AMT (r = 0.14)