Je pense donc je fais: transcranial direct current stimulation modulates brain oscillations associated with motor imagery and movement observation

Abstract

Motor system neural networks are activated during movement imagery, observation and execution, with a neural signature characterized by suppression of the Mu rhythm. In order to investigate the origin of this neurophysiological marker, we tested whether transcranial direct current stimulation (tDCS) modifies Mu rhythm oscillations during tasks involving observation and imagery of biological and non-biological movements. We applied tDCS (anodal, cathodal, and sham) in 21 male participants (mean age 23.8 ± 3.06), over the left M1 with a current of 2 mA for 20 min. Following this, we recorded the EEG at C3, C4, and Cz and surrounding C3 and C4 electrodes. Analyses of C3 and C4 showed significant effects for biological vs. non-biological movement (p = 0.005), and differential hemisphere effects according to the type of stimulation (p = 0.04) and type of movement (p = 0.02). Analyses of surrounding electrodes revealed significant interaction effects considering type of stimulation and imagery or observation of biological or non-biological movement (p = 0.03). The main findings of this study were (1) Mu desynchronization during biological movement of the hand region in the contralateral hemisphere after sham tDCS; (2) polarity-dependent modulation effects of tDCS on the Mu rhythm, i.e., anodal tDCS led to Mu synchronization while cathodal tDCS led to Mu desynchronization during movement observation and imagery (3) specific focal and opposite inter-hemispheric effects, i.e., contrary effects for the surrounding electrodes during imagery condition and also for inter-hemispheric electrodes (C3 vs. C4). These findings provide insights into the cortical oscillations during movement observation and imagery. Furthermore, it shows that tDCS can be highly focal when guided by a behavioral task.

Keywords: EEG; action observation; motor imagery; mu rhythm; primary motor cortex; tDCS.

Figure 1

Experimental Design. The order of observation videos was random and always followed by the imagery correspondent condition. The surrounding electrodes are: surround C3 (EGI electrodes: 29, 30, 35, 37, 41, 42), Cz, surround C4 (EGI electrodes: 87, 93, 103, 105, 110, 111). The sound symbols represent the moment when the metronome stimulus occurs pacing each cycle. For tDCS conditions red represents the anode and black represents cathode. SO represents the supraorbital (reference) area.

Figure 2

TDCS effects on C3 and C4. Bars mean standard errors. ○, sham tDCS resulted in opposing effects for biological and non-biological conditions at C3 (p = 0.005); ■, anodal tDCS resulted in ERS at C3 and ERD at C4 (p = 0.01); ▲, cathodal tDCS resulted in Mu ERD compared to anodal (p = 0.03) and sham (p = 0.02) tDCS at C3 during non-biological movement; ●, anodal tDCS resulted in Mu ERD when compared to cathodal (p = 0.01) and sham (p = 0.02) tDCS at C4 during biological movement; ♦, anodal tDCS resulted in Mu ERS when compared to cathodal (p = 0.001) and sham (p = 0.002) tDCS at C3 during biological movement.

Figure 3

TDCS effects on surrounding electrodes. Bars mean standard errors. ○, after anodal tDCS, biological imagery was significantly different from non-biological imagery (p = 0.028); ●, anodal tDCS resulted in Mu ERD when compared to cathodal tDCS in biological imagery (p = 0.011); ▲, cathodal tDCS resulted in stronger Mu ERD on biological observation when compared to anodal (p = 0.023) and sham (p = 0.05) tDCS; and ■, anodal tDCS resulted in Mu ERS on non-biological observation when compared to non-biological imagery (p = 0.048).