Sensorimotor Rhythm BCI with Simultaneous High Definition-Transcranial Direct Current Stimulation Alters Task Performance

Abstract

Background: Transcranial direct current stimulation (tDCS) has been used to alter the excitability of neurons within the cerebral cortex. Improvements in motor learning have been found in multiple studies when tDCS was applied to the motor cortex before or during task learning. The motor cortex is also active during the performance of motor imagination, a cognitive task during which a person imagines, but does not execute, a movement. Motor imagery can be used with noninvasive brain computer interfaces (BCIs) to control virtual objects in up to three dimensions, but to master control of such devices requires long training times.

Objective: To evaluate the effect of high-definition tDCS on the performance and underlying electrophysiology of motor imagery based BCI.

Methods: We utilize high-definition tDCS to investigate the effect of stimulation on motor imagery-based BCI performance across and within sessions over multiple training days.

Results: We report a decreased time-to-hit with anodal stimulation both within and across sessions. We also found differing electrophysiological changes of the stimulated sensorimotor cortex during online BCI task performance for left vs. right trials. Cathodal stimulation led to a decrease in alpha and beta band power during task performance compared to sham stimulation for right hand imagination trials.

Conclusion: These results suggest that unilateral tDCS over the sensorimotor motor cortex differentially affects cortical areas based on task specific neural activation.

Keywords: BCI; Brain-computer interface; Motor imagery; Sensorimotor rhythm; Transcranial direct current stimulation; tDCS.

Figure 1

Task design and experimental setup. Experimental session: the subject performs 4 runs of 18 trials before stimulation, undergoes 20 minutes of stimulation and BCI trials, performs 4 runs after stimulation, performs a 13 minute visual oddball experiment with right hand response, and performs 4 runs delayed after stimulation (upper left). Setup of HD-tDCS electrodes (black circles) embedded within the 64 channel EEG cap (upper right). Single trial sequence of events: after the target is presented for 3 seconds, a red ball appears on the screen and moves based on the SMR control signal for a maximum of 6 seconds, and followed by a 3 second inter-trial interval (lower panel).

Figure 2

Percent valid correct across time for left and right hand trials with the baseline from each individual first session pre-stimulation block subtracted from the values at later time points. Time points represent session-block PVC values, starting at Pre-stim Session One and ending at Delayed Post-stim Session Three. Group PVC did not significantly change over the three sessions. Values: Mean +/− S.E.

Figure 3

Time to successfully hit right and left hand targets within experimental sessions normalized to subject initial baseline prior to stimulation. Time points represent session-block PVC values, starting at Pre-stim Session One and ending at Delayed Post-stim Session Three. The anodal group displayed a reduced time-to-hit for right hand trials compared to the cathodal group. There was no difference for left hand trials. Values: Mean +/− S.E. *p<0.05 for Kruskal Wallis test.

Figure 4

Time to successfully hit right and left hand targets within experimental sessions normalized to the pre-stimulation baseline for each session. The anodal group had a reduced time-to-hit for right hand trials following stimulation at the delayed time point for right hand but not left hand trials. Values: Mean +/− S.E. *p<0.05 for Wilcoxon rank sum test.

Figure 5

Mean alpha power during task performance in the task blocks after stimulation normalized to the pre-stimulation power for left and right hand trials. For right hand trials, cathodal stimulation decreased the alpha power in the stimulated hemisphere during task performance immediately following stimulation compared to the anodal and sham groups. Colors represent power normalized to pre-stimulation baseline. Black circles represent tDCS electrodes located over the left sensorimotor cortex.

Figure 6

Mean alpha power during task performance normalized to pre-stimulation trial power for successful right hand trials in the C3 and CP3 electrodes across the alpha (8-13Hz) and beta (15-30Hz) frequency bands. There is decreased alpha and beta power in the cathodal group compared to the sham group during right hand task performance in C3 immediately after stimulation and at the delayed post-stimulation time point and in CP3 at the delayed post-stimulation time point. There is no significant difference for left hand trials. Values: Mean +/− S.E. *p<0.0125 for Wilcoxon rank sum test. **p<0.05 for Kruskal Wallis test.