Delineating the effects of anodal transcranial direct current stimulation on myoelectric control based on slow cortical potentials

Active cortical participation in rehabilitation procedures may be facilitated by modulating neuromuscular electrical stimulation (NMES) with electromyogram (EMG) and electroencephalogram (EEG) derived biopotentials, that represent simultaneous volitional effort. Here, the ability of the nervous system to respond to intrinsic or extrinsic stimuli by reorganizing its structure, function, and connections is called neuroplasticity. Neuroplasticity is involved in post-stroke functional disturbances, but also in rehabilitation. Beneficial neuroplastic changes may be facilitated with an adjuvant treatment with non-invasive brain stimulation (NIBS). This paper presents the results from a motor cortex anodal tDCS-EEG/EMG study in healthy volunteers. We investigated slow cortical potentials (SCP) during self-initiated movements. In this preliminary study, we found that anodal tDCS increased baseline-normalized post-tDCS mean power in the Theta band (4-8Hz) of resting state EEG (60.71% vs. 8.36%; p<;0.01), and decreased the slope of post-tDCS SCP from motor task-related EEG (-6.43 au/sec vs. -4.86au/sec; p=0.021) when compared to sham tDCS. These preliminary results are discussed based on an accumulator model for spontaneous neural activity which postulates that a decision threshold applied to auto-correlated noise - in this case the output of a leaky stochastic accumulator - can account for the specific shape of the SCP prior to movement. We postulate that the anodal tDCS facilitated change in the slope of SCP may be related to the reaction times during a cued movement task since our prior work showed that anodal tDCS decreases the delay in initiation of muscle contraction and increases the delay in termination of muscle activity.