Motor evoked potential changes in response to transcranial direct current stimulation correlate with quantitative EEG changes in subjects with chronic spinal cord injury

Abstract

Background : ranscranial direct current stimulation (tDCS) is emerging as a promising method to treat motor and sensory impairment following neurological disorders such as spinal cord injury (SCI). However, the degree to which responses to tDCS are personalized from subject‐to‐subject, and how this affects physiological response, is largely unknown.

Methods: We conducted a randomized, single‐blind, sham‐controlled, cross‐over study in seven chronic SCI subjects with cervical lesions. We applied 20‐ minutes of anodal tDCS over the left primary motor cortex (M1). Subjects were randomized to receive either 1mA or sham stimulation using the StarStim 8‐channel system, with the anodal electrode placed at C3, and the return electrode placed at AF8 (Neuroelectrics, inc). Five‐minute EEG recordings were performed pre‐ and post‐tDCS protocols at F3, F4, Cz, C3, C4, P3, P4 and AF8 recording sites. Motor evoked potential (MEP) amplitude (peak‐to‐peak; Extensor Carpi Radialis muscle) was measured following single‐pulse TMS set at 120% of the Resting Motor Threshold over the C3 electrode.

Results: A very strong negative correlation (r = ‐0.93; p<0.008; Pearson’s correlation) was noted between pre‐/post‐tDCSMEP amplitude changes and pre‐/ post‐tDCS Beta‐2 coherence changes at the AF8 electrodes. Similar correlations existed at electrodes surrounding AF8, but weakened with increased distance from AF8. No such correlations were seen during the sham condition.

Significance: These findings provide initial evidence that tDCS is capable of inducing modulation of oscillatory brain rhythms that correlate with individualized physiological responses to tDCS. This sets the stage for techniques that can optimize tDCS protocols targeting motor cortex.