The effect of transcranial direct current stimulation on the behavioral and neurophysiological performance of healthy subjects during reaching

It is well established that cathodal transcranial direct current stimulation (tDCS) can decrease the excitability of the primary motor cortex (M1) in humans. Despite the cortical inhibition caused by cathodal tDCS, it remains unknown how this intervention alters unrestrained dynamic reaching movements qualitatively. Accordingly, we designed this study to examine how cathodal tDCS impacts unrestrained dynamic reaching as measured by qualitative kinematic features and electromyography (EMG). Ten young, healthy adult subjects were recruited to participate in a two day protocol involving repetitively reaching to two different targets (large and small) both before and following cathodal tDCS applied over the contralateral M1 during one session and before and following sham tDCS over the same brain region during another session. We discovered that cathodal tDCS was not able to alter the kinematic features of reaching in these subjects but did degrade the EMG performance, specifically by increasing the amount of co-contraction between muscle pairs. Because co-contraction is an indicator of relatively unskilled performance, these results seem to indicate that cathodal tDCS of M1 preferentially disrupts the learning or execution of highly coordinated muscle firing patterns during dynamic reaching. This work adds to the growing body of knowledge about how tDCS applied over M1 affects our movements. Moreover, it leads us to believe that tDCS can be utilized to assist in rehabilitation of patient populations who suffer from neurological dysfunctions but EMG assessments may need to be included in order to more effectively assess the patient performance.