Effects of simultaneous bilateral tDCS of the human motor cortex

Background: Transcranial direct current stimulation (tDCS) is a noninvasive technique that has been investigated as a therapeutic tool for different neurologic disorders. Neuronal excitability can be modified by application of DC in a polarity-specific manner: anodal tDCS increases excitability, while cathodal tDCS decreases excitability. Previous research has shown that simultaneous bilateral tDCS of the human motor cortex facilitates motor performance in the anodal stimulated hemisphere much more than when the same hemisphere is stimulated using unilateral anodal motor cortex tDCS.

Objective: The main purpose of this study was to determine whether simultaneous bilateral tDCS is able to increase cortical excitability in one hemisphere whereas decreasing cortical excitability in the contralateral hemisphere. To test our hypothesis, cortical excitability before and after bilateral motor cortex tDCS was evaluated. Moreover, the effects of bilateral tDCS were compared with those of unilateral motor cortex tDCS.

Methods: We evaluated cortical excitability in healthy volunteers before and after unilateral or bilateral tDCS using transcranial magnetic stimulation.

Results: We demonstrated that simultaneous application of anodal tDCS over the motor cortex and cathodal tDCS over the contralateral motor cortex induces an increase in cortical excitability on the anodal-stimulated side and a decrease in the cathodal stimulated side. We also used the electrode montage (motor cortex-contralateral orbit) method to compare the bilateral tDCS montage with unilateral tDCS montage. The simultaneous bilateral tDCS induced similar effects to the unilateral montage on the cathode-stimulated side. On the anodal tDCS side, the simultaneous bilateral tDCS seems to be a slightly less robust electrode arrangement compared with the placement of electrodes in the motor cortex-contralateral orbit montage. We also found that intersubject variability of the excitability changes that were induced by the anodal motor cortex tDCS using the bilateral montage was lower than that with the unilateral montage.

Conclusions: This is the first study in which cortical excitability before and after bilateral motor cortex tDCS was extensively evaluated, and the effects of bilateral tDCS were compared with unilateral motor cortex tDCS. Simultaneous bilateral tDCS seems to be a useful tool to obtain increases in cortical excitability of one hemisphere whereas causing decreases of cortical excitability in the contralateral hemisphere (e.g.,to treat stroke).