Interhemispheric modulation induced by cortical stimulation and motor training.

BACKGROUND Interhemispheric inhibition might be a beneficial cortico-cortical interaction, but also might be maladaptive in people with neurological disorders. One recently revisited technique that has been shown to be effective in improving motor function in people with stroke using interhemispheric modulation is transcranial direct current stimulation (tDCS). OBJECTIVE The aim of this study was to investigate the effects of tDCS combined with unilateral motor training with contralateral hand restraint on interhemispheric inhibition between the dominant and nondominant hemispheres of the brain and on motor performance in participants who were healthy. DESIGN This was a double-blind, prospective, single-center study with participants who were healthy. METHODS Twenty participants who were healthy were randomly assigned to receive either active or sham tDCS of the primary motor cortex (M1) bilaterally combined with unilateral motor training and contralateral hand restraint. A blinded rater assessed motor function and cortical excitability, including assessment of transcallosal inhibition (TCI). RESULTS There was a larger increase in motor performance in the nondominant hand for the active tDCS group compared with the sham tDCS group. In addition, a decrease in cortical excitability in the dominant hemisphere and a decrease in TCI from the dominant to nondominant hemisphere were observed for the active tDCS group only. The TCI decrease in the active tDCS group was correlated with motor performance improvement for the nondominant hand. Limitations Limitations of this study included missing the effect of intracortical inhibition due to a floor effect, not using the optimal tDCS montage, and not being able to assess the effects of other variables such as gender due to the small sample size. CONCLUSIONS The results indicate that tDCS enhances the effects of unilateral motor training and contralateral hand restraint on motor function, and this benefit is associated with a different mechanism of action characterized by bihemispheric modulation in which TCI from the dominant to the nondominant hemisphere is decreased. Transcranial direct current stimulation might be a useful tool to enhance the motor effects of constraint-induced movement therapy.