Modulation of interhemispheric inhibition and motor learning by different intensities of bilateral transcranial direct current stimulation in healthy adults

Interhemispheric inhibition (IHI) plays a crucial role in performing unilateral hand movements. Transcranial direct current stimulation (tDCS) has been shown to modulate IHI and enhance motor learning, but the most effective stimulation intensity remains uncertain. This study aimed to investigate how different intensities (1.0 mA, 1.5 mA, 2.0 mA, and sham) of bilateral tDCS affect IHI and motor learning in healthy adults. Twenty right-handed participants completed four randomized sessions, each involving bilateral tDCS application while performing a serial reaction time task. Resting-state electroencephalogram (EEG) was recorded before and after stimulation to assess cortical activity. The brain symmetry index (BSI) was used as an indicator of IHI. The result demonstrated that 2.0 mA produced the most prominent changes in BSI, particularly in frontal and central brain regions. Correspondingly, reaction times improved during and after stimulation at 2.0 mA. Furthermore, the degree of BSI modulation was associated with improvements in reaction time. These findings indicate that 2.0 mA may be a choice for enhancing both IHI and motor learning in healthy individuals. This study also highlights the potential of using BSI as a biomarker to monitor the effects of brain stimulation and guide the selection of intensity in both research and clinical applications.