Direction of TDCS current flow in human sensorimotor cortex influences behavioural learning

BACKGROUND: Recent studies have shown that neurophysiological outcomes of transcranial direct current stimulation (TDCS) are influenced by current flow in brain regions between the electrodes, and in particular the orientation of current flow relative to the cortical surface. OBJECTIVE: We asked whether the directional effects of TDCS on physiological measures in the motor system would also be observed on motor behaviours. METHODS: We applied TDCS during the practice of a ballistic movement task to test whether it affected learning or the retention of learning 48 h later. TDCS electrodes were oriented perpendicular to the central sulcus and two current orientations were used (posterior-anterior, TDCS(PA); and anterior-posterior, TDCS(AP)). Transcranial magnetic stimulation (TMS) was used to assess whether changes in corticospinal excitability reflected any behavioural changes. RESULTS: Directional TDCS(AP) impaired the retention of learning on the ballistic movement task compared to TDCS(PA) and a sham condition. Although TDCS(PA) had no effect on learning or retention, it blocked the typical increase in corticospinal excitability after a period of motor practice. CONCLUSIONS: Our results extend on previous reports of TDCS producing directionally specific changes in neurophysiological outcomes by showing that current direction through a cortical target also impacts upon behavioural outcomes. In addition, changes in corticospinal excitability after a period of motor practice are not causally linked to behavioural learning.