Abstract
BackgroundRecent 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. ObjectiveWe asked whether the directional effects of TDCS on physiological measures in the motor system would also be observed on motor behaviours. MethodsWe 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, TDCSPA; and anterior-posterior, TDCSAP). Transcranial magnetic stimulation (TMS) was used to assess whether changes in corticospinal excitability reflected any behavioural changes. ResultsDirectional TDCSAP impaired the retention of learning on the ballistic movement task compared to TDCSPA and a sham condition. Although TDCSPA had no effect on learning or retention, it blocked the typical increase in corticospinal excitability after a period of motor practice. ConclusionsOur 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.
Highlights
Methods of transcranial electrical stimulation of the brain, transcranial direct current stimulation (TDCS), which is low cost and easy to apply, have achieved widespread popularity
There was a main effect of TDCS group (F[2,45] 1⁄4 4.779, P 1⁄4 0.013, hp2 1⁄4 0.175), with post hoc tests confirming a greater increase in motor evoked potential (MEP) amplitudes for TDCSSham compared to the TDCSPA (P 1⁄4 0.006), with no differences between TDCSPA and TDCSAP groups (P 1⁄4 0.210) or between TDCSSham and TDCSAP groups (P 1⁄4 0.589)
In the absence of real TDCS (i.e. TDCSSham), MEP amplitudes increased in the abductor pollicis brevis (APB) and were unchanged in the abductor digiti minimi (ADM) after motor practice (Fig. 4), as would be expected [19]
Summary
Methods of transcranial electrical stimulation of the brain, transcranial direct current stimulation (TDCS), which is low cost and easy to apply, have achieved widespread popularity. One factor that we have studied recently is the effect of applied field direction with respect to the orientation of cortical neurones [10,11]. 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. Results: Directional TDCSAP impaired the retention of learning on the ballistic movement task compared to TDCSPA and a sham condition. TDCSPA 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 impacts upon behavioural outcomes.
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