Confined and persisting site-specific offline target engagement of pericentral cortex following bi-hemispheric TDCS of both primary motor hand areas

Background:Transcranial Direct Current Stimulation (TDCS) of the primary motor hand area (M1-HAND) has been shown to modulate corticomotor excitability, with aftereffects lasting up to several hours. Specifically, bi-polar TDCS targeting the right and left M1-HAND has been used to shift the interhemispheric balance. Pseudo-continuous Arterial Spin Labeling (pc-ASL) maps regional cerebral blood flow (rCBF) and can capture the rCBF changes that are caused by a change in regional neuronal activity in M1-HAND and connected areas during and after TDCS.
Objective: To elucidate how TDCS with a bi-hemispheric M1-HAND montage engages the bi-hemispheric pericentral cortex and shifts in interhemispheric balance.
Methods:In separate sessions, 20 healthy right-handed participants received 10 minutes of active TDCS (30 sec ramp-up and down) or sham TDCS (30 sec ramp-up, immediate ramp-down) at 2.0 mA with the anode placed over left M1-HAND and the cathode placed over right M1-HAND. We used 7 x 5 cm rubber electrodes with conductive gel. Real and sham TDCS were applied at two different days in counterbalanced order with subjects being blinded to the TDCS condition. We performed pc-ASL fMRI at 3 T concurrently with TDCS, to measure effects on rCBF both during and for 24 minutes after stimulation. We focused on the post-stimulation period of TDCS (offline effects) and used FSL’s FMRI Expert Analysis Tool (FEAT) to model rCBF changes after TDCS.
Results: Compared to sham TDCS, real TDCS induced a relative decrease in rCBF in the deep part of left M1-HAND underlying the anode. This effect persisted during the entire post-TDCS period, reaching the highest effect size 9-16 minutes after the end of TDCS (peak voxel: -30, -30, 52, z-score = 4.67). In contrast, there were no outlasting rCBF changes in the right M1-HAND underlying the cathode.
Conclusion: Our preliminary results show that bi-hemispheric TDCS with standard non-focal electrodes targeting left M1-HAND (anode) and right M1-HAND (cathode) induces lasting site-specific offline effects of TDCS. Aftereffects are regionally confined to the deep area of left M1-HAND underlying the anode. This unilateral aftereffect might underpin the previously reported shift in bi-hemispheric balance between the two M1-HAND.