Modulation of iTBS after-effects via concurrent directional TDCS: A proof of principle study

Background: Polarising currents can modulate membrane potentials in animals, affecting the after-effect of theta burst stimulation (TBS) on synaptic strength. Objective: We examined whether a similar phenomenon could also be observed in human motor cortex (M1) using transcranial direct current stimulation (TDCS) during monophasic intermittent TBS (iTBS). Methods: TDCS was applied during posterior-anterior iTBS using three different conditions: posterior anterior TDCS (anode 3.5 cm posterior to Ml, cathode 3.5 cm anterior to M1), anterior-posterior TDCS (cathode 3.5 cm posterior to Ml, anode 3.5 cm anterior to M1), and sham TDCS. Results: When the direction of TDCS (posterior-anterior) matched the direction of the electrical field induced by iTBS, we found a 19% non-significant increase in excitability changes in comparison with iTBS combined with sham TDCS. When the TDCS was reversed (anterior-posterior), the excitatory effect of iTBS was abolished. Conclusion: Our findings suggest that excitatory aftereffects of iTBS can be modulated by directionally specific TDCS. (C) 2017 The Authors. Published by Elsevier Inc.