Effects of electrode angle-orientation on the impact of transcranial direct current stimulation on motor cortex excitability

Background: For effects of transcranial direct current stimulation (tDCS), electrical field distribution and coverage of the target areas play a decisive role.

Methods: We explored the effect of different angle-orientations of tDCS electrodes applied over the upper limb motor cortex (M1) on motor cortex excitability in healthy volunteers. Sixteen individuals received 1 mA anodal or cathodal tDCS through 35 cm² electrodes over M1 for 15 min. Transcranial magnetic stimulation was used to examine tDCS-generated cortical excitability effects. The M1 electrode-orientation was following the right-left longitudinal plane, or positioned with 45° deviation from the midsagittal plane. Coverage of underlying brain and electrical field orientation were also investigated.

Results: Cortical excitability modulation was observed only when the electrode was aligned with 45° angle, which covered a larger area of the motor cortex.

Conclusion: an electrode angle-orientation of 45° induces superior neuroplastic effects of M1 due to a better alignment with the motor cortex.