Anodic Electrode Displacement Affect the Efficiency of Transcranial Direct Current Stimulation: a Modeling Study on the Electrode Sizes

Transcranial Direct Current Stimulation (tDCS) is a non-invasive treatment approach aiming at modulating brain functions. Conventional large patch electrodes and emerging dense electrode arrays are used in clinical trials and results show great promise of tDCS in both cognitive enhancement for healthy subjects and disease treatment for patients. However, inconsistent outcomes on the efficacy of tDCS were recently reported. Besides the anatomical differences of participants, it is plausible that the inevitable displacement of the stimulation electrode caused the inconsistency. To verify this hypothesis and analysis how the size of electrodes affect stimulation efficiency, a human head model was constructed with displaced electrodes of different sizes. Current density induced by the original C3-Fp2 and directional shifting montages were computed by finite element method. Our results showed that minor displacement of the stimulation electrode led to non-negligible changes of the current density distribution to the targeting cortex. Peak variance was up to 45 percent for electrodes with different sizes. For accurate targeting in tDCS, this study indicates that precise placement is essential for both conventional large patch electrodes and small-size electrodes of high-density arrays.