Inconsistent outcomes of transcranial direct current stimulation (tDCS) may be originated from the anatomical differences among individuals: A simulation study using individual MRI data

Transcranial direct current stimulation (tDCS) is a kind of neuromodulation protocol, which transmits small amount of DC currents through scalp electrodes to facilitate or inhibit particular areas of the brain. Although many studies have demonstrated that tDCS can effectively modulate excitability of various brain sites, the outcomes of the tDCS treatment are not consistent among subjects to whom identical electrode montages were applied. So far, no studies have clearly elucidated the main cause of this individual variability. The hypothesis of our study was that the individual variability in the tDCS effect might be originated due to the anatomical differences among subjects. To verify our hypothesis, we investigated the relationship between the current density value at dorsolateral prefrontal cortex (DLPFC) simulated using finite element method (FEM) and the behavioral outcomes of a simple working memory (WM) task. A 3-back WM task experiment was conducted with twenty-five healthy subjects before and after the DC stimulation, when the cathode and anode electrodes were attached to right supraorbital area and F3 location, respectively, for all subjects. The results showed that participants who showed enhanced WM task performance after tDCS had a significantly larger current density on DLPFC, suggesting that the inconsistent behavioral outcomes of tDCS might be partially due to the anatomical differences among subjects.