Simulating individually targeted transcranial electric stimulation for experimental application

Transcranial electric stimulation (tES) induces electric fields that are subject to a complex interaction with individual anatomical properties, such as the low-conducting human skull, the distribution of cerebrospinal fluid or the sulcal depth, as well as stimulation target location and orientation. This complex interaction might contribute to the heterogenous results that are commonly observed in applications of tES in humans. Targeted tES, on the other hand, might be able to account for some of these individual factors. In the present study, we used the finite-element method (FEM) and head models of twenty-one participants to evaluate the effect of individually targeted tES on simulated intracranial current densities. Head models were based on an automated segmentation algorithm to facilitate processing in experimental sample sizes. We compared a standard stimulation montage to two individually optimized tES montages using an Alternating Direction Method of Multipliers (ADMM) and a Constrained Maximum Intensity (CMI) approach. A right parietal target was defined with three different orientations. Individual current densities showed varying intensity and spatial extent near the lower limit at which physiological efficacy of electric fields can be assumed. Both individually optimized targeting algorithms were able to control the electric field properties, with respect to intensities and/or spatial extent of the electric fields. Still, across head models, intensity in the stimulation target was constrained by individual anatomical properties. Thus, our results underline the importance of targeted tES in enhancing the effectiveness of future tES applications and in elucidating the underlying mechanisms.