In silico exploration of electric field distribution in tDCS: integrating white matter anisotropy and subject-specific structural connectivity

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique with promising application in the treatment of neurological and psychiatric disorders. However, its effectiveness is often limited by the high inter-subject variability of the induced effects, mainly attributable to individual anatomical differences, which are not considered in the design of the stimulation protocols. Among these, structural connectivity plays a crucial role but remains often overlooked in tDCS research. Objective—This study aims to evaluate how variations in structural connectivity influence the distribution of the electric field (EF) during tDCS session. In particular, we analyse how the inclusion of white matter anisotropy affects the EF distribution and spread compared to classical isotropic models, and how the strength of connection across cortical parcels affects the EF spread. Approach—The study proposes an advancement in the computational modelling of tDCS through the integration of white matter anisotropy into finite element method (FEM) simulations. By combining advanced computational approaches, we explore the relationship between EF strength and cortical connectivity. Main results—Neglecting white matter anisotropy in electromagnetic simulations lead to a relative error in EF magnitude greater than 10% and to an orientation error of the EF vector of almost 20 degrees. The DTI-informed simulations lead to a more focalized EF distribution, moreover it was found a positive and significant (p < 0.05) correlation between EF focality and the strength of connectivity between cortical areas below P2 and P1 electrodes. Significance—These findings highlight the importance of including white matter anisotropy into tDCS simulation to prevent distortions in EF distribution and suggest the need to integrate structural connectivity information into the definition of subject-specific dose in tDCS protocols.