Linking electric field directionality to treatment outcome in OCD: Insights from patient-specific tDCS modeling

Transcranial direct current stimulation (tDCS) is a promising intervention for treatment-resistant obsessive-compulsive disorder (OCD), yet clinical outcomes remain inconsistent. To investigate the neural mechanisms underlying therapeutic variability, we conducted a patient-specific finite element (FE) modeling study of electric fields (EF) induced by tDCS in OCD patients. Forty-two patients from a double-blind, randomized clinical trial received active tDCS with the cathode over the pre-supplementary motor area and the anode over the right supraorbital region. Clinical response was assessed using the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), and responders were defined as those achieving ≥35% score reduction. Individual head models were created using SimNIBS, and current density directionality ( $J_n$ ) and magnitude ( $J_{\mathrm{magn}}$ ) were analyzed. Voxel-wise comparisons revealed significantly greater depolarization ( $J_n$ > 0) in the left anterior prefrontal cortex (BA10) and right frontal eye field (BA 8) associated with a reduction of Y-BOCS. A link between hyperpolarization of right pars orbitalis (BA47) and improvement in symptoms was also found. Notably, no significant findings emerged using EF magnitude ( $J_{\mathrm{magn}}$ ), underscoring the relevance of current directionality in treatment response. To our knowledge, this is the first study to associate directional EF modeling with clinical outcomes in OCD. Our findings highlight the importance of considering both EF direction and anatomical variability when optimizing tDCS protocols. This approach may contribute to more personalized and effective neuromodulation strategies for psychiatric disorders.