A multiple electrode scheme for optimal non-invasive electrical stimulation

Transcranial electrical stimulation involves the delivery of weak electrical currents to the brain via scalp electrodes to elicit neuromodulatory effects. The current is conventionally passed through two large electrodes resulting in diffused electric fields. In this paper, we propose a novel paradigm in which multiple small electrodes with independent current controls are systematically optimized to yield targeted and effective stimulation under safety constraints. We employ the finite element method, in conjunction with a magnetic resonance imagery based model of the human head, to formulate a linear system relating the applied scalp current to the resulting electric field. Optimization techniques are then applied to derive stimulation parameters which maximize either intensity or focality at the target location. Results demonstrate that the optimal electrode configuration is strongly dependent on both the desired field orientation and the optimization criterion. The proposed scheme yields improvements of 98% in target intensity and 80% in focality compared to the conventional two-electrode montage. Additionally, the presented framework effectively optimizes electrode placement in the classical bipolar configuration, which is useful if only a single channel current source is available. Consequently, the proposed scheme promises to deliver increased efficacy and improved patient safety to clinical settings in which the target site is identified by a clinician.