HingePlace: Focused transcranial electrical current stimulation that allows subthreshold fields outside the stimulation target.

Transcranial Electrical Stimulation (TES) is a promising tool for treating many neurological disorders, but it classically results in diffused stimulation. Many optimization algorithms have been proposed for focusing TES, commonly by creating multi-electrode arrangements and choosing current amplitudes such that the resulting current fields in the brain are focused in the target region, and are as small as possible outside the target region. Consequently, it is likely that such optimization does not harness the non-linear nature of neural dynamics, particularly their thresholding phenomenon, i.e., the observation that neurons fire only when the stimulating currents are above a certain threshold. In this work, we propose HingePlace which explicitly harnesses this thresholding phenomenon by designing multi-electrode arrangements which allow the electric fields outside the target region to be non-zero but still below the stimulation threshold. In idealized simulated models, we compare HingePlace with existing algorithms and find that HingePlace performs strictly better, in some cases providing ~20% reduction in stimulated area for a specified limit on maximum injected current.