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Abstract

Introduction Transcranial weak current stimulation (tCS) uses scalp electrodes to generate electrical currents in the brain and to modulate cortical excitability. Methods for optimizing the focality of the generated electric fields have been proposed. The electric fields, however, follow the Laplace Equation, which places a fundamental constraint on their focality. Here, we study the trade-off between stimulation intensity and focality. Methods Realistic head models (generated by www.simnibs.org) and simple spherical models were used to calculate fields generated by superpositions of many small electrodes (see Figure). An optimization procedure was applied to get the most focal stimulation while guaranteeing a specified minimal field strength at the target with a limited injected current Results The Laplace equation forces the maximum electrical field generated by tCS to be at the tissue boundaries for a homogeneous conductor, resulting in non-focal stimulation of deep cortical targets under normal circumstances. In addition, there is a natural trade-off between field intensity at the target and focality for a limited amount of injected current. These theoretical observations are confirmed by the results of the optimization approach. Conclusion Optimizing tCS montages for specific cortical targets can significantly improve electric field focality and intensity at the target, but fundamental physical limitations prevent very focal stimulations.