Dipole forward simulation guides transcranial electric stimulation of the hand knob area

Transcranial electric stimulation (TES) is a non-invasive technique driving small currents to the brain to modulate neuronal activity. Applications of TES in therapy of neurophysiological disorders require implementations of focused TES to specific target areas in the brain. We introduce a new technique based on the idea of reciprocity, where targeted TES is guided by resampling electric field features originating from an electric current dipole in the target area. We compute the current density in the brain and the electric scalar potential on the scalp surface of a volunteer produced by a dipole in the target area. The resulting minima of the current density distribution are used to guide a bipolar TES stimulation. The maximum and minimum of the potential on the scalp surface serve for polarity selection of the TES electrodes. We compare our results to those obtained with a distributed beam former TES simulation for the same target area. We found that the beam former based TES produced the maximum of the current density in a different brain area than the target area, while our new method produced the maximum at the target area. In the target area, the beam former based TES produced a factor 3 lower current densities than our new method. Our new approach provides a simple means for guiding targeted TES in clinical studies.