A modular system for multichannel Transcranial Magnetic Stimulation: physical and physiological characterization of electric field profiles.

BACKGROUND: Transcranial Magnetic Stimulation (TMS) is an established non-invasive neuromodulation technique, with growing evidence suggesting that targeting multiple interconnected nodes may enable more selective network-level control. However, multisite stimulation remains constrained by the fixed field geometry and limited focality of conventional coils. OBJECTIVE: To introduce and characterize a modular multichannel transcranial magnetic stimulation (mTMS) array capable of electronically steering the induced electric field (E-field) without mechanical coil movement. METHODS: The array consists of two custom-made 3-axis TMS coils arranged with a slight tilt to approximate head curvature and enhance stimulation depth and efficiency. This modular architecture allows flexible adjustment of coils spacing and orientation. Computational simulations and in vivo experiments demonstrate that independently driven coil elements can be combined to form distinct "virtual coils," enabling controlled electronic shifts of the E-field hotspot. RESULTS: Using a physical-versus-electronic hotspot displacement paradigm with repeated resting motor threshold (rMT) estimation, we show that electronic E-field shifts of ±1 cm produce effects comparable to physically moving the coil. Computational modeling analysis confirms that the electronically synthesized virtual coil configurations elicit systematic E-field shifts in precentral gyrus that are spatially consistent with the measured rMT at each physical coil array location. The system achieved functional resolution consistent with the known spatial accuracy of TMS, with variability within expected limits of neuronavigation and calibration errors. CONCLUSION: These findings establish the feasibility and physiological relevance of electronically controlled E-field steering using a modular coil array. This platform provides a scalable foundation for next generation mTMS systems supporting multifocal stimulation of distributed brain networks.