Equivalent circuit simulation of the induced artifacts resulted from transcranial magnetic stimulation on human electroencephalography

A new method is proposed for describing the shape of the induced artifact in the electroencephalography (EEG) applied transcranial magnetic stimulation (TMS) using two equivalent circuit models, the TMS equipment model and the equivalent circuit model of bioelectric measurement system. The TMS equipment circuit is switched into three modes, and one of them is the oscillation mode by a capacitance in the equipment and a TMS coil. The electromagnetic induction from the TMS coil to the brain in this mode corresponds to the TMS. The bioelectric measurement system forms a closed circuit by a bioelectric equivalent circuit and an EEG measurement system, and the TMS induces the electromotive force into the closed circuit. The artifact induced by the TMS is measured as the TMS artifact, which is appended to the measured EEG. Under some simplified approximations, the TMS artifact is expressed by two modes, the TMS oscillation mode and the discharge mode, and the solutions of the TMS artifact are solved from circuit equations for each mode. The simulation output calculated from these solutions successfully describes the TMS artifact in the measured EEG data by considering the frequency property of the bioelectric circuit parameters.