Precise coil positioning system using multi-articular arm for location of stimulated brain area in transcranial magnetic stimulation

We developed a precise coil positioning system for determining stimulated sites in transcranial magnetic stimulation (TMS) by using a multi-articular mechanical arm. A figure-of-eight coil used in TMS can induce a focal electric field beneath the coils center in the cortex; therefore, the point on the cortex beneath the center of the figure-of-eight coil has been conventionally regarded as a stimulating site. However, a stimulating site determined in this manner is not believed to be plausible, because the electric field induced in TMS is dispersed over the brain vectorially. Our system was developed on the basis of the cortical structure of the brain cortex and the direction of the induced electric field. In order to verify the system accuracy, Motor Evoked Potentials (MEPs) elicited by TMS at the hand muscles innervated by the primary motor cortex (M1) were measured as electromyogram (EMG). And functional vectorial map of the excitability of M1 in the resting state was obtained by rotating the stimulation coil at a constant location.