Improved reproducibility of magnetic stimulation-evoked motor potentials in the human masseter by a new method for locating stimulation sites on the scalp

In magnetic-stimulation studies where motor-evoked potentials (MEP) are measured on different occasions, accurate relocation of the stimulation site on the scalp is essential. Here a novel method of locating neural stimulation sites was tested and the reproducibility of MEPs in the human masseter assessed. The heads of the participants were immobilized in a plastic mask that incorporated a grid overlying the left cerebral hemisphere. A figure-of-eight coil was oriented with a micromanipulator. Electrodes were placed over the right masseter using a standardized method. Two discrete sites on the grid were stimulated alternately at threshold +10% whilst participants clenched their teeth. Recordings were repeated after the mask and electrodes had been removed and replaced. Within individual participants, the latency and amplitude of the MEP were reproducible between trials and when the mask and electrodes were replaced. The new method appears to be accurate and practical when recording longitudinal MEP data in the masseter.