Figure 1. Methodology for a reproducible tooth, mucosa, and skin stimulation under static conditions. The subject sits comfortably with the upper teeth held in fixed relation to a probe mounted on the moving coil of a vibrator and bites into an impression of the upper and lower teeth on a metal bite bar. The impression material on the top part of the bite bar is cut away from around the area of stimulation (in this Fig., it is the upper central incisor tooth) so that the probe can 'move' the tooth. The shape, amplitude, and frequency of the stimulus wave are determined by means of a wave-generating computer program. The computer program is set to initiate random stimuli, with the interstimulus interval varying between 1 and 5 sec. Electrodes are placed to record the surface EMG from the ipsi- and contralateral masseter and digastric muscles. Force applied to the tooth is measured with strain gauges mounted in series with the stimulating probe. For feedback, the ipsilateral EMG signal is rectified and low-pass-filtered at 0.1 Hz, and displayed on an oscilloscope screen. The subject is asked to bite in such a way as to keep the level of activity of the ipsilateral masseter muscle at a pre-determined level (10-20% of the maximal voluntary contraction). In all experiments, the sound of the mechanical stimulus is masked by white noise played into earphones at 80-90 dB.