EmmaWe use an Electromagnetic Midsagittal Articulometer system that was designed at MIT and is known by the name EMMA (Perkell, Cohen, Svirsky, Matthies, Garabieta & Jackson, 1992). EMMA is designed for adult subjects who are seated in a chair. The system uses a three-coil transmitter assembly attached to the head by means of a headband. These transmitter coils produce alternating magnetic fields at three frequencies in the range from 50-75 kHz. Undue stress on the head and neck from the weight of the transmitter assembly is prevented by a counterbalancing weight acting through a pulley system attached to the ceiling. Receiver coils, approximately 1.5 x 1.5 x 1.0 mm in size, are attached to the subject's tongue, lips, jaw etc. by means of a dental adhesive. The magnitudes of the alternating voltages induced by the three transmitter coils provide position information which is conveyed to the inputs of electronic amplifiers and discriminators by fine wire leads. The rectified output voltages of the magnetometer, which represent the distances of each receiver from each of the transmitter coils, are subsequently input to a computer via an analog-to-digital converter. Computer software is then employed to convert these voltages into sample by sample positions in a Cartesian coordinate system.

The EMMA system provides inputs for ten receiver coils. Two of the ten receivers are usually attached to fixed points on the subject's head such as the maxilla and nasal bridge and are exclusively employed as reference points. These reference receivers are monitored by the conversion software which uses changes in their position to detect any unwanted head movement with respect to the helmet and to correct the data from the other eight receiver coils. In addition, the software provides the ability to rotate and/or translate the origin of the coordinate system. Thus, if position data are input from two other receivers attached to a Lucite bite plane (roughly 30mm apart) which is held between the subject's teeth (so defining the occlusal plane), this information can be used to compute a transformation that will relocate one of the bite-plane receivers to the origin of the coordinate system and rotate the subject's occlusal plane into coincidence with the x-axis. Subsequently this same transformation can be used to translate and rotate all the x,y data obtained from an experiment to the new coordinate location.

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