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MAGNETOMETRY
 We 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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