Electromagnetic Articulography (EMA or EMMA)

Carstens AG501 Articulograph

Carstens AG501 Articulograph

Speech researchers quantify articulatory movement using a variety of techniques, including 3D imaging (x-ray, ultrasound, MRI) and fleshpoint tracking (x-ray microbeam, electropalatography, electromagnetic articulography). These technologies have provided important insights into the speech of healthy and disordered talkers. EMA (or EMMA, Electromagnetic Midsagittal Articulography) refers to kinematic tracking systems that use low field-strength electromagnetic fields to measure the movement of the tongue, lips, jaw, and velum. Two-dimensional (2D) EMA measures movement in the midsagittal plane. Subjects wear a helmet that places three transmitter coils around the head. The transmitters produce alternating magnetic fields which generate currents in tiny sensors placed on the surface of the articulators. As the sensors move through the fields, they are tracked by computer.

The earliest version of an EMA procedure was developed by Sonoda (1974) in which a permanent magnet was fastened to the tongue, with a second magnet located outside of the mouth. In this procedure, only the position of the magnet in the mouth could be determined. Hixon (1971) and van der Giet (1977) used alternating fields and various transmitter signals, enabling one to measure several points within the mouth. This idea was developed further in 1980 by Perkell who developed a unit with two transmitter coils and miniature sensors. The commercial 2D EMA systems found in laboratories today are based on a 1982 prototype developed at the Medical School of the University of Göttingen, Germany. In 1988, Carstens Medizinelektronik GmbH developed the first commercial Articulograph, the AG100.

Beginning in 1995, a 5-dimensional system has been developed by Carstens Medizinelektronik, the Phonetics department of the University of Munich, and NTT of Japan. In this system, six transmitters (AG500) or 9 transmitters (AG501) fixed on a case produce alternating magnetic fields at different frequencies. It is then possible to calculate the XYZ co-ordinates (as well as two angles) to measure, store and display the positions of the sensors. 3D magnetic tracking systems allow for more complete and accurate tongue visualization than previous 2D models. Also, because 3D systems do not require the use of a helmet, they are easier to use with individuals having movement or balance issues (including children, patients with cerebral palsy, Parkinson’s disease, etc.). These 3D systems are currently in use in a number of laboratories throughout the world.