Electromagnetic articulography

Electromagnetic articulography (EMA) is a method of measuring the position of parts of the mouth. EMA uses sensor coils placed on the tongue and other parts of the mouth to measure their position and movement over time during speech and swallowing. Induction coils around the head produce an electromagnetic field that creates, or induces, a current in the sensors in the mouth. Because the current induced is inversely proportional to the cube of the distance, a computer is able to analyse the current produced and determine the sensor coil's location in space. EMA is used in linguistics and speech pathology to study articulation and in medicine to study oropharyngeal dysphagia. Unlike other methods of data collection, EMA does not expose subjects to ionizing radiation and allows for large amounts of data to be collected easily. The ability to observe the movements of articulators has been of great importance to the study of phonetics in order to understand the way sounds are produced. Electromagnetic articulography uses the principle of electromagnetic induction to measure the position and movement of various points in and around the mouth. A helmet containing electromagnetic transmitters creates a variable magnetic field by running currents through the transmitters at different frequencies. Sensor coils placed midsagittally in the mouth produce current as they move through the magnetic field inversely proportional to the cube of the distance from the transmitters. The current induced alternates at the same frequency as the transmitter coil and the composite signal can be separated out to determine the distance from each individual coil, thus determining the position of the sensor in space. In two-dimensional articulography, transmitter coils are placed in an equilateral triangle along the midsagittal plane at the forehead, chin, and neck. Because of the geometric orientation of the transmitter coils, accurate readings are able to be taken as long as the sensor coils placed on the tongue stay within about a centimeter of the midsagittal plane and are not angled at more than 30 degrees. Articulographs able to measure in three dimensions use six transmitter coils organized in a spherical configuration. The transmitters are arranged so that the axis of a sensor coil is never perpendicular to more than three transmitters. Through the transmitter configuration and ability to measure in multiple dimensions, three-dimensional articulographs are able to make measurements outside of the midsagittal plane. 2D articulographs require restrictive headmounts to ensure that the subject's head does not move off the plane of measurement. Because 3D articulographs are able to measure outside of the midsagittal plane, a less restrictive headmount is able to be used. Thomas Hixon was the first to describe the use of electromagnetic principles to measure articulation. In his letter to the editor, published in The Journal of the Acoustical Society of America, he described a setup using two sensor coils and one generator coil. The sensor coils, attached to the forehead and back of the neck, would remain stationary while the generator coil, attached to the jaw, would move creating variable currents in the sensor coils. These currents could then be used to determine the distance in two dimensions. Early EMA systems like Hixon's had problems with accounting for tilting of the tongue during use as tilting of the sensor coils causes changes in the induced current that can skew data. In 1987, Paul Schönle et al. published an improved system that used three transmitter coils (analogous to Hixon's generator coil) and computer software to triangulate the position of sensor coils and account for tilt. However modern two-dimensional systems are still unable to compensate for tilting of sensors past 30 degrees and measurement is distorted if sensor coils are moved off the centerline of the mouth. In 1993, Andreas Zierdt published a description of an articulograph that would be able to measure movement in three dimensions, though three dimensional articulographs have only been commercially available since around 2009. Zierdt's conceptualization placed six transmitter coils equidistant from each other. Because sensor coils are dipoles, when they are perpendicular to a transmitter coil the current induced is zero, so Zierdt angled the transmitter coils so that for any given rotation of a sensor coil, it was not perpendicular to more than three transmitter coils allowing for at least three transmitter coils to triangulate the position of the sensor.