(January 21, 2003) - Bethesda, MD – If the classic musical My Fair Lady had followed the laws of kinetics, the study of motion in the body, then language expert Henry Higgins might have never been able to teach "proper English" to flower girl Eliza Doolittle.
Earlier research findings have demonstrated that pronunciation alone may not be key for kinetics, because the jaw has a natural stiffness, causing variability in motion as speech is made. A new research study reports findings on jaw stiffness, and assesses the relationship between patterns of jaw stiffness and kinematic variability during speech production.
Stiffness, as defined in kinetics, is a measure of resistance to displacement and is affected by central neural inputs, reflexes, muscle properties, and the geometry of muscle attachments. Human jaw stiffness may affect kinematic variability (differences in motion) found in speech. Because stiffness characterizes resistance to displacement, directions in which stiffness is high may exhibit less variability than directions in which stiffness is low. Accordingly, there is a probability that the pattern of variability in jaw position that accompanies the production of speech sounds reflects the directional asymmetries of stiffness of the jaw.
A comparison of the relationship between stiffness and kinematic variation in the jaw against the patterns was observed when subjects compensate for destabilizing loads to the limb. The ability of subjects to compensate for these loads has been shown to depend on the direction of the disturbance relative to the shape of the stiffness field at the hand. Specifically, there is a greater effect on limb position when loads act in directions of lower stiffness and a smaller effect in directions of higher stiffness. Consequently, kinematic variation in speech production may be similarly dependent on the magnitude of jaw stiffness in different directions.
The authors of "Relationship Between Jaw Stiffness and Kinematic Variability in Speech," are Douglas M. Shiller and David J. Ostry, both from McGill University, Montreal, Quebec, Canada (Dr. Ostry is also affiliated with Haskins Laboratories, New Haven, CT) and Rafael Laboissiere at the Max Planck Institute, Munich, Germany, and Institut de la Communication Parle´e, Grenoble, France. Their findings appear in the November 2002 edition of Journal of Neurophysiology. The journal is one of 14 scientific journals published monthly by the American Physiological Society (APS).
Six subjects (four males and two females) participated in the measurement of jaw stiffness. Like measures of kinematic variability were obtained for the same subjects; none reported any history of speech motor disorder or temporo-mandibular joint dysfunction.
Measurement of jaw stiffness: Measures of jaw stiffness were obtained by connecting a computer-controlled robotic device to an acrylic and metal dental appliance that was attached to the mandibular teeth. The appliance was custom molded for each subject and glued to the buccal surface of the teeth using a dental adhesive. The coupling between the robot and the jaw was achieved by using a magnesium and titanium rotary connector that permitted motion of the jaw in all six translational and rotational degrees of freedom. Subjects were thus able to produce naturalistic jaw movements when coupled to the robot.
Measurement of jaw kinematic variability: Measures of jaw kinematic variability were obtained by tracking the motion of the head and jaw. To measure head motion, four infra-red emoting diodes (IREDS) were placed on a dental appliance that was attached to the maxillary teeth. Jaw movement was measured using four additional IREDs that were attached to an appliance on the mandibular teeth. The dental appliances had little effect on the intelligibility of the utterances tested in this study. IRED motion was measured at a sampling rate of 200 Hz. Measures of jaw kinematic variability associated with vowel production were obtained from repetitions of simple speech sequences. Each sequence consisted of a consonant-vowel-consonant (CVC) combination that was embedded in a carrier sentence. Twelve CVC sequences were tested, resulting from the combination of the consonants /s/, /t/, and /k/ with the four vowels that were used in the measurement of jaw stiffness: /i/, /e/, /a/, and /æ/. The consonants were chosen to vary the jaw position at closure. The test sentences were of the form "say sesy again" or "say taty again."
Jaw stiffness: Measures of position and force at the mandibular central incisors were transformed into a common head-centered co-ordinate system. The origin is at the tip of the maxillary incisors. The "horizontal" axis of this coordinate system is aligned with the occlusal axis (the positive direction corresponds to jaw protrusion). The vertical axis passes through the incisors and is orthogonal to the horizontal axis (jaw lowering is in the negative direction).
Kinematic variability: The 3D position data for each IRED were digitally low-pass filtered using a second-order zero phase lag Butterworth filter with a cutoff frequency of 10 Hz. The IRED motions were transformed into a six dimensional rigid-body representation of jaw position and orientation in head-centered coordinates. The origin of this new coordinate system is at the tip of the maxillary incisors in the mid-sagittal plane. The "horizontal" axis is aligned with the occlusal plane. Consistent with jaw stiffness measures, data analysis of jaw motion was restricted to vertical and horizontal position in the sagittal plane. An interactive computer program was used to identify the jaw position associated with the production of the vowel within each CVC. In the kinematic record, CVC production is typically associated with a relatively large-amplitude opening and closing movement of the jaw corresponding in time to the voiced portion of the CVC utterance. The position of maximum opening during the CVC was determined to be the position of the jaw during vowel production.
Simulation studies: The simulations were based on a model of sagittal plane jaw and hyoid movement. The model includes seven muscle groups corresponding to anterior temporalis, posterior temporalis, lateral pterygoid, masseter, anterior digastric, posterior digastric, and sternohyoid. These muscle groups contribute to motion in four kinematic degrees of freedom: horizontal jaw position, sagittal plane jaw orientation (about an axis through the center of the mandibular condyle), horizontal hyoid position, and vertical hyoid position.
In this study, researchers report on direct measures of jaw stiffness in the fore and aft plane that were obtained by applying precise mechanical disturbances to the jaw at positions associated with the production of vowels. The level of jaw stiffness was found to be variable, with higher stiffness observed along the axis of jaw protrusion and retraction. Patterns of kinematic variability were assessed for the same set of speech sounds and were shown to be related in a detailed manner to the directions and magnitudes of jaw stiffness. Simulation studies supported the idea that the pattern of jaw stiffness arises from the balance of muscle-force-generating abilities and musculoskeletal geometry in the orofacial system.
The pattern of kinematic variation was shown to be dependent on jaw geometry and jaw muscle properties. The dependence of variability in tongue movement on similar factors is unknown. However, the present findings underscore the importance of understanding the contribution of muscle mechanical and geometrical factors in the interpretation of patterns of kinematic variation.
Previous work on human arm movement suggests the possibility that the pattern of kinematic variability may be dependent simply on the direction of movement. In this study on the jaw, variability in jaw-raising and lowering movements is greater in the raising and lowering direction and less in the direction of protrusion and retraction. However, the researchers demonstrated that kinematic variability is greatest in the direction of jaw raising and lowering even in the context of jaw-protrusion movements. Kinematic variability in the jaw is thus consistently related to the pattern of jaw stiffness not to the direction of movement.
Source: November 2002 edition of the Journal of Neurophysiology
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.
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