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THE TWO-POINT MODEL OF JAW MOVEMENT DURING SPEECH

Jaw movement during speech was generally described as pure translation or pure rotation of a single point on the jaw. In pure translation, the jaw simply translates in some direction, usually as the principal movement. In the pure rotation model, the jaw rotates about a transverse axis that passes through the mandibular condyles.

While research on jaw movement during speech has examined the movement of a single point on the jaw, the anatomy of the hinge jaw joint allows both rotation and translation, and during speech, the jaw can move with up to three independent degrees of freedom. Therefore, describing jaw position in terms of a single point on the jaw did not provide enough information to predict the position of every other point on the jaw. Moreover, the simple translation and rotation models are anatomically inaccurate for both non-speech gestures and speech-related jaw movements.

The range of jaw opening and closing movements during speech is considerably less than during mastication. Essentially, no lateral movement of the jaw occurs during speech, and the maximal vertical opening of the jaw is two to four times greater for mastication than for speech. Thus, speech-related movements fall within the range of vertical jaw positions.

Edwards and Harris (1990) used a two-dimensional model of jaw movement to describe jaw opening and closing gestures for speech, where jaw movements consisted of three components: rotation about the terminal hinge axis, and the horizontal and vertical translation of that axis. Data was collected for three subjects and statistical analysis was used to examine the relationships among the three kinds of jaw movement. For two subjects, but not for the third, jaw rotation and horizontal translation was inter-dependent. For these two subjects, the first degree of freedom of jaw movement corresponded to a combination of rotation and jaw translation. For the third subject, the first degree of freedom of jaw movement corresponded to rotation alone. The findings were used to address two questions: (1) does jaw movement during speech-related openings and closings use two or three degrees of freedom; and (2) how are these degrees of freedom related to the three jaw movement components?

The results for the three speakers were similar. The three components of jaw movement moved in the predicted direction. For all three speakers, the center of rotation moved down and front for jaw opening and up and back for jaw closing; the angle of jaw rotation became more open for jaw opening and less open for jaw closing; and jaw rotation contributed most to the jaw displacement at the front teeth.

Edwards and Harris (1990) observed both intra-speaker and inter-speaker differences. Their results, like those of Westbury’s (1988) two-point model, indicated that two points of jaw movement were needed to accurately describe jaw movement during speech. Edwards and Harris’s (1990) model of jaw movement was more complex as it combined rotation about the terminal hinge axis with the simultaneous vertical and horizontal translation of that axis. The model could relate movement to jaw muscle activity, and tongue movement to tongue muscle activity. Because the tongue rests on the jaw, tongue movement includes movement due to tongue muscles and to jaw-related movement. The three models (the pure rotation model, the pure translation model, and Edward and Harris’ (1990) two-point model) were compared with respect to their contribution of the first degree of freedom of jaw movement to tongue displacement.

A more accurate description of jaw movement as a combination of rotation and translation also provided insight into patterns of jaw movement for different speakers. Many dentists have noted that speakers of different occlusal classes show differences in their speech-related jaw movements. These differences are used by prosthodontists to determine the occlusal class of edentulous patients. In particular, individuals with Class II occlusions generally exhibit large amounts of movement for opening and closing gestures; individuals with Class I occlusions exhibit smaller amounts of movement; and individuals with Class III occlusions exhibit virtually no anterior-posterior movement.

References

Edwards, J., & Harris, K. S. (1990). Rotation and translation of the jaw during speech. Journal of Speech, Language, and Hearing Research, 33, 550-562. doi:10.1044/jshr.3303.550

Westbury, J. R. (1988). Mandible and hyoid bone movements during speech. Journal of Speech and Hearing Research, 31, 405-416.

Other Resources Related to This Topic

Coker, C. H. (1976). A model of articulatory dynamics and control. Proceedings of the IEEE, 64, 452-460.

Edwards, J. (1985). Mandibular rotation and translation during speech. Unpublished doctoral dissertation, CUNY.

Gentil, M., & Gay, T. (1986). Neuromuscular specialization of the mandibular motor system: Speech versus non-speech gestures. Journal of Speech Communication, S, 69-82.

Gibbs, C. H., & Messerman, T. (1972). Jaw motion during speech. In Orofacial function: Clinical research in dentistry and speech pathology. (ASHA Reports, No. 7). Washington, DC: American Speech and Hearing Association.

Gibbs, C. H., Messerman, T., Reswick, J. B., & Derda, H. J. (1971). Functional movements of the mandible. Journal of Prosthetic Dentistry, 26, 604-620.

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Mermelstein, P. (1973). Articulatory model for the study of speech production. Journal of the Acoustical Society of America, 53, 1070-1082.

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Pound, E. (1977). Let S be your guide. Journal of Prosthetic Dentistry, 38, 482-487.

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This article was reprinted with permission from Reed Invent for Therapy (http://www.reedinvent.com/)

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