ATR Human Inf. Process. Res. Labs., 2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto, 619-02 Japan
McGill Univ., Montreal, Quebec, Canada
Previously, it was reported that, when 3-D jaw motion during speech is decomposed into its three rotation and three translation components, it consists primarily of rotation and translation within the midsagittal plane [E. Vatikiotis-Bateson and D. J. Ostry, J. Acoust. Soc. Am. 92, 2391(A) (1992)]. Although small in magnitude, two nonsagittal components, roll and yaw, also varied consistently with phonetic context. In this paper, it is shown that the behavior of individual jaw motion components depends in part on the choice of coordinate system orientation. In particular, the use of the occlusal bite plane to orient the jaw may not be appropriate for characterizing the active components of jaw motion during speech. For example, when orientation of the coordinate system is rotated 15--20 deg down from the occlusal plane, the roll component is eliminated. Whether or not the frame or reference for jaw motion is function-specific or purely anatomical is addressed using data from a new study comparing mastication and speech.