A model of articulator trajectory formation based on the motor tasks of vocal-tract shapes

This paper presents a modeling study of articulator movements for continuous speech utterances. The model determines the trajectories of the articulator movements that form the vocal tract into phoneme-specific shapes. Vowels, for example, are generated based on the specifications of the tract shape (motor tasks) related to the shape of the tongue and lips. Consonants, such as /p/ and /k/, on the other hand, can be generated by specifying a closure in the tract between the lips, or between the tongue and the palate. In the model, state variables represent the active movements of each articulatory organ as the displacement relative to the neutral position. In addition, tract variables are defined to specify the motor tasks of the lips and tongue as functions of the state variables of the jaw, lips, and tongue: The definition of the tract variables incorporates the representation of the coordinative structure of the articulatory system between the jaw and the lower lip and between the jaw and the tongue. This coordinative structure allows for an infinite number of trajectories that satisfy all of the given motor tasks. This redundancy is overcome by minimizing the cost function so that the movements of each articulatory organ are uniquely determined. Articulator movements were simulated for several utterances, and the simulated movements of the jaw, lips, and tongue agree well with measured data. The results show that it is valid to use the features of vocal-tract shapes to represent motor tasks when modeling articulator movements and that the model accurately represents the coordinated movements of the articulatory organs.