Coarticulation of jaw movements in speech production: is context sensitivity in speech kinematics centrally planned?

Coarticulation in speech production is a phenomenon in which the articulator movements for a given speech sound vary systematically with the surrounding sounds and their associated movements. Although these variations may seem to be planned centrally, without explicit models of the speech articulators, the kinematic patterns that are attributable to central control cannot be distinguished from those that arise because of dynamics and are not represented in the underlying control signals. We address the origins of coarticulation by comparing the results of empirical and modeling studies of jaw motion in speech. The simulated kinematics of sagittal-plane jaw rotation and horizontal jaw translation are compared with the results of empirical studies in which subjects produce speech-like sequences at a normal rate and volume. The simulations examine both "anticipatory" and "carryover" coarticulatory effects. In both cases, the results show that even when no account is taken of context at the level of central control, kinematic patterns vary in amplitude and duration as a function of the magnitude of the preceding or following movement, in the same manner as that observed empirically in coarticulation. Because at least some coarticulatory effects may arise from muscle mechanics and jaw dynamics and not from central control, these factors must be considered before drawing inferences about control in coarticulation.