This study aims to evaluate the impact of anatomical, morphological and biomechanical properties of the human articulators on the kinematic properties of observed movements. We focus here on very specific movements, namely articulatory displacements during speech production (involving jaw, tongue and lips motion). Since a sequence of speech movement can be seen, to a first approximation, as a list of discrete target requirements related to the phonemic message, we consider the study of target-to-target movements to be a central issue, and we therefore assume that these movements are produced in response to a sequence of centrally-specified targets. The question that immediately arises is whether a sequence of targets is the only centrally-controlled task specification, or whether movements are characterised with precise on-line control of articulatory dynamics, along the entire trajectory. This point is particularly important in the case of speech control, since the duration of movements is typically very short (several milliseconds for consonants). As many studies of speech motor control are based on features of the velocity profile (temporal variation of the velocity) observed during articulatory displacement, we have tried to address the following question: "how may we distinguish between kinematic properties arising from the way the articulators are controlled, and those that are strictly linked to the physical properties of the speech apparatus?"
To clarify this, a biomechanical model of the human tongue (Payan & Perrier, 1997) was set up, controlled according to the target-based Equilibrium Point Hypothesis of Feldman (1986). It was evaluated by generating articulatory trajectories for phoneme-to-phoneme sequences. Two different kind of simulations were carried out : vowel-to-vowel gestures and vowel-consonant-vowel productions. In each case, kinematic patterns (displacements and velocity profiles) were analysed and compared. Both lead us to the same conclusion : a proper account of the biological properties of the human system, from the neurophysiology of the control apparatus to the mechanical properties of the peripheral apparatus, is highly recommended to understand from kinematic evidence how movements are controlled.
Feldman, A. G. (1986). Once more on the equilibrium-point hypothesis (l model) for motor control, Journal of Motor Behavior, 18, 17-54.
Payan, Y., & Perrier, P. (1997). Synthesis of V-V sequences with a 2D biomechanical tongue model controlled by the Equilibrium Point hypothesis, Speech Communication, 22, 185-205.