Human Jaw Motion Control in Mastication and Speech

Abstract

Detailed kinematics are presented of two-dimensional jaw motion in mastication and speech. The relationship between jaw translation and jaw rotation is described and experimental records are compared with simulations based on the equilibrium point hypothesis (λ model). In general, in both speech and mastication, jaw rotation and jaw translation were found to start and end simultaneously and their coordination was typically characterized by straight line paths when rotation was plotted against translation. A number of manipulations are described which suggest that jaw rotation and jaw translation can be separately controlled. For example, when jaw movements in speech were examined, the slope of the relationship between rotation and translation varied with the consonant but did not depend on the vowel or speech rate. The kinematic details of jaw motion are well accounted for by the λ model. The model demonstrates that separate central commands can be defined associated with jaw translation, jaw rotation, and co-activation of muscles without motion. Central commands may be superimposed to produce combinations of rotation, translation and muscle coactivation. Empirical patterns can be captured by the model under the assumption of simple constant velocity shifts in equilibrium governed by central commands.