Tongue articulators as muscle synergies

Abstract

Tongue movements during speech production were investigated by means of a simple yet realistic biomechanical model, based on a finite-element modeling of soft tissues, in the framework of the equilibrium-point hypothesis (λ model) of motor control. The model has been exploited to estimate the ‘‘central’’ control commands issued to the muscles, for a data set of mid-sagittal digitized tracings of vocal tract shape, recorded by means of low-intensity x-ray cineradiographies during speech. An analysis of commands needed to achieve the empirically observed tongue shapes has shown that, in spite of the great mobility of the tongue and the highly complex arrangement of tongue muscles, central commands can be described in terms of a simple linear control model, with a small number of factors that can be associated with elementary tongue gestures. This result provides a biomechanical substrate to the concept of ‘‘tongue articulator.’’ Moreover, it is suggested that, because of the peculiar geometrical arrangement of tongue muscles, the central nervous system (CNS) may not need a detailed representation of tongue mechanics but, indeed, may make use of a relatively small number of muscle synergies that are invariant over the whole space of tongue configurations.