Degrees of freedom of tongue movements in speech may be constrained by biomechanics

TL;DR

This study investigates whether the primary degrees of freedom in tongue movements during speech are constrained by biomechanical properties of the vocal tract, using a biomechanical model and PCA analysis.

Contribution

It demonstrates that the main tongue movement axes in speech are likely rooted in biomechanical constraints rather than speech-specific control.

Findings

Two main movement axes explain 84% of tongue shape variance.

The principal components resemble observed speech movement degrees of freedom.

Tongue movement constraints are biomechanically based, not speech-specific.

Abstract

A number of studies carried out on different languages have found that tongue movements in speech are made along two primary degrees of freedom (d.f.s): the high-front to low-back axis and the high-back to low-front axis. We explore the hypothesis that these two main d.f.s could find their origins in the physical properties of the vocal tract. A large set of tongue shapes was generated with a biomechanical tongue model using a Monte-Carlo method to thoroughly sample the muscle control space. The resulting shapes were analyzed with PCA. The first two factors explain 84% of the variance, and they are similar to the two experimentally observed d.f.s. This finding suggests that the d.f.s. are not speech-specific, and that speech takes advantage of biomechanically based tongue properties to form different sounds.