Effects of side cavities and tongue stabilization: Possible extensions of the quantal theory

This article provides experimental evidence for two hypotheses, the “interdental-space effect” and the “stabilization effect”, that may help to understand quantal acoustic properties of certain vowels. The interdental-space effect has its basis on the hypothesis [Stevens, K. N., & Perkell, J. S. (1977). Speech physiology and phonetic features. In M. S. Sawashima, & F. Cooper (Eds.), Dynamic aspect of speech production (pp. 323–341). Tokyo: University of Tokyo Press] that the gap between the upper and lower dental arches is involved in abrupt changes of oral cavity volume between low and high vowels. Our magnetic resonance imaging (MRI) data indicate that this side cavity of the vocal tract is fused with the oral cavity in low vowels such as [a] but forms a bilateral side-branch in non-low vowels such as [i] and [e]. This abrupt change in the cavity's affiliation to the vocal tract contributes a discontinuity in otherwise smooth formant transitions. The stabilization effect [Fujimura, O., & Kakita, Y. (1979). Remarks on quantitative description of the lingual articulation. In B. Lindblom, & S. Öhman (Eds.), Frontiers of speech communication research (pp. 17–24). London: Academic Press] is the hypothesis that co-contraction among bundles of the genioglossus muscle stabilizes the formant pattern for [i], which was supported by analysis of electromyographic data. Our MRI-based measurement of tongue muscle geometry suggests an additional cause of vowel stabilization for [a]: posterior bulging of the tongue base for this vowel by the hyoglossus and styloglossus is mechanically limited by the parapharyngeal tissue that surrounds the extra-lingual styloglossus.