The Effect of Tongue–Jaw Coupling on Phonetic Distinctiveness of Vowels in Amyotrophic Lateral Sclerosis

Purpose The aim of this study was to determine the relation of tongue–jaw coupling to phonetic distinctiveness of vowels in persons at different stages (i.e., early, middle, late) of bulbar motor involvement in amyotrophic lateral sclerosis (ALS) and healthy controls. Method The pattern of spatial tongue–jaw coupling was derived from 11 individuals with ALS and 11 healthy controls using the parallel factor analysis. Two articulatory components, which correspond to tongue displacement independent of the jaw (iTongue) and jaw contribution to tongue displacement (cJaw), were extracted from the composite tongue–jaw displacement. These articulatory components were correlated with F1 (i.e., height) and F2–F1 (i.e., advancement) of 4 vowels (/i/, /u/, /æ/, and /ɔ/) across all participants in each group. In addition, a comprehensive index of functional tongue–jaw coupling was derived as the ratio of cJaw/(iTongue + cJaw), and an acoustic index of vowel distortion (VowelDis) was derived to quantify the overall disease-related changes in phonetic distinctiveness of vowels. Based on these indices, disease-related changes in tongue–jaw coupling and phonetic distinctiveness of vowels were examined in individuals at the early, middle, and late stages of the disease. Results For healthy controls, both iTongue and cJaw contributed to F2–F1, while only cJaw contributed to F1. For individuals with ALS, both iTongue and cJaw contributed to F1, whereas only cJaw contributed to F2–F1. Disease-related changes in tongue–jaw coupling included (a) an overall decrease of the percent contribution of the tongue to the composite tongue–jaw displacement accompanied by an increase of percent contribution of the jaw and (b) several changes in the direction of tongue and jaw displacements occurred at different stages of the disease. These disease-related changes in tongue–jaw coupling had various impacts on phonetic distinctiveness of vowels, resulting in (a) a backward shift of front vowels and reduced front–back vowel contrasts, which occurred early and throughout the disease stages; (b) raising of all vowels during the middle stage of the disease; and (c) reduced high–low vowel contrasts during the late stage of the disease. Overall, phonetic distinctiveness of vowels deteriorated progressively throughout the disease course. Conclusions Different from healthy controls who established optimal functional coupling between the tongue and the jaw during vowel productions, individuals at the early-to-middle stages of bulbar ALS showed various adaptive changes in tongue–jaw coupling in response to the disease-related biomechanical and muscular changes in the articulators (particularly in the tongue). These adaptive changes in tongue–jaw coupling were found to be partially effective in mitigating the negative effect of articulatory involvement on phonetic distinctiveness of vowels. As the disease progressed to the late stage, such adaptations appeared to be no longer evident, resulting in a substantial overall reduction of vowel contrasts.