Quantifying Vowel Distinctiveness in Amyotrophic Lateral Sclerosis: A Kinematic Approach

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that often leads to anarthria. Monitoring articulatory movement changes can provide valuable insights into disease progression (Green et al., 2013). While reduced vowel distinctness in the acoustic space has been linked to speech intelligibility decline (Turner et al., 1995), a direct kinematic approach may be more sensitive for detecting subtle articulatory deficits. This study analyzed 47 speakers with ALS and 15 healthy controls. The ALS group was stratified into asymptomatic (n = 18), mild-moderate (n = 19), and severe bulbar (n = 9) subgroups based on clinical speech performance measures. Participants produced eight English vowels in a /bVb/ context while tongue and lip movements were recorded using electromagnetic articulography. Positional data were used to construct the articulatory vowel distinctiveness space (AVDS) with Procrustes distance and multidimensional scaling (2D area). Vowel classification was assessed using a support vector machine (SVM). Compared to healthy controls (.018 ± .006), the mean AVDS area was larger for both asymptomatic (.024 ± .011) and mild-moderate groups (.024 ± .011), but decreased in the severe group (.020 ± .011). Although these differences were not statistically significant, a negative correlation was shown between AVDS area and intelligible speaking rate, suggesting possible compensatory mechanisms before severe bulbar degeneration occurs. SVM classification performance declined from 96.30% in controls to 94.32%, 92.86% and 80.71% in the asymptomatic, mild-moderate, and severe groups, respectively, with all ALS subgroups showing significantly lower accuracy than controls (p < .05). These findings indicate that articulatory alterations begin in early-ALS and evolve with disease severity. While classification accuracy diminishes with severity, it remains robust for dysarthric speech recognition technologies.

Green, J. R., Yunusova, Y., Kuruvilla, M. S., Wang, J., Pattee, G. L., Synhorst, L., Zinman, L., & Berry, J. D. (2013). Bulbar and speech motor assessment in ALS: Challenges and future directions. Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration, 14(7–8), 494–500.

Turner, G. S., Tjaden, K., & Weismer, G. (1995). The influence of speaking rate on vowel space and speech intelligibility for individuals with amyotrophic lateral sclerosis. Journal of Speech, Language, and Hearing Research, 38(5), 1001–1013.