Assessing Vowel Centralization in Dysarthria: A Comparison of Methods

Purpose: The strength of the relationship between vowel centralization measures and perceptual ratings of dysarthria severity has varied considerably across reports. This article evaluates methods of acoustic-perceptual analysis to determine whether procedural changes can strengthen the association between these measures. Method: Sixty-one speakers (17 healthy individuals and 44 speakers with dysarthria) read a standard passage. To obtain acoustic data, 2 points of formant extraction (midpoint and articulatory point) and 2 frequency measures (Hz and Bark) were trialed. Both vowel space area and an adapted formant centralization ratio were calculated using first and second formants of speakers' corner vowels. Twenty-eight listeners rated speech samples using different prompts: one with a focus on intelligibility, the other on speech precision. Results: Perceptually, listener ratings of speech precision provided the best index of acoustic change. Acoustically, the combined use of an articulatory-based formant extraction point, Bark frequency units, and the formant centralization ratio was most effective in explaining perceptual ratings. This combination of procedures resulted in an increase of 17% to 27% explained variance between measures. Conclusions: The procedures researchers use to assess articulatory impairment can significantly alter the strength of relationship between acoustic and perceptual measures. Procedures that maximize this relationship are recommended.