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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.
Acoustic analysis of vowel sounds offers an objective assessment tool for measuring speech production in people with dysarthria. However, there are significant limitations in using acoustic metrics to infer information about listeners' perceptions of the disorder. Although studies have consistently reported an association between acoustic vowel centralization and perceptual measures, the strength of these relationships is highly variable (Lansford & Liss, 2014a). Linking measurements of the speech signal to perceptual outcomes is an important component of validating acoustic metrics for clinical use. Understanding causes of variation in the relationship between acoustic and perceptual data is a first step toward establishing stronger links between these variables.
Centralization of vowel formants has been associated with reduced intelligibility in both healthy speakers and those with motor speech disorders (e.g., Ferguson & KewleyPort, 2007; Liu, Tsao, & Kuhl, 2005; Neel, 2008; Tjaden & Wilding, 2004). In the motor speech literature, the most common way of measuring vowel centralization is through the calculation of vowel space area (VSA)-using the first and second formants of a dialect's corner vowels. Static...