Vowel contrast and speech intelligibility in dysarthria

Background/Aims: This study examined the spectral characteristics of American English vowels in dysarthria associated with cerebral palsy (CP), and investigated the relationship between a speaker's overall speech intelligibility and vowel contrast. Methods: The data were collected from 12 American English native speakers (9 speakers with a diagnosis of CP and 3 controls). Primary measures were F₁ and F₂ frequencies of 3 corner vowels /i, a, u/ and 3 noncorner vowels /I, 3, */. Six acoustic variables were derived from the formant measures, and were regressed against intelligibility: corner vowel space, noncorner vowel space, mean distance between vowels, F ₁ and F₂ variability, and overlap degree among vowels. Results: First, the effect of vowel was significant for both F₁ and F₂ measures for all speakers, but post hoc analysis revealed a reduced distinction at lower intelligibility. Second, regression functions relating intelligibility and acoustic variables were significant for overlap degree among vowels, F₁ variability, corner vowel space and mean distance between vowels. Overlap degree among vowels accounted for the greatest amount of variance in intelligibility scores. Conclusion: A speaker's overall intelligibility in dysarthric speech is better represented by the overlap degree among vowels than by the vowel space.