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 F(1) and F(2) 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 F(2) variability, and overlap degree among vowels. RESULTS First, the effect of vowel was significant for both F(1) and F(2) 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, F(1) 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.