Vowel space change as a predictor of speech intelligibility gain in Down syndrome

Reduced intelligibility is a frequent characteristic of the speech of children with Down syndrome (DS). While it is documented that compressed vowel space is often associated with low intelligibility, few studies have examined how acoustic vowel space changes during speech enrichment interventions are related to functional intelligibility gains. This study investigated whether change in acoustic vowel space can predict intelligibility improvement in DS. Thirteen children with DS completed a naturalistic speech enrichment intervention and were included in the present analysis. Vowel productions from a structured /hVt/ word task were acoustically analysed at baseline and post-speech enrichment interventions. To measure vowel space change, three metrics were used: articulatory-acoustic vowel space (AAVS), quadrilateral vowel space area (qVSA), and convex hull vowel space area (VSAhull). Intelligibility gains were measured using Growth Scale Values (GSVs) from the Woodcock Camarata Articulation Battery (WCAB). Regression analysis results revealed that changes in AAVS and VSAhull significantly predicted WCAB gains, while qVSA did not. These findings suggest that vowel space change reflects clinically meaningful articulatory adjustments, as indicated by their predictive association with intelligibility improvement following speech enrichment interventions. The lack of predictive power for qVSA highlights limitations of traditional corner-vowel metrics and supports the use of more comprehensive vowel space measures. While preliminary, these findings suggest that vowel space metrics may have future potential for clinical monitoring of speech development in children with DS, pending further validation in larger and more controlled studies.