Severity-Dependent Speech Characteristics and Clear Speech Response in Parkinson’s Disease: Perceptual, Acoustic, and Lingual Kinematic Findings

Purpose: This study examined perceptual, acoustic, and lingual kinematic characteristics in healthy speakers and across dysarthria severity levels in people with Parkinson’s disease, and the impact of clear speech on these measures.Method: Forty speakers, including 22 PwPD and 18 neurologically healthy comparison speakers, participated in this study. PwPD were classified into severity groups based on listener-derived intelligibility ratings. Acoustic corner vowel distance, as well as tongue front and tongue back distance, were measured from phrase-level speech samples. Additionally, 50 naïve listeners provided ratings of intelligibility and articulatory precision via crowdsourcing. The study examined group differences (control vs. dysarthria severity levels) and clear speech gains within each group using multilevel Bayesian modeling.Results: The perceptual and acoustic measures generally declined with increasing severity, with more pronounced differences emerging at higher severity levels. Lingual kinematics revealed a nonlinear pattern, with movement initially exceeding controls before declining at higher severity levels. Finally, clear speech resulted in more intelligible speech (particularly for the most severe dysarthria), more precise speech, and larger, more acoustically distinct vowels, with cue effectiveness tending to decrease as severity worsened.Conclusion: The present severity-specific analysis, including the non-linear lingual kinematic patterns observed, helps reconcile prior conflicting findings on lingual kinematics and support the notion of a posterior-to-anterior progression of lingual articulatory impairment. The use of clear speech is generally well supported by the findings, although the effectiveness of clear speech cues varies across severity levels and individual speakers.