Acoustic and perceptual consequences of articulatory rate change in Parkinson disease.

This study sought to characterize the relationship among voluntary rate modification, vocal tract acoustic output, and perceptual impressions of speech for individuals with idiopathic Parkinson disease (PD). Healthy control speakers were studied for comparison purposes. Four research questions were addressed: (1) How is rate modification evidenced in acoustic measures of segmental and global timing? (2) What is the impact of rate modification on measures of acoustic working space for select vowels and consonants? (3) What is the impact of rate modification on perceptual impressions of severity? (4) Are rate-induced changes in measures of acoustic working space related to perceptual impressions of severity? Speakers read the Farm Passage at habitual, slow, and fast rates. The vowels /i/, /ae/, /u/, and /A/ and the fricatives /s/ and /S/ were of interest. Acoustic measures included articulatory rate, segment durations, vowel formant frequencies, and first moment coefficients. Measures of acoustic working space for vowels and fricatives also were derived. The results indicated that temporal acoustic measures changed in the expected direction across rate conditions, with a tendency toward slightly faster rates for the PD group. In addition, the relative rate change for the Fast and Slow conditions compared to the Habitual condition was similar across groups. Rate did not strongly affect measures of acoustic working space for the PD group as a whole, but there was a tendency for slower rates to be associated with larger measures of acoustic working space. Finally, there was not a strong relationship between perceived severity and measures of acoustic working space across the rate continuum for either group. Rather, the relationship between perceived severity and measures of acoustic working space was such that the PD group exhibited smaller measures of acoustic working space and more severe perceptual estimates than the control speakers, irrespective of rate condition.