An Acoustic Study of Coarticulation in Dysarthric Speakers With Parkinson Disease
Anticipatory vowel coarticulation in obstruent + vowel syllables was studied for 9 dysarthric males with mild to moderate idiopathic Parkinson disease (PD) and a group of 10 healthy males. In addition to studying coarticulatory differences for speakers' habitual rate repetitions, a graded speaking rate task was used to investigate the effect of rate variation on coarticulation. The ratio of F2 onset frequency/F2 target frequency was used to infer coarticulation (i.e., ratios of 1.0 indicate complete vowel harmony at vowel onset, and ratios greater than 1.0 indicate relatively less coarticulation). Because F2 onset frequency is a relatively novel measure of coarticulation, a more common measure of anticipatory vowel coarticulation--fricative F2--was obtained for a subset of the speech stimuli and compared to F2 onset measures. Ratios of fricative F2/F2 target frequency also were computed to infer coarticulation and the results were compared to the F2 onset/F2 target ratios. The results indicated a modest relationship between F2 onset frequency and fricative F2 for both speaking tasks and speaker groups, with a stronger relationship for the graded rate task. Regression analyses for the graded rate task further indicated that longer vowel durations were associated with larger ratios for both speaker groups. Thus, coarticulation tended to increase with faster rates and decrease with slower rates, although the relationship was more systematic for control speakers. Ratios for the habitual task tended to be smaller for speakers with PD, suggesting increased coarticulation relative to control speakers. This effect was not entirely attributable to the more rapid speaking rates for speakers with PD. Because habitual rate F2 onset/F2 target ratios tended to underestimate the extent to which coarticulation was increased for speakers with PD--compared to the fricative F2/F2 target ratios--measures other than F2 onset should be used to infer absolute differences in extent of coarticulation for speakers with PD and healthy controls.