Acoustic and Perceptual Effects of Rate and Loudness Manipulations in Dysarthria
Rate reduction and increased loudness are associated with an increase in the size of the articulatory-acoustic working space, and thus improved acoustic distinctiveness for speakers with dysarthria. Improved intelligibility also has been reported. Few studies have compared rate and loudness effects for speakers with dysarthria, however, although these are common treatment techniques. In the current study, 15 speakers with dysarthria secondary to Multiple Sclerosis, 12 speakers with dysarthria secondary to Parkinson disease (PD), and 15 healthy controls read a passage in Habitual, Loud, and Slow conditions. Supraglottal behavior was inferred from the acoustic measures. Ten listeners also scaled intelligibility of reading passages, and intelligibility estimates were related to acoustic measures. Articulatory rate was reduced in the Slow condition and vocal intensity increased in the Loud condition, relative to the Habitual condition. Vowel acoustic distinctiveness, as indexed by vowel space area, was maximized in the Slow condition, but stop consonant acoustic distinctiveness, as indexed by first moment difference measures, was maximized in the Loud condition. F2 slope measures for diphthongs were not consistently affected by rate or loudness. Intelligibility for speakers with PD also improved in the Loud condition. Intelligibility estimates, however, were typically unrelated to acoustic measures of supraglottal behavior. Results are compared to previous studies and implications for treatment are discussed.