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Vocal Tract Steadiness in Spasmodic Dysphonia Revisited
Michael P, Cannito & Lesya Chorna

Prior studies have disagreed as to whether abnormal formant frequency steadiness is exhibited in sustained vowels produced by speakers with spasmodic dysphonia (SD), depending on whether a natural or artificial voice source was employed. This study directly compares /a/ vowel productions using an artificial source vs. natural phonation. The steadiest 3 s segment of each of two vowel productions was analyzed. Formants were carefully hand edited for tracker errors and screened for outliers, which were replaced by linear interpolation, to minimize bias toward increased variability. High levels of inter-examiner reliability of measurements were observed. LPC-based formant tracks (F1, F2) indicate that variability of first and second formant frequencies and bandwidths was significantly increased in adductor SD, compared with normal control speakers, irrespective of voice source. Abductor SD speakers did not differ from controls. Generally, formant variability was greater in vowels produced naturally than with artificial voice source. There was no interaction of voice source with subject groups. Results support an extrapyramidal (dyskinetic) interpretation of ADSD. Discrepancies among previous studies probably reflect the temporal resolution of analysis methods employed. These will be discussed, along with potential underlying mechanisms of formant variability.

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