Formant centralization ratio: a proposal for a new acoustic measure of dysarthric speech.
OBJECTIVE
The vowel space area (VSA) has been used as an acoustic metric of dysarthric speech, but with varying degrees of success. In this study, the authors aimed to test an alternative metric to the VSA-the formant centralization ratio (FCR), which is hypothesized to more effectively differentiate dysarthric from healthy speech and register treatment effects.


METHODS
Speech recordings of 38 individuals with idiopathic Parkinson's disease and dysarthria (19 of whom received 1 month of intensive speech therapy [Lee Silverman Voice Treatment; LSVT LOUD]) and 14 healthy control participants were acoustically analyzed. Vowels were extracted from short phrases. The same vowel-formant elements were used to construct the FCR, expressed as (F2u + F2a + F1i + F1u) / (F2i + F1a), the VSA, expressed as ABS([F1i x (F2a - F2u) + F1a x (F2u - F2i) + F1u x (F2i - F2a)] / 2), a logarithmically scaled version of the VSA (LnVSA), and the F2i /F2u ratio.


RESULTS
Unlike the VSA and the LnVSA, the FCR and F2i/F2u ratio robustly differentiated dysarthric from healthy speech and were not gender sensitive. All metrics effectively registered treatment effects and were strongly correlated with each other.


CONCLUSIONS
Albeit preliminary, the present findings indicate that the FCR is a sensitive, valid, and reliable acoustic metric for distinguishing dysarthric from unimpaired speech and for monitoring treatment effects, probably because of reduced sensitivity to interspeaker variability and enhanced sensitivity to vowel centralization.
Sapir
Shimon
S
Department of Communication Sciences and Disorders, Faculty of Social Welfare and Health Sciences, University of Haifa, Haifa, Israel. sapir@research.haifa.ac.il
Ramig
Lorraine O
LO
Spielman
Jennifer L
JL
Fox
Cynthia
C
D000368:Aged; D004401:Dysarthria / Q000175:diagnosis* / Q000628:therapy; D005260:Female; D006801:Humans; D008297:Male; D010300:Parkinson Disease / Q000175:diagnosis / Q000628:therapy; D010700:Phonetics; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012737:Sex Factors; D013060:Speech; D013061:Speech Acoustics; D013068:Speech Production Measurement / Q000379:methods*; D013070:Speech Therapy; D013997:Time Factors; D016896:Treatment Outcome