Hybridizing conversational and clear speech to investigate the source of increased intelligibility in speakers with Parkinson's disease

Purpose: A speech analysis-resynthesis paradigm was used to investigate segmental and suprasegmental acoustic variables explaining intelligibility variation for 2 speakers with Parkinson's disease (PD). Method: Sentences were read in conversational and clear styles. Acoustic characteristics from clear sentences were extracted and applied to conversational sentences, yielding 6 hybridized versions of sentences in which segment durations, short-term spectrum, energy characteristics, or fundamental frequency characteristics for clear productions were applied individually or in combination to conversational productions. Listeners (N = 20) judged intelligibility in transcription and scaling tasks. Results: Intelligibility increases above conversation were more robust for transcription, but the pattern of intelligibility improvement was similar across tasks. For 1 speaker, hybridization involving only clear energy characteristics yielded an 8.7% improvement in transcription intelligibility above conversation. For the other speaker, hybridization involving clear spectrum yielded an 18% intelligibility improvement, whereas hybridization involving both clear spectrum and duration yielded a 13.4% improvement. Conclusions: Not all production changes accompanying clear speech explain its improved intelligibility. Suprasegmental adjustments contributed to intelligibility improvements when segmental adjustments, as inferred from vowel space area, were not robust. Hybridization can be used to identify acoustic variables explaining intelligibility variation in mild dysarthria secondary to PD.