Speech naturalness ratings and perceptual correlates of highly natural and unnatural speech in hypokinetic dysarthria secondary to Parkinson’s disease

Abstract

Despite the importance of speech naturalness to treatment outcomes, little research has been done on what constitutes speech naturalness and how to best maximize naturalness in relationship to other treatment goals like intelligibility. This study investigated the speech naturalness ratings of individuals with dysarthria and the associated perceptual correlates of highly natural and unnatural speech. Four speakers with hypokinetic dysarthria secondary to Parkinson’s disease were recorded and rated for naturalness by 69 students in Communication Disorders. Students were presented with 436 speech samples and asked to provide speech naturalness ratings on a 1-9 Likert scale. After rating speech samples, subjects listed perceptual cues associated with samples rated most and least natural and weighted each cue on a visual analog scale. The data on naturalness ratings showed that spontaneous speech was rated the least natural on average, while sentences from a short story were rated slightly more natural and individually read sentences were rated the most natural of all of the utterance types. Thirteen themes emerged from the perceptual cues collected. Of the thirteen themes, intelligibility was rated significantly more important than other cues in highly natural speech and intelligibility and articulation were rated significantly more important than other cues in highly unnatural speech.