Listener Strategies in the Perception of Dysarthric Speech: A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Speech Language Therapy, Department of Communication Disorders, University of Canterbury

Abstract

When listeners are presented with stimuli from multiple speakers versus single speakers in a perception experiment, decreased speech recognition accuracy and increased response time results. These findings have been demonstrated in studies that have employed normal (Creelman, 1957; Mullennix & Pisoni, 1990; Nygaard & Pisoni, 1998) and accented speech (Bradlow & Bent, 2008). It is thought that perceptual normalisation processes are, in part, responsible for this perceptual cost (Bladon, Henton, & Pickering, 1984; Johnson, 2009; Magnuson & Nusbaum, 2007; Mullennix, Pisoni, & Martin, 1989). Interestingly, studies are yet to examine whether these same findings occur when listeners encounter dysarthric speech – a naturally degraded speech signal associated with neurological disorder or disease. It has also been found that when listeners are exposed to multiple speakers with dysarthria, they generally adapt to the dysarthric signal over time; resulting in an improved ability to decipher the signal (Liss, Spitzer, Caviness, & Adler, 2002; Tjaden & Liss, 1995a). However, the rate of this adaption when listeners are exposed to a single speaker is yet to be examined. This study aimed to determine: (1) whether the intelligibility of dysarthric speech (in this case, hypokinetic dysarthria associated with Parkinson‘s disease) varied across single versus multi-speaker conditions; and (2) whether intelligibility increased over time when a listener was exposed to a single speaker with dysarthria. To answer these questions, sixty young healthy listeners were randomly allocated to one of four experimental conditions, one multiple speaker and three single speaker conditions. Each listener transcribed 60 three to five word phrases over one session and the results were examined for percent words correct. Contrary to expectations, there was no significant difference in percent intelligibility scores of the listener group who transcribed in a multi-speaker versus transcriptions from the single speaker listener conditions. In addition, perceptual learning effects across the rating period were identified for two out of the three single speaker listener groups only. The absence of significant findings in the multi-speaker versus single speaker transcripts may be explained by further analysis of within speaker variability. Acoustic analysis of the speakers may also shed light on the reduced perceptual learning that occurred in one of the single speaker groups. Greater numbers of speakers and experimental phrases would be beneficial in expanding trends seen in intelligibility of the single speaker groups.