Restriction lift date: 2024-05-30
Perceptual and acoustic features of speech in individuals with Down syndrome and their impact on speech intelligibility
University College Cork
Speech production involves five major components: respiration, phonation, articulation, resonance, and prosody and number of methods can be used to evaluate these aspects. Perceptual analysis methods are frequently employed; however, their reliability has been called into question. Acoustic analysis methods can provide a more objective view of certain aspects of speech production. For example, mean fundamental frequency can provide an accurate measure for vocal pitch. Due to a number of factors such as anatomical features and neurological involvement, difficulties with all five of the components of speech production have been frequently reported for people with Down syndrome (DS). When producing speech, the five components combine to produce a spoken message via the acoustic signal. The amount of acoustic signal that is transferred from the speaker to the listener is referred to as speech intelligibility. When a speaker’s speech intelligibility is reduced, it can negatively impact on their communication and affect their quality of life. Difficulties with speech intelligibility are common in people with DS. Reduced speech intelligibility can affect people with DS of all ages and may never be fully resolved. Several methods also exist for specifically measuring speech intelligibility. The most commonly used methods are rating scales and orthographic transcription. Some studies have examined how aspects of speech production contribute to speech intelligibility scores in different clinical populations. However, to date, no study has evaluated all five components of speech production in people with DS in one study or examined their impact on speech intelligibility. It is important to understand more about the nature of reduced speech intelligibility for this group in order to plan and deliver appropriate intervention. This thesis includes four studies which aimed to address the gap in knowledge around the nature of speech intelligibility difficulties in people with DS and its measurement. The speakers in all four studies were 30 adolescents and adults with DS aged 16 years and over. Thirty typical adolescent and adult speakers matched for age and gender were included in the second study to provide a comparison for acoustic analysis findings. Sixteen experienced listeners were recruited to judge the speech samples to obtain a measure of speech intelligibility. Another group of 16 listeners who were not experienced, were also recruited to judge speech intelligibility for the fourth study. Recruiting a group of inexperienced (naive) listeners allowed for comparison of speech intelligibility scores when judged by listeners who were exposed and not exposed to hearing unclear speech. Study 1 used perceptual analysis methods to evaluate the five major components of speech production–respiration, phonation, articulation, resonance, and prosody–for the speakers with DS. The experienced listeners then completed orthographic transcription of sentences to obtain a measure of speech intelligibility for the speakers with DS. Regression analysis was used to examine which perceptual parameters impacted on speech intelligibility scores. Results showed that a measure of phonation, breathiness, and a measure of articulatory accuracy, percent consonants correct, were the two strongest predictors of speech intelligibility. Study 2 was similar to the first study in that it also used regression analysis to examine which parameters impacted on speech intelligibility scores for the speakers with DS. However, in this case, acoustic analysis methods were used to evaluate the five components of speech production and a group of typical speakers was included to serve as a comparison. Results showed that the slope of the second formant of /ɑ/ and the voice onset time of /d/ were the strongest acoustic predictors of speech intelligibility for the speakers with DS. Study 3 investigated how language ability impacted on speech intelligibility for the speakers with DS. Regression analysis was used to examine which receptive language measures and expressive language measures contributed to speech intelligibility scores. Results showed that mean length of utterance was the strongest predictor of intelligibility for those with DS. Study 4 compared speech intelligibility scores using two different methods to measure intelligibility; and involved listeners with varying levels of experience to judge intelligibility. Orthographic transcription and visual analogue scale of sentences were used as the measurement methods of intelligibility. Results showed that speech intelligibility scores were significantly higher when orthographic transcription was used as the method of measurement compared to visual analogue scale. Results also showed that speech intelligibility scores were significantly higher when judged by the group of experienced listeners compared to the naive listener group across both measurement methods. This thesis concludes that, when all aspects of speech production are considered, speech and language therapists (SLTs) should be aware that voice quality can play an influential role in the speech intelligibility of those with DS. They should also take into account articulation, motor speech performance, and language ability when assessing and treating speech intelligibility for this group. Additionally, when obtaining a measure of speech intelligibility, SLTs should be aware that the method of measurement they choose to employ can impact on outcome scores and that different methods may be measuring different aspects of speech intelligibility. SLTs should also bear in mind that, as an experienced listener, their judgement of speech intelligibility for a speaker with DS may be different compared to when judged by an inexperienced listener. That is, even if an SLT perceives a speaker with DS to have a high level of speech intelligibility, the same speaker may not be so intelligible to listeners who are not familiar with the speech characteristics of people with DS. These findings have provided more insight into the nature of speech intelligibility difficulties in individuals with DS. If SLTs knew about these difficulties, they would be more informed when making decisions regarding assessment and treatment and may lead to be better communicative performance and increased quality of life for those with DS.
Down syndrome , Speech intelligibility , Motor speech , Acoustic analysis , Perceptual analysis
O'Leary, D. S. 2020. Perceptual and acoustic features of speech in individuals with Down syndrome and their impact on speech intelligibility. PhD Thesis, University College Cork.