Intelligibility in Down syndrome: Effect of measurement method and listener experience

Speech intelligibility is a global indicator of the severity of a speech problem. It is a measure that has been used frequently in research and clinical assessment of speech. Previous studies have shown that factors, such as measurement method and listener experience, can influence speech intelligibility scores. However, these factors of speech intelligibility assessment have not yet been investigated in people with Down syndrome (DS).

To compare the speech intelligibility scores in speakers with DS measured using two methods: orthographic transcription and visual analogue scale (VAS), by two groups of listeners, experienced listeners and naïve listeners. Also, to examine the relationship across the four sets of speech intelligibility scores by means of correlational analysis.

A total of 30 adolescents and adults with DS read or repeated 12 sentences from a standardized test of intelligibility for adults with dysarthria. Each sentence was saved as a separate sound file and the 360 sentences were divided to form eight sets of stimuli. A total of 32 adults (16 experienced and 16 naïve) served as listeners of speech intelligibility. Each listener heard a single set of sentences and independently estimated the level of intelligibility for each sentence using a VAS in one task and wrote down the words perceived (i.e., orthographic transcription) in another task. The order of the two tasks was counterbalanced across listeners and the tasks were completed at least 1 week apart.

Repeated-measures analysis of variance (ANOVA), confirmed by mixed-methods analysis, showed that the scores obtained using orthographic transcription were significantly higher than those obtained using VAS; and the experienced listeners' scores were significantly higher than the naïve listeners' scores. Spearman rank correlation analysis showed that the four sets of scores across all conditions were strongly positively correlated with each other.

Listeners, both experienced and naïve, may udge speech in DS differently when using orthographic transcription versus VAS as the method of measurement. In addition, experienced listeners can judge speech intelligibility differently compared with listeners who are less exposed to unclear speech, which may not represent 'real-world' functional communicative ability. Speech and language therapists should be aware of the effect of these factors when measuring intelligibility scores and direct comparison of scores obtained using different procedures and by different groups of listeners is not recommended. What this paper adds What is already known on the subject Previous research on other clinical groups (e.g., Parkinson's disease) has shown that speech intelligibility scores can vary across different measurement methods and when judged by listeners with different experience. However, these factors have not yet been investigated in people with DS. What this paper adds to existing knowledge Similar to the findings reported for other clinical groups, using an impressionistic measurement method, such as VAS, can result in different speech intelligibility scores compared with scores obtained from orthographic transcription in speakers with DS. Furthermore, experienced listeners can perceive intelligibility as better compared with naïve (untrained) listeners for this group. What are the potential or actual clinical implications of this work? When measuring speech intelligibility, speech and language therapists should be aware that scores obtained using orthographic transcription can be higher than those obtained using VAS. They should also be aware that their increased exposure to hearing atypical speech may cause them to judge the speech difficulty as less severe and lead to an inaccurate representation of speech performance. Speech and language therapists should consider these factors when interpreting assessment results and especially when using intelligibility measures for treatment outcomes.