Assessing speech intelligibility of pathological speech in sentences and word lists: The contribution of phoneme-level measures.

Introduction: Speech intelligibility is an important indicator of the degree of speech impairment in pathological speech. Articulation, as a key feature of dysarthria, has been found to be a stronger contributor to intelligibility of dysarthric speech compared to voice quality, nasality, and prosody. In fact, therapy addressing articulation is often used by speech-language pathologists. Since phoneme-level measures are more directly related to articulation, they may contribute to better evaluating articulation imprecision in speakers with dysarthria and to monitoring the effectiveness of therapy.

Method: We collected two types of phoneme-level measures: a) Accuracy of Phonemes, the percentage of correctly transcribed phonemes, and b) Phonetic Distance, from orthographic transcriptions obtained from expert raters in two types of speech materials (i.e., meaningful sentences and word lists). We first examined the measures' interrater reliability using Generalizability Theory. Then we studied the validity of the measures by correlating them to three criterion variables. Following this, we explored their ability in distinguishing speakers in two classification tasks according to speakers' types (i.e., healthy vs dysarthric) and their severity levels of dysarthria, respectively.

Results: The results showed that both types of phoneme-level measures are highly reliable and valid in two different speech materials. They also showed acceptable results for both classification tasks in different speech materials, with word lists performing better than meaningful sentences. The differences between the two speech materials may be largely caused by differences in word structures and contextual cues in the materials.

Conclusion: The results indicate that both types of phoneme-level measures show largely similar reliability and validity in both speech materials. These measures perform better in word lists than in meaningful sentences, suggesting an advantage for using word lists in clinical practice and research. On the other hand, meaningful sentences can be used for classifying healthy and dysarthric speakers. Our results suggest that using different speech materials gives a better overview of the speakers' intelligibility at the segmental level and the implications of their articulation impairments.