Tone-vowel interaction and co-articulation in Cantonese speakers with apraxia of speech and co-existing aphasia : a preliminary study

Abstract

Background: Speakers with apraxia of speech (AOS) usually produce segmental and prosodic errors that influence their speech intelligibility. Literature on AOS in tonal language speakers is sparse compared to that in non-tonal language. Also, the existing research often made static, isolated analyses, leaving the production and co-articulation between segments and suprasegmental entities in tonal languages under-investigated. Aims: This preliminary study aims to fulfill the aforementioned research gaps by investigating vowel-tone interaction and tonal and vocalic co-articulation in Cantonese post-stroke speakers with AOS. Methods: Five Cantonese adults with AOS post-stroke, five adults without AOS post-stroke, and five healthy controls performed the Tone Sequencing Task (TST), a task adapted from oral diadochokinetic tasks that required five rapid repetitions of 3-syllable items formed by three different Cantonese vowels and three different Cantonese tones. The quality of vowels was indexed by midpoint formant values and euclidean distances between the vowels. Within-speaker variation was assessed by coefficients of variance. Co-articulation was indexed by onset and offset formant or f0 values. The effects of the participant groups, the positions of tone-syllable in the TST stimuli, and the tones carried by vowels/carrying vowels were evaluated with linear mixed effect models. Results: Cantonese-speaking adults with AOS had difficulty in producing distinctive vowels and tones. They also showed large within-speaker variation in vowel production but reduced tone contrast, especially at the final positions. The disrupted anticipatory co-articulation between vowels as well as between tones further suggested that the speakers with AOS could not sequence segments and suprasegments simultaneously.