Interarticulatory coordination of the lips and jaw in childhood apraxia of speech

Childhood apraxia of speech (CAS) is often characterized by an ability to produce phonemes in isolation but not in more complex phonetic sequences. This has led to the hypothesis that articulator coordination is impaired in CAS. This study explored whether coordination between the lips and jaw during speech production is impaired in this group. We used two methods to investigate interarticulatory relationships. Cross-correlation analysis directly measures spatial-temporal coupling of articulator movements. The spatiotemporal index (STI; Smith, Goffman, Zelaznik, Ying, & McGillem, 1995) measures repetition stability and has also been used as an indirect measure of interarticulatory coordination by providing an index of the coordinative consistency of the relationship between articulators within a pair (Smith & Zelaznik, 2004). Three groups of children were included: children with CAS; children with a speech sound disorder involving articulation, phonological errors, or both (the SD group); and typically developing (TD) children. A facial motion capture system was used to track upper lip, lower lip, and jaw movement during a naming task in which stimuli varied by word length. The CAS, SD, and TD children did not significantly differ in spatial-temporal coupling; however, coefficients of variation of the spatial and temporal coupling measures did differentiate the CAS and SD groups. Additionally, the CAS children were distinguished from the SD children by higher lip aperture STI values, indicating that the CAS group had more difficulty generating stable movement plans.