Acoustic and Kinematic Methods of Indexing Spatiotemporal Stability in Children With Developmental Language Disorder

Purpose: The spatiotemporal index (STI) is a standard metric for quantifying the stability and patterning of speech movements. The STI has often been applied to individual speech articulators, but an STI derived from the acoustic signal offers a composite and easily obtained measure that incorporates multiple components of the speech production complex. In this work, we examine the relationship between kinematic and acoustic STIs in children with and without developmental language disorder (DLD), with the aim of determining whether the acoustic and kinematic STIs reflect similar degrees of production variability. Method: A total of 85 children with DLD and with typical language development (or typically developing [TD] children), aged 4-8 years, were studied. In this methodological article, two experiments were conducted: one deliberately selected because group differences were observed in the kinematic STI (i.e., sentence production) and one in which there were no group differences in the kinematic STI (i.e., nonword production). These two experiments are representative of speech stability studies. The aim was to determine whether the acoustic STI (i.e., amplitude envelope) results aligned with those obtained via the kinematic STI (i.e., lip motion). Results: In sentence production, most group differences aligned across kinematic and acoustic STI measures. The acoustic, but not the kinematic, STI showed higher variability in children with DLD compared with the 6-year-old TD group. In nonword production, neither the kinematic STI nor the acoustic STI differentiated children with DLD from TD children. In each experiment, the kinematic and acoustic STIs showed a moderate-to-strong correlation. Conclusions: The kinematic and acoustic STIs assess different components of speech movement patterning. However, the relationship between acoustic and kinematic spatiotemporal stability is strong in two tasks of varying linguistic complexity in children with and without DLD. These findings are promising for future experimental work in this area.