Developmental changes in the effects of utterance length and complexity on speech movement variability.

PURPOSE The authors examined the effects of utterance length and linguistic complexity on speech movement consistency for 210 participants between the ages of 5 and 22 years. Variability and durational analyses were conducted to (a) determine a more complete picture of the developmental course of earlier observations of the effects of linguistic constructs on speech motor variability and (b) describe trends for duration of the same sequence of words in different sentential contexts across development. METHOD Lower-lip movement was recorded during the production of "buy Bobby a puppy" spoken in isolation as well as embedded as a phrase in 2 longer, more complex sentences. RESULTS Compared with young adults, children demonstrated higher variability in producing repeated movement sequences for the target word sequence across all conditions. Also, for all age groups except young adults, increased processing demands resulted in significantly increased movement trajectory variability. Duration analyses suggest that around age 9 years, children begin to use adult-like pre-speech processes to plan the timing of sentence internal phrases, and maturation of these planning processes continues through late adolescence. CONCLUSION These results provide further evidence for language-motor interactions and for a protracted course of speech motor development that continues well into adolescence.