Influences of utterance length and complexity on speech motor performance in children and adults.

The possible influences of utterance length and complexity on speech motor performance were examined by assessing the effects of increased processing demands on articulatory movement stability. Eight 5-year-old children and 8 young adults repeated a 6-syllable phrase in isolation (baseline condition) and embedded in sentences of low and high syntactic complexity. Lower lip movements for the target phrase were analyzed to produce the spatiotemporal index (STI), an index that reflects the stability of lip movement over 10 repetitions of the phrase. It was predicted that movement stability would be lower (reflected by higher values of the STI) for the phrase when it was spoken embedded in complex sentences and that, compared to adults, children's movement output would be more negatively affected by increased processing demands. The STI was significantly increased for the phrase spoken in the complex sentences compared to the baseline condition, and STIs of the children were consistently higher than those of the adults across conditions. These findings provide novel evidence that speech motor planning, execution, or both are affected by processes often considered to be relatively remote from the motor output stage.