Speech motor development during acquisition of the voicing contrast.

Lip and jaw movements were studied longitudinally in 19-month-old children as they acquired the voicing contrast for /p/ and /b/. A movement tracking system obtained lip and jaw kinematics as participants produced the target utterances /papa/ and /baba/. Laryngeal adjustments were also tracked through acoustically recorded voice onset time (VOT) of the consonants. Across this period of developmental phonological change, the children began to produce VOTs in 2 distinct categories for voiced and voiceless plosives. Specific kinematic differences were observed during oral opening and closing and between spatial and temporal parameters of movement. The development of the voicing contrast was most closely associated with changes in jaw kinematics for oral opening in comparison to that of the lip. Conversely, movements into oral closing were not accompanied by significant increases in jaw, upper lip, or lower lip displacement or velocity, although a decrease in jaw movement variability was found. There was no evidence of phoneme-specific movement differences between /p/ and /b/ in the children or in the adults studied. Spatial coupling between the jaw and upper lip changed significantly across sessions, whereas changes in temporal coupling were not observed. Findings indicate that oral opening and closing have different task requirements and that children modify their articulatory movements to meet the demands of each task. Overall, the findings illustrate how orofacial movements and laryngeal function change in parallel during linguistic development.