Speech Motor Development During Acquisition of the Voicing Contrast

Abstract

Lip and jaw movements were studied longitudinally in 19-month-old children as theyacquired the voicing contrast for /p/ and /b/. A movement tracking system obtainedlip and jaw kinematics as participants produced the target utterances /papa/ and/baba/. Laryngeal adjustments were also tracked through acoustically recordedvoice onset time (VOT) of the consonants. Across this period of developmentalphonological change, the children began to produce VOTs in 2 distinct categoriesfor voiced and voiceless plosives. Specific kinematic differences were observedduring oral opening and closing and between spatial and temporal parametersof movement. The development of the voicing contrast was most closely associatedwith changes in jaw kinematics for oral opening in comparison to that of the lip.Conversely, movements into oral closing were not accompanied by significantincreases in jaw, upper lip, or lower lip displacement or velocity, although a decreasein jaw movement variability was found. There was no evidence of phoneme-specificmovement 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 thatoral opening and closing have different task requirements and that children modifytheir articulatory movements to meet the demands of each task. Overall, the findingsillustrate how orofacial movements and laryngeal function change in parallel duringlinguistic development.