Speech motor control during acquisition of the voicing contrast
By M.I. Grigos, J.H. Saxman, and A.M. Gordon
Upper lip, lower lip and jaw movements in children as they acquired the voicing contrast for /p/ and /b/, were studied longitudinally and compared to adult speakers. A movement tracking system obtained lip and jaw kinematics as subjects produced the target utterances. Laryngeal adjustments were tracked simultaneously through acoustically recorded voice onset time (VOT) of the consonants. Lip and jaw displacement, peak velocity and duration were analyzed over this developmental period. Spatial and temporal coordination of articulator movements were also examined. Results revealed an increase in the duration of lip and jaw movements as the voicing contrast was acquired. The amplitude of lip and jaw displacement also increased from session to session. The children produced faster articulatory movements across sessions, as reflected in an increase in peak velocity. Variability of lip and jaw displacement and velocity decreased as children began to produce the voiceless plosive yet increased for productions of the voiced plosive. Movement time variability increased. Spatial and temporal coupling of the lips and jaw improved over time. The results of the present study support the hypothesis that upper lip, lower lip and jaw movements become more coordinated and less variable as the voicing contrast is acquired.