Babbling and Chewing: Jaw Kinematics from 8 to 22 months.

Empirical gaps remain regarding infant mandibular kinematics observed during naturally occurring episodes of chewing and pre-linguistic vocalizations during the first 2-years of life. Vertical jaw displacement was measured from a typically developing infant from 8 to 22 months. Infant jaw kinematics was measured for vowel babble, non-variegated and variegated babble, and chewing. Results indicated that measures of kinematic variability were significantly less for chewing than all babble categories. These measures changed across age for chewing: (a) peak vertical jaw elevation decreased in variability, while (b) jaw displacement and (c) speed of movement increased in variability. Kinematics for vowel babble were characterized as exhibiting less jaw displacement with higher average vertical jaw position than other babble types and chewing. Developmentally, jaw kinematics for babble changed for jaw displacement and average vertical jaw position. These changes were related to decreased episodes for vowel babble productions and increased episodes for variegated babble and reduplicative syllables. These results suggest that developmental processes such as non-overlapping task-demands likely differentiate trajectories of jaw movement for infant chewing and babble. Infant jaw kinematics for babble cannot be predicted from observations of adult speakers or from non-speech behaviors observed for infants or adults.