Developmental change in variability of lip muscle activity during speech.
Compared to adults, children's speech production measures sometimes show higher trial-to-trial variability in both kinematic and acoustic analyses. A reasonable hypothesis is that this variability reflects variations in neural drive to muscles as the developing system explores different solutions to achieving vocal tract goals. We investigated that hypothesis in the present study by analyzing EMG waveforms produced across repetitions of a phrase spoken by 7-year-olds, 12-year-olds, and young adults. The EMG waveforms recorded via surface electrodes at upper lip sites were clearly modulated in a consistent manner corresponding to lip closure for the bilabial consonants in the utterance. Thus we were able to analyze the amplitude envelope of the rectified EMG with a phrase-level variability index previously used with kinematic data. Both the 7- and 12-year-old children were significantly more variable on repeated productions than the young adults. These results support the idea that children are using varying combinations of muscle activity to achieve phonetic goals. Even at age 12 years, these children were not adult-like in their performance. These and earlier kinematic studies of the oral motor system suggest that children retain their flexibility, employing more degrees of freedom than adults, to dynamically control lip aperture during speech. This strategy is adaptive given the many neurophysiological and biomechanical changes that occur during the transition from adolescence to adulthood.