Contemporary models of the physiological development of speech are based primarily on acoustic observations and orofacial kinematics derived from the vocalizations of infants and toddlers. There is a paucity of corresponding data on the development of vocal control in infants, even though infants exhibit a remarkable range of frequency and intensity modulation. Controlled modulation of these speech parameters entails coordinated activity of the respiratory and laryngeal system, which are quite different in infants and adults. he proposed research project will investigate the development of respiratory and laryngeal control of vocal fundamental frequency (f0) and intensity in groups of six-month olds, one-year olds, two-year olds, and adults. Using procedures that are well-established for adult subjects, changes in f0 and intensity will be elucidated using auditory feedback with perturbed f0, intensity during a vowel imitation task. Intensity changes will also be elicited by changes in the intensity of pink noise presented to the subject. The amplitude and rate of change of f0 and intensity responses will be evaluated for directional sensitivity (i.e., rising versus falling contours) and to reveal when these low-level responses emerge developmentally. Although changes will be elicited in only one parameter at a time (e.g., fo), corresponding changes in the non-target parameter (e.g., intensity) will reveal the extent to which f0 and intensity can e differentially adjusted. In addition to revealing the developmental course for specific autogenic feedback in speech motor control, the proposed investigation will reveal differences in chest wall motion during changes in f0 and intensity. Specific differences in chest wall motion for f0 and intensity modulation will also be evaluated.
