The physiological development of speech production has remained largely unexplored in spite of Us importance to understanding developmental speech disorders and to models of normal speech motor control. In theoretical models and clinical applications, speech coordination, like other complex motor behaviors, has been assumed to be derived from rhythmic, homeostatic, centrally patterned functions such as respiration, chewing, and sucking. This assumption has not been tested, and despite its intuitive appeal, might be doubted because of demonstrated differences between centrally patterned behaviors and speech production in adults (Moore, Smith, & Ringel, 1988). An empirical evaluation of this assumption has implications both for models of normal speech motor control and for the rationale underlying diagnosis and treatment of children with developmental speech disorders. The proposed Investigation of normally developing children contrasts the coordination of centrally patterned oral and respiratory behaviors with speech coordination. This contrast will provide a quantitative description of speech development in the context of well-understood motor behaviors. Physiological development of speech motor control in primary speech subsystems in young children (9-48 months of age) will be described using a cross-sectional (70 children) and longitudinal (20 children) design. Coordination of systems for respiratory drive, mandibular movement, phonation, and articulation will be quantified during speech and during automatic, rhythmic, non-speech behaviors such as chewing and resting breathing. Electromyographic (EMG), acoustic, and kinematic signals will be analyzed. Articulatory coordination and laryngeal control will be inferred from formant (F1 and F2) trajectories and fundamental frequency (f-o) contours. Relative timing of muscle activity will be quantified using correlational analysis of E G signals. Signal coherence arising from paired speech muscles will be computed in order to detect activity arising from a common neural source. Non-zero coherence within paired EMG signals has been shown to be an observable characteristic of rhythmic motor behaviors, including mastication and respiration, both of which are mediated by central pattern generators (CPGs) in many species. Developmental changes in the temporal and spectral characteristics of EMG signals will delineate the role of automatic, non-speech coordinative organization in speech maturation.
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