| Physiologic investigations of the organization of mandibular muscle activity in infants support the notion that motor control for speech emerges from a coordinative organization distinct from those underlying nonspeech behaviors, such as chewing (e.g., Moore & Ruark, 1996). Differences have also been reported between babble and true words (Moore & Ruark, 1996). Grillner (1982) and others (MacNeilage, 1998) have suggested that the development of motor control for speech may originate from the recombination of innate motor programs such as that for chewing. This hypothesis has promoted specific assumptions regarding development of lexical and phonetic motor planning, the ontogeny of motor control for speech, and jaw coordination and the resultant kinematics during emergent babble (e.g., Locke, 2000; MacNeilage, 1998; MacNeilage & Davis 2001). The current investigation used six indices of mandibular coordination at three levels of analysis: mandibular kinematics, the association of EMG and jaw movement, and the association of muscle groups (EMG). EMG and kinematic data were recorded from an infant, from 8 to 22 months of age, at 4 to 6 week intervals. The parents were native English speakers. Nonspeech tasks included chewing and jaw oscillation; speech tasks included multisyllabic syllables, levels one (i.e., vowel babble), two (e.g., reduplicative syllables), and three (i.e., variegated syllables), and monosyllables. Rate and complexity of jaw movement were significantly different between nonspeech and speech tasks, and differences in complexity were noted among speech tasks and across levels of multisyllables. Correlated changes in EMG and subsequent jaw movement, and in EMG modulation and jaw movement, were different between nonspeech and speech tasks, and among nonspeech tasks, speech, and multisyllables. Only nonspeech tasks exhibited significant linear relationships between peak frequencies for EMG modulation, for all muscle groups, with that for associated jaw movement. Differences in the organization of muscle activity and in the correlation of modulation across muscles groups were noted between nonspeech and speech tasks, and among nonspeech tasks, speech, and multisyllables. Differences between and among speech and nonspeech tasks suggested that motor control develops as the product of emerging, interdependent capacities of the infant, as well as by the requirements imposed on it by the demands of the given task. |
