Development of functional synergies for speech motor coordination in childhood and adolescence

Adults produce rapid, interleaved sequences of speech sounds by controlling the relative motions in time and space of the oral articulators. The control and coordination of these effectors appear to be automatic, effortless, and usually error free. If speech production is viewed within the framework of classical motor control theories, we can infer that adults have organized functional synergies (consistent patterns of activation of muscle collectives) that act as stable subunits. In this study, the development of these stable functional synergies is examined. Motion of the upper lip, lower lip, and jaw was recorded during sentence production in 180 children and adults (aged 4-22 years). Two indices of oral motor coordination were computed, which reflect the degree of trial-to-trial consistency in intereffector relationships and thus the stability of the underlying functional synergies. Major findings are: (a) The time course of development for speech motor coordination is protracted, (b) speech motor control processes are not adultlike until after age 14 years for both females and males, (c) boys (until age 5 years) show a slower maturational course of speech motor development, and (d) late childhood (the 7- to 12-year period) is characterized by a plateau in the development of these coordinative synergies for speech production. We posit that multiple factors are likely to contribute to the protracted development of oral motor coordination for speech, including maturation of components of the motor system itself and maturation of the brain subsystems for language processing.