Physiologic development of speech motor control: articulatory coordination of lips and jaw

Abstract

The objective of this investigation was to describe development of lip and jaw coordination during speech. The potential influence of labiomandibular coordination on phonologic acquisition was also considered. A computer-based movement tracking system was used to transduce movement of the upper lip, lower lip, and jaw. Productions of syllables containing bilabial consonants were obtained from four age groups (i.e., one-, two-, six-years, and young adult). Two complementary analytic techniques were used to quantify interarticulator coordination, one reflecting spatial and temporal coupling of articulatory pairs and the other isolating each articulator's contribution to oral closure.The coordinative organization of these articulatory gestures was shown to change dramatically during the first several years of life and to continue to undergo refinement st age six. At one year of age, jaw displacement contributed the most to oral closure. The contribution of the lower lip increased gradually with age, whereas the contribution the upper lip was greater for two-year-olds than for any other group.Spatial and temporal coupling of movement of the upper lip, lower lip, and jaw were shown to increase with maturation. A similar developmental trend was exhibited for each measure. Coupling of upper lip and lower lip movement was rigid in even the youngest subject groups. In contrast, coupling of lip and jaw pairs was initially weak and gradually increased with age.The present results can be interpreted as representing three primary phases in the development of lip and jaw coordination for speech: integration, differentiation, and refinement. Each of these developmental processes entails the existence of distinct coordinative constraints on early articulatory movement. It is suggested that these constraints will have predictable consequences for the sequence of phonologic development.