Oohashi, H. 1, 2 , Watanabe, H. 1 & Taga, G. 1
1 Graduate School of Education, The University of Tokyo
2 The Japan Society for the Promotion of Science
Development of speech production is attributed to changes in anatomy of articulatory organs and motor coordination. Although previous studies have revealed changes in acoustical features of speech sounds, limited information is available as to articulatory states during speech production in children. Here, we applied an acoustic-to-articulatory inversion technique to estimate articulatory states from vowel sounds produced by Japanese children aged from 6 to 60 months. We assumed seven articulatory parameters, that is, jaw, tongue position, tongue shape, tongue tip, lip aperture, lip protrusion and larynx height, and two scaling factors determining a length of oral and pharyngeal cavities. Three lower spectral peaks of each vowel were used to calculate the parameters of articulatory states by using pseudo-inverse Jacobian matrix that maps an acoustical space to an articulatory space. The stepwise discriminant analysis with vowel categories as the predictor and the estimated articulatory parameters as the independent variables showed that the development of children's articulation went through three stages. At the first stage (6-9 months of age), coordination of the tongue position and the lip aperture formed three vowels, that is, front, back and central vowels. At the second state (10-17 months of age), recruitments of jaw and tongue tip enabled differentiation of three vowels into the five Japanese vowels. At the third stage (18- months of age), coordination among articulatory controls including tongue shape produced more distinct vowels specific to Japanese. Our result suggests that initial vowel articulations are founded on coordination between lip and tongue, and that the articulations are differentiated and refined into those adjusted to the native language along development. In particular, the changes in the recruitment of tongue parameters are critical in the development of vowel production.