Acquisition of vowel articulation in childhood investigated by acoustic-to-articulatory inversion.

While the acoustical features of speech sounds in children have been extensively studied, limited information is available as to their articulation during speech production. Instead of directly measuring articulatory movements, this study used an acoustic-to-articulatory inversion model with scalable vocal tract size to estimate developmental changes in articulatory state during vowel production. Using a pseudo-inverse Jacobian matrix of a model mapping seven articulatory parameters to acoustic ones, the formant frequencies of each vowel produced by three Japanese children over time at ages between 6 and 60 months were transformed into articulatory parameters. We conducted the discriminant analysis to reveal differences in articulatory states for production of each vowel. The analysis suggested that development of vowel production went through gradual functionalization of articulatory parameters. At 6-9 months, the coordination of position of tongue body and lip aperture forms three vowels: front, back, and central. At 10-17 months, recruitments of jaw and tongue apex enable differentiation of these three vowels into five. At 18 months and older, recruitment of tongue shape produces more distinct vowels specific to Japanese. These results suggest that the jaw and tongue apex contributed to speech production by young children regardless of kinds of vowel. Moreover, initial articulatory states for each vowel could be distinguished by the manner of coordination between lip and tongue, and these initial states are differentiated and refined into articulations adjusted to the native language over the course of development.

Decomposition of spontaneous movements of infants as combinations of limb synergies.

We hypothesized that a variety of limb movements in infants, including spontaneous movements and movements during interactions with the environment, can be represented as combinations of limb synergies, which are building blocks for generating coordinated movements of multiple limbs. A decomposition algorithm based on a nonnegative matrix factorization was applied to the discrete data segments taken from continuous data of limb movements in 298 infants (age, 3-4 months). The data were linearly decomposed into bases, which were referred to as synergies. The results showed that approximately 70% of the variance in the velocity profiles of the data segments of the four limbs can be explained by a set of five simple synergies that represent single-limb movements and the synchronous movement of all limbs. The present method showed that the complex properties of limb movements can be represented as combinations of synergies. Furthermore, comparisons of movement patterns across different age groups showed that in older infants, the contribution ratios of each synergy were different between spontaneous movements and movements during playing with a toy, whereas in younger infants, there were no differences in the contribution ratios between the different movement conditions. These results demonstrate that decomposition into limb synergies is useful for determining the spatiotemporal properties of interlimb coordination during spontaneous movements and task-constrained movements in infants.

Precursors of dancing and singing to music in three- to four-months-old infants.

Dancing and singing to music involve auditory-motor coordination and have been essential to our human culture since ancient times. Although scholars have been trying to understand the evolutionary and developmental origin of music, early human developmental manifestations of auditory-motor interactions in music have not been fully investigated. Here we report limb movements and vocalizations in three- to four-months-old infants while they listened to music and were in silence. In the group analysis, we found no significant increase in the amount of movement or in the relative power spectrum density around the musical tempo in the music condition compared to the silent condition. Intriguingly, however, there were two infants who demonstrated striking increases in the rhythmic movements via kicking or arm-waving around the musical tempo during listening to music. Monte-Carlo statistics with phase-randomized surrogate data revealed that the limb movements of these individuals were significantly synchronized to the musical beat. Moreover, we found a clear increase in the formant variability of vocalizations in the group during music perception. These results suggest that infants at this age are already primed with their bodies to interact with music via limb movements and vocalizations.

Development of a serial order in speech constrained by articulatory coordination.

Universal linguistic constraints seem to govern the organization of sound sequences in words. However, our understanding of the origin and development of these constraints is incomplete. One possibility is that the development of neuromuscular control of articulators acts as a constraint for the emergence of sequences in words. Repetitions of the same consonant observed in early infancy and an increase in variation of consonantal sequences over months of age have been interpreted as a consequence of the development of neuromuscular control. Yet, it is not clear how sequential coordination of articulators such as lips, tongue apex and tongue dorsum constrains sequences of labial, coronal and dorsal consonants in words over the course of development. We examined longitudinal development of consonant-vowel-consonant(-vowel) sequences produced by Japanese children between 7 and 60 months of age. The sequences were classified according to places of articulation for corresponding consonants. The analyses of individual and group data show that infants prefer repetitive and fronting articulations, as shown in previous studies. Furthermore, we reveal that serial order of different places of articulations within the same organ appears earlier and then gradually develops, whereas serial order of different articulatory organs appears later and then rapidly develops. In the same way, we also analyzed the sequences produced by English children and obtained similar developmental trends. These results suggest that the development of intra- and inter-articulator coordination constrains the acquisition of serial orders in speech with the complexity that characterizes adult language.