Model of the mechanical linkage of the upper lip-jaw for the articulatory coordination

The present study considers the muscle impedance control of upper lip-jaw coordination for jaw mechanical perturbations during bilabial utterances. When we perturbed jaw movement for the/Φ/utterance in the carrier sentences of "kono a a a mitai", labial distance was recovered quickly by the downward shift of the upper lip [8]. Initial downward shift (40ms after the load onset) of the upper lip by the perturbation was larger when the load was supplied at the closing phase when preparing for the first/Φ/than that at the opening phase when preparing for the second/a/in the sentence. This result suggests that the stiffness between upper lip and jaw varies depending on speech tasks. To characterize this difference, we estimated the stiffness changes of both utterances by using an upper lip-jaw spring model with positional shifts of the first/Φ/and the second/a/. The estimated ratio of the upper lip-jaw stiffness for/Φ/to that for/a/indicates that the stiffness increased in bilabial utterances. In addition, temporal variation of stiffness highly correlated with that of muscle activity. Moreover, we have succeeded to reproduce such compensatory movements by using a dynamical simulation of the mechanical linkage model with the perturbed jaw responses and estimated stiffness-change. These results suggest that the observed compensatory movements are generated by regulating passive muscle-linkages.