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Abstract:
The present study shows experimental results concerning
compensatory movements for jaw mechanical perturbations during
bilabial utterances. First experiment compares the differences in
compensatory responses during sustained utterances of Φ, for
which lip constriction is required, and of /a/, for which lip and
jaw are relatively relaxed. The upper lip downward displacement
caused by jaw-load for Φ was larger than that for /a/,
whereas jaw was less shifted for Φ than was for /a/.
Considering that the muscle activity of upper lip (OOS) for Φ
was larger than for /a/, this observation suggests that initial
quick compensatory response of upper lip is dominantly achieved
by increased stiffness between articulators. In the second
experiment, we perturbed jaw movement associated with
labial-mandibular coordination for Φ and /p/ in the carrier
sentences of ``kono afafa mitai'' and ``kono apapa mitai''. As
observed previously, labial distance was recovered quickly by the
downward shift of upper lip, and then slowly by lower lip.
Initial downward displacement of upper lip by perturbation was
larger when the load was supplied at the closing phases preparing
for the fast Φ and /p/ than that at the opening phase
preparing for /a/ in the sentences. Although no distinctive
change in EMG (electromyogram) by perturbation was observed in
the half of subjects, the EMG itself nicely correlated with the
downward shift of upper lip by perturbation. The estimated ratio
of the upper lip-jaw stiffness for Φ to that for /a/
indicates that the stiffness increased in bilabial utterances.
The results suggest that not only instantaneous change in muscle
activity delivered by some nonautogenic neural mechanisms
demonstrated in previous studies is important in generating quick
cooperative relationships in articulatory movements, but also is
change in passive stiffness regulated by muscle activation
level.
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