Compensatory articulation during bilabial fricative production by regulating muscle stiffness

The cooperative mechanisms in articulatory movements were examined by using mechanical perturbations during bilabial phonemic tasks. The first experiment compares the differences in compensatory responses during sustained productions of the bilabial fricative /Φ/ for which lip constriction is required, and /a/, for which the lips and jaw are relatively relaxed. In the second experiment, we perturbed jaw movement with different load-onsets in the sentence "kono /aΦaΦa/ mitai". In both experiments, labial distances were recovered partly or fully by the downward shifts of the upper lip. The upper lip response was frequently prior to the EMG response observed in the sustained task. Additionally, initial downward displacement of the upper lip was frequently larger when the load was supplied during /Φ/ than when it was supplied during /a/ in the sustained and sentence tasks, respectively. The stiffness variation estimated by using a muscle linkage model indicates that the stiffness increases for the bilabial phonemic task in order to robustly configure a labial constriction. The results suggest that the change in passive stiffness regulated by the muscle activation level is important in generating quick cooperative articulation.