Dynamics of intergestural timing: a perturbation study of lip-larynx coordination

In this study, downward-directed mechanical perturbations were applied to t he lower lip during both repetitive (/...(sic).../) and discrete (/(sic)/) utterances in order to examine the perturbation-induced changes of interges tural timing between syllables (i.e., between the bilabial and laryngeal ge stures for successive Ipl's) and within phonemes (i.e., between the bilabia l and laryngeal gestures within single Ipl's). Our findings led us to sever al conclusions. First, steady-state (phase-resetting) analyses of the repet itive utterances indicated both that "permanent" phase shifts existed for b oth the lips and the larynx after the system returned to its pre-perturbati on rhythm and that smaller steady-state shifts occurred in the relative pha sing of these gestures. These results support che hypothesis that central i ntergestural dynamics can be reset by peripheral articulatory events. Such resetting was strongest when the perturbation was delivered within a "sensi tive phase" of the cycle, during which the downwardly directed lower-lip pe rturbation opposed the just-initiated, actively controlled bilabial closing gesture for Ipl. Although changes in syllable duration were found for ethe r perturbed phases, these changes were simply transient, effects and did no t indicate a resetting of the central "clock." Second, analyses of the tran sient portions of the perturbed cycles of the repetitive utterances indicat ed that the perturbation-induced steady-state phase shifts are almost total ly attributable to changes occurring during the first two perturbed cycles. Finally, the transient changes in speech timing induced by perturbations i n the discrete sequences appeared to share a common dynamical basis with th e changes to the repetitive sequences. We conclude by speculating on the ty pe of dynamical system that could generate these temporal patterns.