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Speech Motor Control during Unanticipated Perturbation of the Lip:
Replication and Extension of Gracco & Abbs 1985
Kirrie J. Ballard, Jerald Moon, Donald A. Robin, & John W. Folkins

The purpose of this study was to replicate and extend previous work (Gracco & Abbs 1985 (G&A)) demonstrating that sensory information is critical for controlling movement to achieve acoustically accurate speech production goals. Response of the speech motor system to unexpected mechanical perturbation of the lower lip during the lip closing gesture for “p” production was analyzed. A larger sample of subjects was included and, unlike the original study, the effect of perturbation on the acoustic speech signal was empirically tested. Perturbations were in the from of a 30 g load applied to the lower lip in a downward direction -120 to -10 ms prior to onset of lower lip muscle activity (early) or -10 ms before to +120 ms after onset of muscle activity (late). Results were consistent with G&A in that early perturbations yielded primarily autogenic responses, reflecting flexibility of motor programming processes. Late perturbations yielded primarily nonautogenic responses reflecting feedforward processes involved in movement execution. Unexpected, both early and late perturbations increased voice onset time for “p” and syllable duration, but the prolonged VOT has no perceptual consequence.

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