The functional specificity of articulatory control and coordination

Abstract

Favored interpretations of “compensatory” movements of the lips to unanticipated resistances applied to the jaw during a closing gesture have focused on afferent feedback control [e.g., Folkins and Abbs (1975)] or feedforward, open‐loop control processes [e.g., Abbs and Cole (1982)]. The present experiments sought to identify and analyze, using dynamic perturbation techniques, functional constraints among several articulators when they cooperate to produce n desired utterance. A brushless DC torque motor coupled to the subject's lower teeth via a custom‐made prosthesis was programmed to inject a 500‐g torque load to the jaw for 1.5 s during the vowel‐consonant transition in /baeb/ and /baez/ on 20%–25% of trials. Infrared L.E.D.s were attached to the lips and jaw and monitored using a modified SELSPOT optical tracking system. In experiment 1, increased downward displacement of the upper lip occurred in perturbed conditions for /b/ (p < 0.01) but not for /z/. Experiment 2 replicated this result and also showed, through hooked wire electrode recordings from a tongue muscle, a significant increase in electromyographic activity for /z/ prior to occlusion, but not for /b/. Closure and frication were attained on all trials without any obvious changes in timing. These highly distinctive patterns suggest that the jaw, lips, and tongue may be controlled and coordinated as a single, functional unit (a coordinative structure) that is unique and specific to the intended act. [Work supported by NINCDS and BRSG.]