Compensatory responses to low magnitude loads applied to the lower lip during speech

Unanticipated inferiorly directed loads were applied to the lower lip during speech using a torque motor under force feedback control. Loads well within the physiological force range of the labial musculature (10–15 gms) were chosen to ensure meaningful interpretation of ‘normal’ compensatory processes. Loads were applied on less than 15% of the utterance repetitions so as to minimize performance artifacts due to perturbation-induced conscious or unconscious motor adaptation. Compensatory EMG changes reflecting both closed-loop feedback as well as open-loop feedforward control processes were observed. These compensations, seen the first time the load was applied, included increased agonistic muscle activity (001, Mentalis), reductions in antagonistic muscles (DLI) as well as goal-oriented increases of upper lip muscle activity (OOS). Observation of EMG compensations from one load trial to another also revealed tradeoffs between muscles, apparently reflecting motor equivalence capabilities. These findings will be discussed relative to the role of afferent mechanisms in the autogenic control of individual movements and in the coordination of multiple speech gestures. [Supported in part by NINCDS program Project Grant NS-13274.]