Dynamic control of the perioral system during speech: kinematic analyses of autogenic and nonautogenic sensorimotor processes

Afferent contributions to the motor control of speech were evaluated by applying unanticipated loads to the lower lip during the combined upper lip-lower lip gesture associated with the oral closing movements for a "b" sound. Loads were introduced randomly in approximately 15% of the trials to minimize subject anticipation or adaptation. A total of 490 load trials (in five naive subjects) were distributed within a restricted interval (100 ms) centered on the initiation of agonist muscle contraction associated with the lip-closing movements. Kinematic adjustments of the upper and lower lips to these perturbations were examined in detail. In all subjects, load-induced changes in upper and lower lip displacement, movement time, and closing velocity were statistically significant and observed the first time a perturbation was introduced. Load timing variations within the target interval resulted in systematic changes in the site of the compensatory adjustments (upper versus lower lip) and in the magnitude of the kinematic responses. These kinematic changes appeared to reflect the dynamic nature of underlying control processes and clearly contrasted the different response characteristics of autogenic (lower lip) and nonautogenic (upper lip) compensatory actions. Although both upper and lower lip adjustments contributed to perturbation compensations, autogenic responses were found to predominate when loads occurred 20-55 ms before muscle activation. For these early loads, autogenic responses provided approximately 75% of the total compensation. For later loads, when the evolving speech motor action was more time constrained, nonautogenic (open-loop) compensations predominated, providing approximately 65% of the total compensation. The variations in upper and lower lip compensatory response magnitude did not parallel the time course of facial muscle activation. Lower lip kinematic adjustments were reduced 10-15 ms prior to the onset of agonist muscle activation, whereas upper lip adjustments increased in magnitude 10-20 ms after agonist onset. Apparently the dynamic modulation of these responses is controlled independently from facial motoneuron excitation, possibly involving sensorimotor processing via supranuclear centers. Overall the compensatory movement displacements were highly related to the magnitude of the perturbation displacement, especially for loads introduced prior to agonist muscle onset, reflecting a well-calibrated readjustment.(ABSTRACT TRUNCATED AT 400 WORDS)