Lip muscle reflex and intentional response levels in a simple speech task

Sensorimotor integration in human lip muscle was studied by recording muscle activity while subjects produced simple speech utterances in response to mechanical stimulation. On each trial subjects were instructed either to produce the syllable "pa" or not respond when they detected movement of a small paddle held between the lips. Mechanical stimuli were adequate to elicit reflexes over poststimulus intervals of 15-30 ms (R1) and 30-50 ms (R2). EMG recordings were obtained from upper and lower lip muscles, and EMG levels were calculated for individual trials over several poststimulus time intervals. The independent effects of stimulus magnitude, prestimulus EMG, and reaction time on poststimulus response levels were assessed using multiple regression analysis. R1 and R2 levels were positively correlated with stimulus magnitude, but stimulus magnitude had little modulating effect on intentional lip muscle responses. Both reflex and intentional response levels showed positive associations with prestimulus EMG level. Instructional set had significant modulating effects on reflex responses in 9 of 10 subjects, but the nature of these effects varied among subjects. These various findings are discussed in relation to similar studies on limb motor systems and lip motor control for speech.