Lip and jaw interaction during speech: responses to perturbation of lower-lip movement prior to bilabial closure.

Electrical stimulation was used to produce unexpected, involuntary depression of the lower lip in three normal young adults. Stimulation was timed to begin 500 to 40 ms prior to voice offset in [aep] and (Ip]. Upper lip, lower lip, and jaw movements were measured with a strain gauge system. Movements in 104 syllables with lower-lip stimulation were compared to the preceding normal syllable. Both the jaw and upper lip compensated for the involuntary perturbations in lower-lip movement. Compensatory movements did not occur as additional, discrete gestures following stimulation onset, but appeared as an increase in the size of closing movements. Bilabial closure was produced at the typical time (within - 10 to + 20 ms of voice offset) in 68% of the perturbed syllables, but it was delayed (a mean of 61 ms) in the remaining 32%. Neither the incidence nor the magnitude of this delay appeared to be related to the jaw position at stimulation onset or to the time between stimulation onset and voice offset.