Evidence for suppression of lip muscle reflexes prior to speech.

The early component of the mechanically evoked lip muscle reflexes was studied in order to determine whether a consistent form of modulation occurs in associated neural pathways during the preparation to speak. Electromyograms (EMG) were obtained of upper and lower lip muscle while subjects were presented simultaneously with a visual instruction stimulus and a mechanical reflex stimulus every 4 s. On alternating trials, they were to produce either a single word or maintain their lip posture when they felt the mechanical stimulus. Consistent reductions in reflex amplitude were observed across subjects in lower lip muscle in the speech condition compared with the no-response condition. Statistical analyses suggest that this suppression effect occurred independently of variations in prestimulus EMG. These results are consistent with the idea that trigeminal sensory pathways innervating the perioral region are suppressed in preparation for orofacial movements associated with simple speech utterances.