Variation in perioral reflex amplitude prior to lip muscle contraction for speech.

The purpose of this study was to examine variations in the amplitude of the perioral reflex that may occur during the reaction time interval before voluntary muscle contraction for speech. Four normal adults produced the syllable /wi/ as quickly as possible in response to a low level electric shock while electromyographic recordings of the orbicularis oris inferior muscle (OOI) were made. During the reaction time interval a precise mechanical stretch was applied at the corner of the mouth to elicit the first component of the perioral reflex. Analysis of the resulting data revealed a significant increase in perioral reflex amplitude occurring between 30 and 50 msec before the onset of voluntary OOI contraction. It is concluded that the observed increases in reflex amplitude were the result of increased excitability of brainstem interneurons and/or motoneurons mediating the perioral reflex. The central mechanisms underlying such excitability increases and their significance for understanding speech motor control are discussed.