Some effects of auditory stimulation on perioral motor unit discharge and their implications for speech production

The objective of this investigation was to describe the electrophysiological properties of short-latency reflexive pathways existing between the auditory system and the motoneurons involved in the control of lower lip movements for speech. The general procedure involved binaural presentation of auditory clicks (60--75 dB SL) at constant rates while subjects maintained a steady firing rate in a perioral motor unit. Post-stimulus time histograms were used to assess the effects of stimulation on the probability of firing of individual motor units. Data collected on 33 motor units in four subjects revealed a significant short-latency change in the probability of firing of 16 of these units. The mean latency of this effect was 20.4 ms. There was some indication that the direction and latency of the initial response to stimulation was dependent on the location and function of individual motor units. The likely neural pathways mediating these effects and their potential role in speech production are discussed.