The purpose of this study was to describe the activation characteristics of lip-muscle single motor units in relation to speech rate and phonetic structure. Repeated experiments were carried out on three adult subjects from whom recordings of lower lip EMG and two-dimensional displacement were obtained. Single motor unit recordings were obtained from the orbicularis oris inferior (OOI), depressor labii inferior (DLI), and mentalis (MENT) muscles. Subjects' tasks involved repeating CV syllables at 1 to 4 syllables per second (syl/sec). The distribution of interspike intervals and corresponding firing rates were obtained on 11 motor units. The firing rates of OOI and MENT motor units increased as syllable rate changed from 1 to 3 syl/sec, but firing rates tended to be equivalent at 3 and 4 syl/sec. DLI and tonic motor units showed little or no modulation in their firing rates with speech rate. Firing rate data and related observations on lip movement and EMG spike count levels suggest that distinct neuromechanical processes control lip movements at low and high speech rates. Both kinematic and EMG data support the expectation that phonetic structure has its greatest effects on lip opening compared to lip closing movements in CV syllables. OOI and MENT activation levels tended to be highest for /p/ productions compared to /w/ and /f/. This may be related to the requirements for complete lip closure and elevated levels of intraoral pressure for production of /p/.

Hearing adults produced signed and spoken monologues under conditions of quiet or 80 dB SPL of noise and with their vision unobstructed or obstructed. Their signs were videotaped and a random sample of 24 frames was analyzed in each condition through a computer program that determined the overall distance of the hand from a marker placed on the signer's torso. Vocal intensity was digitized from the tape recordings and analyzed by computer for 1 min of continuous speech in each condition. The visual obstruction had no effect either on the distance of the signs or on the vocal intensity of their speech. The subjects increased vocal intensity by about 55% when the noise was introduced (the usual Lombard effect), but the noise had no effect on the distance of the signs. Sign performance was not influenced by visual feedback, and sign and speech were independent communication systems, even in bilingual speaking and signing subjects.

This study investigated the effects of reduced visibility on the distance of signing hands from the bodies of adult signers who are deaf and on their rates of sign production. Subjects were videotaped as they signed with partners in each of three experimental conditions: unimpeded, moderately impeded, and severely impeded visibility. Visual impedance was created by placing screens between signing partners. The distance of a hand from the body, in terms of lateral, vertical, and forward-depth dimensions, was determined from digitized measurements of single video frames randomly selected from those in which the subject was actively signing. Sign rate was calculated as the number of signs produced in a given segment of time. Signs produced in the severely impeded visibility condition were found to be made at a slower rate and with a mean hand position further forward and vertically higher than signs produced in the unimpeded visibility condition.

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.

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