ACTIVATION OF LIP MOTOR UNITS WITH VARIATIONS IN SPEECH RATE AND PHONETIC STRUCTURE

The purpose of this study was to describe the activation characteristi cs of lip-muscle single motor units in relation to speech rate and pho netic structure. Repeated experiments were carried out an three adult subjects from whom recordings of lower lip EMG and two-dimensional dis placement were obtained. Single motor unit recordings were obtained fr om the orbicularis oris inferior (OOI), depressor labii inferior (DLI) , and menta[is (MENT) muscles. Subjects' tasks involved repeating CV s yllables 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. Firin g rate data and related observations on lip movement and EMG spike cou nt levels suggest that distinct neuromechanical processes control lip movements at low and high speech rates. Both kinematic and EMG data su pport 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 c ompared to /w/ and /f/. This may be related to the requirements for co mplete lip closure and elevated levels of intraoral pressure for produ ction of /p/.