Md. Mcclean et Jl. Clay, ACTIVATION OF LIP MOTOR UNITS WITH VARIATIONS IN SPEECH RATE AND PHONETIC STRUCTURE, Journal of speech and hearing research, 38(4), 1995, pp. 772-782
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/.