Muscle Activation for Labial Speech Gestures: An Electromyographic and Acoustic Study of Normal Speakers

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Abstract
The present study demonstrates the existence of several electromyographic and acoustic characteristics common to a normal-speaking group during labial articulation: Prespeech, background and articulatory activity have been identified and distinguished from each other. The muscles are recruited in two functionally antagonistic groups with reciprocal innervation. There is clear evidence that the EMG activity is context-dependent. Vowel duration varied systematically as a function of both vowel identity and consonantal environment. The duration of labial closure varied in a complementary fashion with the duration of the vowel segment. The dependence of vowel duration on the voicing or voicelessness of the following consonant was correlated with a difference in the timing of the EMG activity associated with the labial closure gestures. This timing was also found to depend significantly on the identity of the vowel. To account for these latter results a model of timing is proposed according to which the moment of gesture initiation (= onset of EMG activity) is a function of the articulatory and acoustic features of the gesture to be executed, such as its spatial extent and duration. Although more research is necessary on the neurophysiological correlates of such a hypothetical mechanism, the present findings for normal-speakers may provide a preliminary baseline for clinical applications and the further study of articulatory disorders.

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