On-line reflex control for tongue posture stabilization during speech production
Résumé
Tongue posture can be controlled using on-line sensory feedback for the stabilization of produced speech sounds. Although reflex mechanisms such as stretch reflex have been known to be involved in neural feedback control for the limb system, previous studies have been failed to provide evidence for any short-latency reflex in the human tongue. In our behavioral study using tongue-stretch perturbation, a relatively quick compensatory response (< 130 ms) was found for tongue posture stabilization in vowel production, which might be driven by short-latency reflex mechanisms. To further investigate this idea, this study aims to show neurophysiological evidence for reflex in the tongue during vowel production by the recording of tongue muscle activity. Due to the technical difficulty to combine movement recording with electromyographic (EMG) recording, we also carried out a computer simulation to examine whether the recorded EMG response can reproduce the previously observed articulatory response with the appropriate latency. In the test, we applied tongue-stretch perturbation using a robotic device during the production of the vowel /i/. EMG was recorded from the anterior part of the mouth floor using a uni-polar surface Ag-AgCl electrode. EMG signals from this site reflect in large part the activations of the Genioglossus and Geniohyoid muscles, which are largely involved in the production of /i/. We examined whether EMG signal from the tongue increased in response to tongue-stretch perturbation during the speech task. We also examined whether this response was modulated at resting state, in a voluntary reaction and in a non-speech task respectively. In speech condition, we found an increase of muscle activation in response to tongue-stretch perturbation. The latency was around 50ms. In the resting condition, a response was also induced with a similar latency, but with a reduced amplitude. This response was also induced earlier than the muscle activation induced by voluntarily reaction. A similar response was also seen in non-speech condition, but its peak amplitude was smaller than in the speech condition. All these observations indicate that the observed EMG response was induced by a reflex mechanism and that this reflex is task-dependent. Computer simulations considering muscle dynamics between EMG and force showed that the articulatory response cannot be reproduced by a passive component alone, and that the simulated movement was compatible with the previously observed articulatory response. The results demonstrate the existence and clear involvement of reflex mechanisms in tongue posture control during speech production.
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