Tongue posture stabilization by reflex mechanism for speech production
Résumé
The stability of tongue posture control is important to produce speech sounds with enough accuracy. Stretch reflex is known as one mechanism that contributes to the posture stabilization in human sensorimotor control. While a previous study failed to examine tonic stretch reflex in the tongue, we have found behavioral evidence of relatively quick compensatory response of the tongue for the stabilization in vowel production, which can be driven by short-latency reflex mechanism. To support this idea, this study aims to show neurophysiological evidence of the tongue stretch reflex during vowel production by the recording of the tongue muscle activity. In the test, electromyographic response (EMG) was recorded from the anterior part of the mouth floor using a uni-polar surface Ag-AgCl electrode. The recorded EMG signal can reflect activation of the Genioglossus and Geniohyoid muscles, which are involved in the production of front-high vowels such as /i/. We applied a sudden tongue stretch using a precisely-controlled robotic device during the production of vowel /i/, since our previous study has showed a clear compensatory response in this vowel production. To verify whether the response is different in speech and non-speech tasks, we also tested a non-speech condition, in which participants were asked to produce the same amplitude of background muscle activation, without articulating any speech sound, and a resting condition associated with little or no muscle activation. To verify whether the target response can be modified voluntarily, we also included a reaction task, in which the subjects were asked to produce a strong muscle activation once they perceived the perturbation force during the production of /i/. In speech condition, we found an increase of muscle activation in response to the perturbation force. The latency was around 50 ms, which was longer than the possible latency by mono-synaptic connection of reflex (8-12 ms in the jaw-jerk reflex) and was comparable with the cortical reflex for lip compensation in speech (48 ms). A response was also induced in non-speech condition, but the peak amplitude of the response was smaller than in the speech condition, indicating that the reflex gain was modulated depending on the imposed task. In the resting condition, the response was induced with similar latency, indicating no evidence of typical mono-synaptic loop of reflex. The response was also induced earlier than the one induced by voluntary reaction, indicating that the response was not a voluntary one. These results showed that a reflex mechanism exists in the tongue and support the hypothesis that this reflex mechanism is involved in speech motor control for posture stabilization in vowel production. Considering the reflex latency, the reflex can result from a poly-synaptic reflex-arc and possibly mediate within the cortical loop.
Origine | Fichiers produits par l'(les) auteur(s) |
---|