Inducing vocal register transition in an in vivo evoked phonation canine model

Background and purpose: The nature of vocal registers is still a subject of controversy. The purpose of this study was to demonstrate the induction of timbre transition of vocal register in an in vivo evoked phonation canine model and thereby confirm vocal register transition as a laryngeal event. Materials: A canine midbrain stimulation evoked phonation model was used in this study. To repeat a low-pitched evoked phonation in the model, the low activity of the thyroarytenoid (TA) muscle and coordinate actions of other intrinsic laryngeal muscles were kept in a consistent condition by stimula ting the same midbrain point with the same electric current intensity at th e same timing in the respiratory cycle. The cricothyroid (CT) muscle was ac tivated with an electrical current delivered directly to the muscle during the evoked phonation. Under constant subglottal pressure, CT muscle activit y was varied while changes in vocal register of the evoked phonation were m onitored. Results. The fundamental frequency (F0) of the evoked phonation increased a s the stimulating current to the CT muscle increased. In addition to the in crease in F0, data collected from six animals demonstrated that timbre regi ster transition was induced by a stepwise increase of current to the CT mus cle. The abrupt escalation of F0 and sudden change in sound quality, which could be verified perceptually, manifested the register transition. Frequen cy spectrum analysis showed that the sound in the modal register contained abundant harmonics that were different from those of the sound in the false tto register, which contained fewer harmonics. Conclusion: The results of this Study indicated that intrinsic laryngeal mu scles (especially CT and TA muscle interactions) regulate timbre register t ransition in a canine model.