Observations of phonation threshold pressure and fundamental frequency in the in vivo canine larynx using graded neuromuscular stimulation

Phonation threshold pressure is an objective measure of the relative ease of phonation. Previous studies of the in vivo canine larynx have reported values of phonation threshold pressure in the range of 3.5-5.5 kPa. Such studies are often cited to argue that canine and human vocal fold physiology may differ significantly, since threshold pressures in human phonation are often measured to be as low as 0.2-0.3 kPa. Similarly, the frequency range observed in canine phonation has generally been reported to be more limited than that found in human phonation. In this study, our hypothesis was that through the use of a newly-developed method of graded stimulation to both the superior laryngeal nerve and the individual branches of the recurrent laryngeal nerve, the ranges of phonation threshold pressure and fundamental frequency observed in the in vivo canine model would overlap significantly with that of human phonation. This hypothesis was confirmed. In particular, phonation threshold pressures were observed to be as low as 0.2-0.3 kPa, and the fundamental frequency range was observed to span nearly four octaves. Previous studies of in vivo canine phonation may have been limited by an inadvertent, exclusive focus on hyper-stimulation of the thyroarytenoid muscle.