Fig. 5.

Heparitinase treatment alters the air-sac pressure required to produce phonation (sound onset pressure) in a non-frequency-dependent manner. A: pressure at phonation onset is changed 24 h after heparitinase treatment. At least a 5% increase in phonation onset pressure was found in 22 of 26 measured syllables. B: in enzyme-treated birds, the average increase in phonation onset pressure was 56.1% greater (Student's t-test, *P < 0.05) than pretreatment levels (measured as relative voltages). No significant difference in phonation onset pressure was found in vehicle-treated birds. C: heparitinase treatment did not significantly alter the fundamental frequency (FF) of sound at 24 h after enzyme injection compared with vehicle controls when injected into the ventral syringeal and ventral tracheobronchial muscles.