MIDDLE-EAR DEVELOPMENT .4. UMBO MOTION IN NEONATAL MICE

Laser interferometry was used to measure umbo velocity in the developi ng BALB/c mouse middle ear at 133 pure-tone frequencies between 2.0 kH z and 40.0 kHz, all at a constant 100 dB sound pressure level. Umbo ve locities increased with age across the entire frequency range, and rea ched adult-like levels by about 19 days between 2.0 and 22.0 kHz. Velo cities at 28.0 and 34.0 kHz took 27 and 52 days respectively to reach adultlike levels. A simple middle-ear model utilizing compliance, resi stance, and inertia elements matched the general trends of our velocit y results and provided an indication of the anatomical basis for the g rowth in umbo velocity. The model suggested that velocity development at the lowest frequencies may be attributed to increases in tympanic m embrane compliance. The model also indicated that both the frictional resistance of the middle ear and the inertia of the tympanic membrane and ossicles decreased during the growth period. At frequencies below 20.0 kHz, age-related increases in umbo velocity coincided with improv ements in N-1 thresholds recorded from the round window and evoked pot ential thresholds obtained from the cochlear nucleus. These results in dicated that the functional development of the middle-ear plays a majo r role in the development of hearing in the mouse.