Middle-ear development VII: umbo velocity in the neonatal rat.

Laser interferometry was used to measure umbo velocity in the developing rat. The tympanic membrane was stimulated with pure tones between 0.4 and 40.0 kHz, at intensity levels between 50 and 130 dB SPL. The corresponding umbo velocity response was measured. Umbo velocity responded linearly with respect to sound pressure throughout development. When the stimulus level was held constant at 100 dB SPL, all animals displayed a velocity response that increased with frequency until a peak response was reached at about 20.0 kHz. Above this frequency the response decreased in all age groups. Umbo velocity increased with age at all frequencies, and at 1.0, 2.0, 4.0, 8.0, 16.0, and 32.0 kHz the velocity reached 90% of its mature value by 68, 24, 24, 15, 19, and 50 days after birth, respectively. These age-related increases in tympanic membrane velocity coincided with improvements in compound action potential (CAP) thresholds (as measured by other investigators) at similar frequencies. Both umbo velocity and CAP thresholds showed substantial growth after 10 days of age. The role of middle-ear functional development with respect to overall auditory sensitivity is discussed.