MIDDLE-EAR DEVELOPMENT .7. UMBO VELOCITY IN THE NEONATAL RAT

Laser interferometry was used to measure umbo velocity in the developi ng 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 co rresponding umbo velocity response was measured. Umbo velocity respond ed linearly with respect to sound pressure throughout development. Whe n the stimulus level was held constant at 100 dB SPL, all animals disp layed a velocity response that increased with frequency until a peak r esponse was reached at about 20.0 kHz. Above this frequency the respon se decreased in all age groups. Umbo velocity increased with age at al l frequencies, and at 1.0, 2.0, 4.0, 8.0, 16.0, and 32.0 kHz the veloc ity reached 90% of its mature value by 68, 24, 24, 15, 19, and 50 days after birth, respectively. These age-related increases in tympanic me mbrane velocity coincided with improvements in compound action potenti al (CAP) thresholds (as measured by other investigators) at similar fr equencies. Both umbo velocity and CAP thresholds showed substantial gr owth after 10 days of age. The role of middle-ear functional developme nt with respect to overall auditory sensitivity is discussed. (C) 1996 Acoustical Society of America.