ASA 125th Meeting Ottawa 1993 May

4pAB2. Hearing recovery in neonatal chicks after exposure to intense pure tones.

James C. Saunders

Henry J. Adler

Daryl E. Doan

Dept. of Otorhinolaryngol. Head and Neck Surgery, Univ. of Pennsylvania, Philadelphia, PA 19104

Neonatal chicks exposed to a 120-dB SPL pure tone at 0.9 kHz for 48 to 200 h exhibited a severe loss in auditory function. The growth and recovery of hearing loss was traced with a variety of peripheral evoked-potential and single-cell recordings. The growth of threshold shift and the loss of frequency selectivity reached asymptotic levels after 40 h and then remained constant to 200 h. At 0 days of recovery the magnitude of threshold shift in the most affected frequencies was 55--60 dB and tuning curves showed a 50% loss in selectivity. Nearly complete recovery of sensitivity and selectivity was noted 12--15 days post exposure. Severe recruitment was also seen in functions relating sound level to evoked-potential amplitude or single cell rate-intensity functions. The abnormal coding of intensity returned to normal within 3 days. The loss in function was associated with significant cochlear damage, and recovery was associated with considerable cochlear repair. The repaired cochlea, however, does not have a normal appearance. It remains to be explained how auditory function can completely recover in a cochlea with severe permanent structural defects. [Work supported by the NIDCD, PLHRF, and NOAR.]