RECOVERY OF AUDITORY FUNCTION AND STRUCTURE IN THE CHICK AFTER 2 INTENSE PURE-TONE EXPOSURES

Groups of neonatal chicks were examined in three experimental conditio ns that differed in the age and number of times they were exposed to a pure tone of 0.9 kHz at 120 dB SPL for 48 h. Several animals were exp osed once at 2 or 16 days of age, while others were subjected twice to the above stimulus, first at 2 days and then at 16 days. Evoked poten tial measures of threshold shift, obtained at 0, 12 or 26 days post-ex posure, were used to determine the degree of hearing loss and recovery . The average threshold loss in the mid-range frequencies was about 57 dB at 0 days for all three conditions. This level was reduced to abou t 15 dB in all three groups at 12 days of recovery, while in birds exp osed once at 2 days, but allowed 26 days to recover, the post-exposure thresholds returned to pre-exposure levels. Scanning electron microsc opic analysis of cochlear structure was conducted in groups of similar ly exposed and recovered animals. Twelve days post-exposure, the struc tural analysis revealed regeneration of a single honeycomb-like tector ial membrane layer in both the once and twice-exposed cochleae. Howeve r, damage to, and repair of, the tectorial membrane after the second e xposure revealed the production of a second honeycomb layer in about h alf the animals examined. The results indicated that chicks retain the capacity to repair receptor epithelium damage and recover considerabl y from hearing loss after multiple exposures to intense sound.