Recovery of auditory function and structure in the chick after two intense pure tone exposures

Groups of neonatal chicks were examined in three experimental conditions 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 exposed 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 potential measures of threshold shift, obtained at 0, 12 or 26 days post-exposure, 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 about 15 dB in all three groups at 12 days of recovery, while in birds exposed once at 2 days, but allowed 26 days to recover, the post-exposure thresholds returned to pre-exposure levels. Scanning electron microscopic analysis of cochlear structure was conducted in groups of similarly exposed and recovered animals. Twelve days post-exposure, the structural analysis revealed regeneration of a single honeycomb-like tectorial membrane layer in both the once and twice-exposed cochleae. However, damage to, and repair of, the tectorial membrane after the second exposure revealed the production of a second honeycomb layer in about half the animals examined. The results indicated that chicks retain the capacity to repair receptor epithelium damage and recover considerably from hearing loss after multiple exposures to intense sound.