Hair cell damage produced by acoustic trauma in the chick cochlea.

Examination of pure-tone acoustic damage in the chick basilar papilla revealed that the location and extent of hair cell damage was a function of both the stimulus intensity and the age at which the chicks were exposed. Scanning electron microscopic evaluation of noise-exposed cochleae at post-hatching days 1, 10 and 30 permitted the identification of discrete regions of damage, including hair cells with stereocilia injuries as well as those lost from the epithelium. The hair cell damage was tonotopically distributed along the cochlea according to frequency. However, for each exposure frequency two distinct sites of damage were often produced, and their locations were correlated with stimulus intensity. At low intensities, a longitudinal strip of hair cell damage ran along the superior edge of the basilar papilla. As exposure intensity increased, a second damage site developed along the inferior edge of the basilar papilla, distal to the longitudinal strip. This second type of damage initially took the form of a series of laterally-oriented wedges, but at higher intensities, the wedges coalesced to form a large crescent-shaped patch of damage. The location of the damage sites for each frequency did not shift with age. However, there were differences in the extent and position of the damage which could be correlated with stimulus intensity and with changes in middle ear admittance during development [(1983) Development of Auditory and Vestibular Systems, pp. 3-25. Editor: R. Romand. Academic Press, New York]. These results suggest that developmental changes in the location and extent of hair cell damage depend on the effective stimulus intensity reaching the cochlea, rather than on alterations in the frequency coding of the hair cells.