Ontogenetic changes in the position of hair cell loss after acoustic overstimulation in avian basilar papilla

High intensity sound was used to produce localized hair cell damage within the basilar papilla of chicks at three different ages: embryonic day 20, post-hatch day 10 and post-hatch day 30. At each age separate groups of animals were exposed to broadband white noise or pure tones at 500, 1500 or 3000 Hz for 12 h at 125 dB SPL. Chicks were killed 10 days later. Their basilar papillae were then fixed, dissected free, osmicated, embedded in Epon, sectioned serially and stained. Hair cells were counted at 100 micron intervals throughout the length of the papilla. There was a systematic developmental shift in the position of damage produced by each of the acoustic stimuli. Broadband white noise produced damage only in the basal one half of the cochlea in the embryonic animals while at later ages it produced damage throughout the length of the papilla. Exposure with each of the pure tones produced a discrete area of hair cell loss. However, with each frequency the region of damage shifted apically as a function of the age of the animal at the time of sound exposure. These results suggest that the frequency representation along the basilar papilla is not fixed, but changes during the development of hearing.