Incomplete recovery of chicken distortion product otoacoustic emissions following acoustic overstimulation.

Distortion product otoacoustic emissions (DPOAEs) were measured in chickens before and after exposure to a 525-Hz pure tone (120 dB SPL, 48 h). The exposure caused extensive hair cell loss and destroyed the tectorial membrane along the abneural edge of the basilar papilla in the low-to-mid-frequency region of the cochlea. Although the lesion was restricted, DPOAEs were greatly depressed at all frequencies immediately after the exposure. The high-frequency DPOAEs gradually recovered to preexposure values after the exposure; however, there was little or no improvement in DPOAEs at test frequencies equal to or slightly above the exposure frequency even after 16 weeks of recovery. By 28 days of recovery, the previously damaged region of the basilar papilla had been repopulated by hair cells and the lower honeycomb layer of the tectorial membrane had regenerated, but not the upper fibrous layer. The upper fibrous layer of the tectorial membrane was still missing after 16 weeks of recovery and the region of damage corresponded closely to the frequency regions where the DPOAEs were depressed.

Cochlear frequency-place map in adult chickens: intracellular biocytin labeling.

A cochlear frequency-place map was developed for adult chickens by labeling cochlear ganglion neurons with biocytin and correlating the location of each labeled fiber along the basilar papilla with the characteristic frequency of the unit's tuning curve. Labeled fibers showed little or no branching within the sensory epithelium and most fibers appeared to terminate on a single hair cell along the neural side of the basilar papilla. The CFs of the labeled neurons ranged from 353 Hz to 3145 Hz and the location of the labeled neurons ranged from 30.1% to 74.4% of the total distance from the apex of the papilla. CFs increased in an orderly manner from the apex towards the base of the papilla. The cochlear frequency map for adult chickens was similar to that estimated from previous cochlear lesion studies carried out on 30 day old chicks, although the predicted frequencies in the adults were slightly higher in some regions of the basilar papilla than in 30 day old animals. However, previous maps developed in young animals (< or = 21 days) using lesion or labeling data predict significantly lower frequencies for a given location than in adult animals particularly in the basal half of the cochlea.