The quantitative relation between sensory cell loss and hearing thresholds.

On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amount of sensory cell loss has been quantitatively related to the amount of permanent threshold shift at eight audiometric test frequencies between 0.125 and 16 kHz. The noise exposures, which varied extensively in spectrum, intensity and duration, produced permanent threshold shifts that ranged from 0 to 70 dB across a broad range of test frequencies. These data show: (1) consistent outer hair cell losses with less than 5 dB permanent threshold shifts (PTS) across all the test frequencies; (2) the first approximately 30 dB of PTS is established by losses of primarily outer hair cells; (3) in regions of the cochlea that transduce frequencies higher than or equal to 2 kHz, the three rows of outer hair cells show the same degree of loss for a given PTS, while in the 0.5 to 1.0 kHz region of the cochlea, the third row of outer hair cells (OHC) consistently shows less loss than do rows one and two; (4) appreciable inner hair cell (IHC) loss does not begin to appear until PTS exceeds approximately 30 dB; (5) in the virtual absence of OHC, hearing thresholds are least sensitive to IHC loss in the octave band centered at 4 kHz, i.e., the 4 kHz region can be as functional as other areas of the cochlea in spite of a greater amount of damage. The quantitative relation between cell loss and PTS varies as a function of test frequency in an orderly fashion.