Noise-induced hair-cell loss and total exposure energy: analysis of a large data set.

The relation between total noise-exposure energy, recovery time, or rest during the exposure and amount of hair-cell loss was examined in 416 chinchillas. The exposures were octave bands of noise (OBN) with a center frequency of either 4 kHz at 47-108 dB sound pressure level (SPL) for 0.5 h to 36 d, or 0.5 kHz at 65-128 dB SPL for 3.5 h to 432 d. Recovery times varied from 0 to 365 d. With both OBNs, some animals were exposed on interrupted schedules. Hair-cell loss as a function of age in nonexposed animals (N = 117) was used to correct for sensory-cell loss due to aging. For both OBNs, the ears (N = 607) were separated into three subsets to characterize the primary hair-cell loss from noise and the secondary post-exposure loss and to determine if rest during the exposure decreased loss. Cluster and regression analyses were performed on data from the basal and apical halves of the cochlea to determine the specific rates for these three factors. It was found that: (1) when the OBN was above a critical level, there was no relation between total energy and hair-cell loss; (2) below a critical level, there were highly significant log-linear relations between total energy and hair-cell loss, but not at rates predicted by the equal-energy hypothesis; (3) rest periods during either OBN exposure reduced hair-cell loss; more so for the 4 kHz OBN than the 0.5 kHz OBN; (4) except for the highest exposure levels, the majority of outer hair cell loss from the 4 kHz OBN occurred after the exposure had terminated, while that from the 0.5 kHz OBN occurred during the exposure; and (5) a majority of the inner hair cell loss from both OBNs occurred post-exposure.