Animal model for the 4-kHz tonal dip.

In humans, noise exposures produce permanent hearing losses which usually begin at 4 kHz. In chinchillas, a similar pattern of hearing loss was observed following exposure for 9 or 18 days to an octave band of noise with a center frequency of 0.5 kHz. Histopathological observations of cellular degeneration showed that this exposure produced different types of damage in the basal and apical turns of the cochlea. Behavioral measures of auditory function showed that damage in the basal turn was associated with permanent threshold shifts for one to several of the high frequencies. However, moderate losses of outer hair cells commonly appeared in the apical turn without permanent threshold shifts for low-frequency tones. These findings indicate that the pure-tone audiogram may not accurately reflect the condition of the organ of Corti after noise exposure.