Objective tonal tinnitus induced by noise exposure in chinchillas

Although objective tonal tinnitus (OTT) is very common in normal-hearing humans, we have not found any such instances in ten ears of six healthy chinchillas. However, we have succeeded in inducing OTT in one chinchilla (while failing in several others) by exposing it to an octave band of noise centered at 500 Hz, at 95 dB SPL, for nine days. The OTT, which was not present immediately after exposure but was seen after 12 days of recovery, is found variously at either 4.6 or 5.8 kHz at an amplitude of 40 dB SPL in the occluded ear canal. Another noise-exposed chinchilla possessed an OTT at about 6.4 kHz but, because this animal was not tested prior to exposure, we can only surmise that the OTT was caused by the acoustic trauma. Histological examination of this latter ear showed a punctate but complete lesion of the organ of Corti near the cochlear location characteristic of the frequency of the OTT. Observations on the suppression of these OTT's by external sounds leads us to conclude that, as with humans, the vibratory source of these signals lies within the cochlea. [Work supported by NIH.]