Comparison of behavioral and auditory brainstem measures of hearing loss in rats exposed to loud sound

Abstract

Behavioral thresholds for 400-ms pure-tone pulses (2/s, 10 ms rise-decay) were determined for monaural rats using the method of conditioned suppression. Auditory brainstem response (ABR) thresholds for 1-ms tone pulses (27.7/s, 0.5 ms rise-decay) were determined in the same animals for the same frequencies (2, 4, 8, 16, and 45 kHz). A hearing loss was then induced by exposing an isoflurane-anesthetized animal to a loud tone (110 or 120 dB) for 10 min. A behavioral pure-tone threshold was determined one hour after exposure followed immediately by an ABR threshold for the same frequency. The animals were retested on subsequent days until their thresholds stabilized. So far (n=10), no simple relationship has been found between the ABR-estimated hearing loss and the actual hearing loss. Although the ABR tended to overestimate the hearing loss, the range of both over- and under-estimates was more than 25 dB. The difference between the two measures may be because of the different stimulus parameters used in the two procedures or because the ABR is a measure of neural synchrony as well as absolute sensitivity. Work is in progress to determine if a more reliable estimate of sensorineural hearing loss can be obtained from the ABR.