A method for determining interaural attenuation in animal models of asymmetric hearing loss

Asymmetric or unilateral sensorineural hearing loss is an important hall-mark of various forms of sensorineural hearing loss. Animal research regarding the etiology and mechanism of these disorders often requires hearing estimates in each ear of experimental animals. Monaural auditory testing of animals with experimentally induced unilateral hearing loss therefore requires prior knowledge of interaural attenuation (IAA) to facilitate contralateral masking. The purpose of this study is to describe a method of determining frequency-specific IAA data and to present relevant information obtained in the rats--a frequently used animal in studies of acquired sensorineural hearing loss. A custom-made sound source was designed to accomplish threshold determination at important frequencies in the dynamic range of rats. Six male Long-Evans rats were surgically monauralized by ablation/obliteration of the cochlea. Auditory brainstem response (ABR) thresholds were determined for ipsilateral and contralateral presentations of 2-kHz, 10-kHz, and 40-kHz toneburst. IAA was calculated by subtracting the frequency-specific ABR threshold obtained from the normal ear from that obtained following tone presentation to the 'dead' ear, and was found to average 65.0 +/- 10.5 dB at 2 kHz, 45.0 +/- 8.4 dB at 10 kHz, and 47 +/- 15.1 dB at 40 kHz (+/- standard deviation). Using data obtained from the animal demonstrating the smallest IAA, masking is not needed until a threshold asymmetry of 50 dB at 2 kHz and 30 dB at 10 and 40 kHz is observed. In order to obtain bilateral auditory threshold information in any animal model of asymmetric hearing loss, data regarding IAA are needed in order to know when to apply contralateral masking and therefore avoid crossover stimulation of the non-test ear. The protocol presented herein provides guidelines for use in any animal model of sensorineural hearing loss which may demonstrate unilateral or asymmetric deficits.