AUDITORY BRAIN-STEM RESPONSE CORRELATES OF RESISTANCE TO NOISE-INDUCED HEARING-LOSS IN MONGOLIAN GERBILS

The auditory brain-stem response (ABR) was recorded from young adult Mongolian gerbils exposed to noise (octave band of noise centered at 4 kHz, 80 dB SPL, 6 h on, 18 h off) for 12 days. Temporary threshold shift (TTS) of 20-50 dB was measured at 4-8 kHz and TTS of 10 dB or less was measured at 1-2 and 16 kHz immediately after the initial exposure. Immediately following the final (12th) exposure, each animal had 10 dB or less threshold shift at all frequencies, demonstrating as much as 40-dB resistance to TTS. Because significant TTS was limited to the high frequencies, the apical portion of the cochlea was left relatively unaffected by the exposure. Amplitudes of waves ii-iii and iv of the ABR were unaffected at low frequencies and reduced at all stimulus levels for 8 kHz on the first day of exposure; the amplitudes recovered to near-baseline levels by the 12th day of exposure. ABR latencies of waves ii and iv were prolonged at low stimulus levels on days one and six of exposure, but recovered to baseline levels by the 12th day of exposure. Because resistance to noise exposure was observed in all subjects and resistance was limited in spectrum, the results suggest that the gerbil is an excellent model for examining mechanisms of resistance to noise-induced hearing loss.