Active ultrasonic vocal communication channel found in Mongolian gerbils through the cochlear microphonics with noise exposure

Mongolian gerbils (Meriones unguiculatus) use ultrasonic vocal communication in frequency range of 22–45 kHz. However, hearing threshold of this frequency range reported has been very high, which is not suited for vocal communication. We examined possible active amplification created by the outer hair cells for frequency range of 22–45 kHz. In this study, we evaluated the amount of active amplification by the cochlear microphonics (CM) combined with temporary damage created by noise exposure. Adult gerbils received surgical implantation of a silver wire electrode on the round window of their cochlea through the middle ear to record CM. They were exposed to broadband noise (0.5 to 45 kHz) at 90 dB SPL for 5 min. CMs were recorded for tone bursts of 1 to 45 kHz. The following results were obtained. First, we observed the largest CM reduction just after the noise exposure. Second, decrements in CM amplitude depended on frequency. Low sensitivity frequency range above 22 kHz produced large reduction in CM amplitude. Third, decrease in CM amplitude was greater for lower stimulus intensities. Fourth, for testing frequencies, which produced large CM decrements, it took a longer period to recover back to pre-noise exposure amplitude levels. These findings indicate that reduction in CM amplitude appeared to be related to the cochlear nonlinearity generated by the outer hair cells.