Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns.

Abstract

Conditioned reductions in hearing sensitivity were elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 20, 40, or 80 kHz. Hearing was assessed via noninvasive measurement of auditory brainstem responses (ABRs) to 20 - to 133-kHz tone bursts presented at randomized intervals from 1 to 3 ms. ABRs within each trial were obtained by averaging the instantaneous electroencephalogram, time-locked to tone burst onsets, over 2- to 3-s time intervals. In initial testing, ABR amplitudes were reduced (relative to baseline values) in one dolphin after the conditioned stimulus, but before the unconditioned stimulus, demonstrating conditioned hearing attenuation. In subsequent testing with both dolphins, ABRs were attenuated throughout the entire 31-s trial. Maximum ABR threshold shifts occurred at and above the unconditioned stimulus frequency and were above 40 dB for some conditions. The results (1) confirm that dolphins can be conditioned to reduce hearing sensitivity when warned of an impending noise exposure, (2) show that hearing attenuation occurs within the cochlea or auditory nerve, and (3) support the hypothesis that toothed whales can "self-mitigate" some effects of noise if warned of an impending exposure.