Summary

The possibility of conditioned dampening of whale hearing thresholds, when a loud sound is preceded by a warning sound, was investigated. The loud sound was a tone of 20 kHz, 170 dB re 1 μPa, 5s. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. The same test sounds served as warning sounds. The durations of the warning sounds were varied randomly to avoid locking an anticipated conditioning effect to the timing immediately before the loud sound. When the test/warning sound pairing varied within a range of 1 to 9 s or from 5 to 35 s, hearing thresholds before the loud sound increased relative to the baseline, respectively, by 10.5 and 13.2 dB. When the test/warning sound precedence varied within a range of 20 to 140 s, the threshold increase was negligible (2.9 dB). The observed hearing threshold increase was not a result of the unconditioned effect of the loud sound like a TTS, so it is considered as a manifestation of conditioned dampening of hearing when the subject anticipated the quick appearance of a loud sound, most likely to protect its hearing.

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