Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus).

The frequency specificity of conditioned dampening of hearing, when a loud sound is preceded by a warning sound, was investigated in a bottlenose dolphin. The loud sounds were 5 s tones of 16, 22.5 or 32 kHz, sound pressure level of 165 dB root mean square (RMS) re. 1 µPa. Hearing sensitivity was tested at the same three frequencies. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. The test sound stimuli served also as warning sounds. The durations of the warning sounds were varied randomly to avoid locking a conditioning effect to the timing immediately before the loud sound. Hearing thresholds before the loud sound increased, relative to the baseline, at test frequencies equal to or higher than the loud sound frequency. The highest threshold increase appeared at test frequencies of 0.5 octaves above the loud sound frequencies.