Hearing sensitivity during target presence and absence while a whale echolocates

Hearing sensitivity was measured in a false killer whale during echolocation. Sensitivity was measured using probe stimuli as sinusoidally amplitude modulated signals with a 22.5-kHz carrier frequency and recording auditory evoked potentials as envelope-following responses. The probes were presented and responses were recorded during short 2-s periods when the animal echolocated to detect the presence or absence of a target in a go/no-go paradigm. In the target-absent trials, a hearing threshold of 90.4 dB re 1 muPa was found; in the target-present trials, the threshold was 109.8 dB. Thus, a 19.4-dB difference was found between thresholds in the target-present and target-absent trials. To check the possibility that this difference was the result of different masking degree of the probe by the emitted sonar clicks, click statistics were investigated in similar trials. No indication was found that the energy of the emitted clicks was higher in the target-present than in target-absent trials; on the contrary, mean click level, mean number of clicks per train, and overall train energy was slightly higher in the target-absent trials. Thus the data indicate that the hearing sensitivity of the whale varied depending on target presence or absence.