Quantifying differences in dolphin hearing thresholds obtained with behavioral and auditory evoked potential methods

Different methods of producing the auditory steady state response (ASSR) are used to test dolphin hearing, but each method affects the resulting ASSR threshold. Since behavioral thresholds are often desired, this study, using common ASSR methods, compared differences between ASSR and behavioral hearing thresholds in five dolphins. Sinusoidal amplitude modulated (SAM) tones or tone pip trains were presented to the dolphins through a contact transducer while they were in air or partially submerged under water. Underwater behavioral hearing thresholds were obtained with pure tone stimuli on the same days as ASSR testing. Independent of the test medium, SAM tone stimuli yielded thresholds that consistently overestimated (i.e., were higher than) behavioral thresholds. Tone pip trains consistently underestimated thresholds when presented in air, and while they underestimated thresholds at lower test frequencies, they overestimated thresholds at higher test frequencies when presented under water. The mean differences between ASSR and behavioral thresholds were almost always lower when using tone pip train stimuli, but were exaggerated up to -47 dB when testing frequencies just above the upper-frequency limit of hearing. Knowing the relationship between ASSR and behavioral thresholds enables better approximations of behavioral thresholds in dolphins for which only ASSR thresholds exist.