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Abstract
Comparisons between click-evoked auditory brainstem responses (ABR) and auditory steady-state responses (ASSR) were performed to determine if the click-evoked ABR could be used to predict hearing loss in the bottlenose dolphin (Tursiops truncatus). The ASSR was evoked using sinusoidal amplitude modulated tones at half octave frequency intervals from 20 to 160 kHz and utilized to determine the upper-frequency limit of hearing in each dolphin (i.e., the frequency at which threshold was equal to 120 dB re 1 µPa). The click-evoked ABR was then recorded following exposure to a moderate-amplitude click (peak-peak equivalent sound pressure level of 122 dB re 1 µPa, 5 to 100 µs duration) and examined to determine if relationships existed among the upper-frequency limit of hearing and the amplitude/latency characteristics of the click-evoked ABR. The ASSR and clickevoked ABR were measured in six dolphins (4 males and 2 females, from 13 to 49 y of age) with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upperfrequency limit of hearing, although the number of waves showing the relationship varied with the duration of the click. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ~45 min and 1 min, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in delphinids.
Key Words: odontocet, bottlenose dolphin, Tursiops truncatus, hearing assessment, auditory steady-state response, auditory brainstem response, presbycusis
Introduction
Auditory evoked potentials (AEPs) are small changes in voltage representing neural synchrony within the auditory nervous system in response to acoustic stimuli. The auditory brainstem response (ABR) is an AEP which has been generated specifically from the auditory nerve and within the auditory brainstem. In odontocetes, the ABR to a click or tone-burst stimulus is a robust and replicable response quantified by amplitude and latency values of seven waveforms, all occurring within about 6 ms of the stimulus onset (Bullock et al., 1968; Ridgway, 1980). As stimulus intensity decreases, the ABR waveform amplitudes decrease and latency values increase. Although tone-burst ABRs may be reliably obtained in odontocetes (toothed whales), most estimates of frequency-specific thresholds have utilized the auditory steady-state response (ASSR; also termed the envelope-following response or...