Bottlenose dolphins (Tursiops truncatus) and other odontocete cetaceans rely on sound for communication, navigation, and foraging. Therefore, hearing is one of their primary sensory modalities. Both natural and anthropogenic noise in the marine environment could mask the ability of free-ranging dolphins to detect sounds, and chronic noise exposure could cause permanent hearing losses. In addition, several mass strandings of odontocete cetaceans, especially beaked whales, have been correlated with military exercises involving mid-frequency sonar, highlighting unknowns regarding hearing sensitivity in these animals.Auditory evoked potential (AEP) methods are attractive over traditional behavioral methods for measuring the hearing of marine mammals because they allow rapid assessments of hearing sensitivity and can be used on untrained animals. The goals of this study were to 1.) investigate the differences among underwater AEP, in-air AEP, and underwater behavioral heari ng measurements using two captive bottlenose dolphins, 2.) investigate the hearing abilities of a population of free-ranging bottlenose dolphins in Sarasota Bay, Florida, using AEP techniques, and 3.) report the hearing abilities of a stranded juvenile beaked whale (Mesoplodon europaeus) measured using AEP techniques.For the two captive dolphins, there was generally good agreement among the hearing thresholds determined by the three test methods at frequencies above 20 kHz. At 10 and 20 kHz, in-air AEP audiograms were substantially higher (about 15 dB) than underwater behavioral and underwater AEP audiograms.For the free-ranging dolphins of Sarasota Bay, Florida, there was considerable individual variation, up to 80 dB between individuals, in hearing abilities. There was no relationship between age, gender, or PCB load and hearing sensitivities. Hearing measured in a 52-year-old captive-born bottlenose dolphin showed similar hearing thresholds to the Sarasota dolphins up to 80 kHz, but exhibited a 50 dB drop in sensitivity at 120 kHz.Finally, the beaked whale was most sensitive to high frequency signals between 40 and 80 kHz, but produced smaller evoked potentials to 5 kHz, the lowest frequency tested. The beaked whale hearing range and sensitivity were similar to other odontocetes that have been measured.

<p dir="ltr">Sensorineural hearing loss (SNHL) resulting from cochlear damage has been shown to elevate hearing thresholds (reduces audibility), broaden auditory filters (reduces frequency selectivity which in turn reduces word recognition ability-particularly in adverse listening conditions), and produce abnormal growth of loudness. The use of hearing aids with prescribed amplification has been shown to improve both audibility and word recognition ability in most SNHL individuals, but not all. The neural bases for these perceptual deficits are not well understood. The few published reports evaluating auditory nerve single unit responses; and ensemble brainstem phase-locked neural activity appear to suggest that these perceptual deficits may be due, at least in part, to altered and/or degraded neural representation of certain acoustic features preserved in the temporal fine structure (TFS), and/or the envelope periodicity (ENV) of complex sounds like speech. In an effort to address this knowledge gap, we propose to evaluate the consequences of SNHL on the neural representation of the ENV and TFS of consonant-vowel syllables (CV) as reflected in the scalp-recorded brainstem envelope following response (FFR_ENV) and the fine structure following response (FFR_TFS), respectively. The concurrent recording of these brainstem responses (FFR_ENV and FFR_TFS) and the cortical acoustic change complex (ACC) will allow us to compare the nature of neural representation at two processing levels (auditory cortex and rostral brainstem) along the auditory neuraxis. Comparison of: (i) neural responses elicited by unprocessed and optimal hearing-aid processed stimuli will enable us to determine if prescribed hearing-aid processing will improve the neural representation of acoustic features important for speech perception. Such an outcome can potentially suggest the use of these neural metrics as an objective clinical tool to provide hearing aid outcome measures, which in turn could be used to tailor optimal signal processing in hearing aids to improve benefits at the individual level; and (ii) since one major complaint of hearing impaired listeners is increased difficulty in adverse listening conditions it would be interesting to evaluate if individuals with SNHL with degraded neural representation is relatively more susceptible to greater degradation in adverse listening conditions. Finally, measures of neural representation with and without visual cues will enable us to evaluate the influence of visual cues on the neural representation of the acoustic features of complex sounds in normal and impaired ears.</p>