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1,AEPs (auditory brainstem responses, ABRs) were picked up keyed from the individual outgoing echolocation clicks. The basic experimental methodĪs can be seen in Fig. This ability to measure what a whale hears of its outgoing echolocation click and its returning echoes has allowed the opportunity to ask very basic questions about odontocete echolocation. While there has been a good amount published on the characteristics of the outgoing signals of odontocetes( Nachtigall and Moore, 1988 Au, 1993 Thomas et al., 2004), the direct measurement of odontocete hearing during actual echolocation required the development of a technique to measure the AEPs in response to both the outgoing clicks and the returning echoes during an active echolocation task( Supin et al., 2003). Echolocation involves the ability to send signals and listen for the returning echoes from the environment. High frequency hearing appears to have evolved along with the ability to echolocate. Generally, as well as being able to follow modulated sound very fast, the dolphins and small toothed whales hear very high frequency sound under water( Johnson, 1966 Nachtigall et al., 2000). Thresholds can be obtained by modulating the carrier frequency of interest at rates near one-thousand times per second,transforming the responses to various carrier frequency levels viafast Fourier transforms, performing a linear regression on the peaks, and determining thresholds where the regression line crosses zero( Nachtigall et al.,2007a). The toothed whales and dolphins appear especially adapted for following fast changing sounds, as observed with modulation rate measurements, and this makes the technique particularly useful for odontocetes ( Dolphin et al.,1995 Supin and Popov,1995 Mooney et al.,2006). The direct comparison of physiological and behavioural measurements was made possible through the use of the envelope following response method of obtaining toothed whale hearing thresholds. While there were proponents of the direct comparison of electrophysiological and behaviourally obtained psychophysical( Stevens, 1970) sensory thresholds, only recently, due to the expense, time and difficulty of obtaining experimental subjects, has the use of auditory evoked potential(AEP) threshold measurement in dolphins and whales been emphasized and shown to be directly comparable to behaviourally obtained audiometrics( Yuen et al., 2005 Houser and Finneran, 2006). Overall, hearing during echolocation appears to be a very active process. We have found that: (1) the whale may hear her loud outgoing clicks and much quieter returning echoes at comparable levels, (2) the whale has protective mechanisms that dampen the intensity of her outgoing signals – she hears her outgoing signals at a level about 40 dB lower than similar signals presented directly in front of her, (3) when echo return levels are lowered either by making the targets smaller or by placing the targets farther away – without changing the levels of her outgoing signals – the hearing of these echoes remains at almost the same level, (4) if targets are made much smaller and harder to echolocate, the animal will modify what she hears of her outgoing signal – as if to heighten overall hearing sensitivity to keep the echo level hearable, (5) the animal has an active `automatic gain control' mechanism in her hearing based on both forward masking that balances outgoing pulse intensity and time between pulse and echo, and active hearing control.
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Working with a well-trained echolocating false killer whale( Pseudorca crassidens) wearing latex surface suction cup electrodes,we have measured echolocation hearing AEPs in response to outgoing echolocation clicks, returning echoes, and comparable simulated whale clicks and echoes in a variety of situations. While much has previously been learned about the echolocation performance and characteristics of the outgoing signals of echolocating dolphins and small whales, the hearing processes occurring while these animals actively echolocate have not previously been examined. Most small toothed whales echolocate and hear very high frequency sounds underwater. We have recently measured the hearing of a stranded infant Risso's dolphin, the audiograms of white-beaked dolphins temporarily caught and released, and the hearing of anaesthetized polar bears. The use of auditory evoked potential (AEP) measurements has added considerably to knowledge of the hearing mechanisms of marine mammals.