Modulation rate transfer functions in bottlenose dolphins (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>) with normal hearing and high-frequency hearing loss |
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Authors: | James J Finneran Hollis R London Dorian S Houser |
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Institution: | (1) US Navy Marine Mammal Program, Space and Naval Warfare Systems Center, San Diego, Code 2351, 53560 Hull St., San Diego, CA 92152, USA;(2) University of California, San Diego, La Jolla, CA 92093, USA;(3) Biomimetica, 7951 Shantung Drive, Santee, CA 92071, USA |
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Abstract: | Envelope following responses were measured in two bottlenose dolphins in response to sinusoidal amplitude modulated tones
with carrier frequencies from 20 to 60 kHz and modulation rates from 100 to 5,000 Hz. One subject had elevated hearing thresholds
at higher frequencies, with threshold differences between subjects varying from ±4 dB at 20 and 30 kHz to +40 dB at 50 and
60 kHz. At each carrier frequency, evoked response amplitudes and phase angles were plotted with respect to modulation frequency
to construct modulation rate transfer functions. Results showed that both subjects could follow the stimulus envelope components
up to at least 2,000 Hz, regardless of carrier frequency. There were no substantial differences in modulation rate transfer
functions for the two subjects suggesting that reductions in hearing sensitivity did not result in reduced temporal processing
ability. In contrast to earlier studies, phase data showed group delays of approximately 3.5 ms across the tested frequency
range, implying generation site(s) within the brainstem rather than the periphery at modulation rates from 100 to 1,600 Hz.
This discrepancy is believed to be the result of undersampling of the modulation rate during previous phase measurements. |
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Keywords: | Dolphin Hearing Evoked potential MRTF Envelope following response |
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