34
WWL. Au
sure at the subject's threshold. The acoustic stimulus is usually fairly long,
up to several seconds, and its frequency is well established. On the other
hand, ABR responses are onset responses that are triggered by the beginning portion of a brief acoustic signal. It is not clear how this onset level
can be related to the rms value of a tone burst. Secondly, ABR stimulii are
brief in duration and therefore relatively broadband. The auditory nervous system is not necessarily responding to the peak frequency in the stimulus but to all the frequencies contained in the stimulus. Therefore, there
is a lack of frequency specificity in the ABR stimulus and corresponding response. Finally, ABR measurements with dolphins often involves a
restrained animal placed in a small bath, making the acoustic propagation
condition extremely complex, and the size of the bath often makes it difficult to place the transducers along the major axis of the animal's receiving
beam. These factors are often overlooked or ignored when comparisons are
made between thresholds obtained with the ABR technique and a behavoral technique.
An auditory evoked potential technique that avoids some of the problems associated with the ABR technique for measuring hearing threshold is the envelope following response (EFR) technique. Scalp-recorded
responses have been found to follow the envelope of continuous sinusoidally amplitude-modulated stimuli in humans and in gerbils (Dolphin
and Mountain 1993). The neurons of the auditory system are actually
responding to the carrier frequency but are firing at the rate of the modulation frequency, so that the evoked potential response will have a peak frequency corresponding to the modulating frequency. The use of continuous
amplitude modulated stimuli in conjunction with EFR measurements have
been performed on dolphins by Dolphin et al. (1994) and Dolphin (1996)
at low frequencies «10kHz) and by Popov et al. (1995) for high frequencies (64 to 128kHz). Dolphin et al. (1995) measured the EFR of three
different species, T. tursiops, D.leucas, and P. erassidens. Since the EFR stimulus can be considered continuous, its rms value can be readily measured.
The response of the auditory system is a steady-state response and is more
closely related to the response of a subject in a behavioral study. Therefore,
this technique provides a rapid technique to estimate the audiogram of a
dolphin.
7. Summary
Cetaceans are clearly acoustically oriented animals. Sounds are important
to them for their survival in the aquatic environment. These animals emit
a wide variety of sounds. The large baleen whale emits low-frequency
sounds that are generally below several hundred Hertz but with frequency
components that can extend to about 3kHz. These low-frequency sounds
can travel relatively long distances depending on the propagation condition
WWL. Au
sure at the subject's threshold. The acoustic stimulus is usually fairly long,
up to several seconds, and its frequency is well established. On the other
hand, ABR responses are onset responses that are triggered by the beginning portion of a brief acoustic signal. It is not clear how this onset level
can be related to the rms value of a tone burst. Secondly, ABR stimulii are
brief in duration and therefore relatively broadband. The auditory nervous system is not necessarily responding to the peak frequency in the stimulus but to all the frequencies contained in the stimulus. Therefore, there
is a lack of frequency specificity in the ABR stimulus and corresponding response. Finally, ABR measurements with dolphins often involves a
restrained animal placed in a small bath, making the acoustic propagation
condition extremely complex, and the size of the bath often makes it difficult to place the transducers along the major axis of the animal's receiving
beam. These factors are often overlooked or ignored when comparisons are
made between thresholds obtained with the ABR technique and a behavoral technique.
An auditory evoked potential technique that avoids some of the problems associated with the ABR technique for measuring hearing threshold is the envelope following response (EFR) technique. Scalp-recorded
responses have been found to follow the envelope of continuous sinusoidally amplitude-modulated stimuli in humans and in gerbils (Dolphin
and Mountain 1993). The neurons of the auditory system are actually
responding to the carrier frequency but are firing at the rate of the modulation frequency, so that the evoked potential response will have a peak frequency corresponding to the modulating frequency. The use of continuous
amplitude modulated stimuli in conjunction with EFR measurements have
been performed on dolphins by Dolphin et al. (1994) and Dolphin (1996)
at low frequencies «10kHz) and by Popov et al. (1995) for high frequencies (64 to 128kHz). Dolphin et al. (1995) measured the EFR of three
different species, T. tursiops, D.leucas, and P. erassidens. Since the EFR stimulus can be considered continuous, its rms value can be readily measured.
The response of the auditory system is a steady-state response and is more
closely related to the response of a subject in a behavioral study. Therefore,
this technique provides a rapid technique to estimate the audiogram of a
dolphin.
7. Summary
Cetaceans are clearly acoustically oriented animals. Sounds are important
to them for their survival in the aquatic environment. These animals emit
a wide variety of sounds. The large baleen whale emits low-frequency
sounds that are generally below several hundred Hertz but with frequency
components that can extend to about 3kHz. These low-frequency sounds
can travel relatively long distances depending on the propagation condition