336
PE. Nachtigall et al.
stem of the dolphin and recorded from the head surface. The pattern of
brain waves recorded from the dolphin varies as a function of sounds played
to the animal, and the presence of a particular pattern in the animal's electroencephalogram (EEG) can be used as evidence that the animal has
detected the signal. In Popov and Klishin's study, the resulting audiogram
resembled closely the general form obtained using more standard psychophysical measures. One of the reasons that this study is so interesting is
that the animal studied had been found beached and in distress. Its brain
wave activity was being monitored as a health measure. Popov and Klishin
were able to obtain the brain stem response data using the same electrodes
used to monitor the status of the animal and they were able to obtaip an
audiogram without any training. The success of this measure demonstrates
the promise of this procedure for finding out about the hearing of other
cetaceans that may be difficult or impossible to bring into the laboratory.
A related technique used the auditory brain stem response to investigate
the modulation rate transfer function, which is an estimate of the dolphin's
sensitivity to rapidly changing stimuli (Dolphin et al. 1995; Dolphin,
Chapter 7). These electrophysiological measurements promise to add significantly to our ability to measure quickly the hearing capabilities of dolphins and other animals because they require little time to obtain a
threshold and no explicit training. Current work is underway to evaluate
more completely the comparability of the measures obtained using
this technique and those obtained using more traditional psychophysical
measures.
1.4 Early Psychoacoustic Studies with Dolphins
Fraser (1947) published a short article in Nature in which he collected a
number of earlier references and observations regarding the sound production of various cetacean species. He reviewed several reports of
cetaceans emitting high-frequency noises, and recounted his own experience of observing dolphins emitting a fine stream of bubbles while at a
depth of a few fathoms.
Kellogg and Kohler (1952) reported preliminary observations on auditory sensitivity in two dolphin species, Tursiops truncatus and Stenella
plagiodon, housed at the Marine Studios at Marineland, Florida. Based on
earlier field studies reporting high-frequency sound production and sensitivity, and neuroanatomical evidence supporting a highly developed auditory sense, this simple study examined the reactions of dolphins to sounds
over a wide frequency range. The sound stimuli consisted of 2 to 3s bursts
of tonal signals ranging from 20 to 200,000 Hz. The measured behavioral
response was defined as an increase in the rate of locomotion, as judged by
two to four observers. Based on this criterion, it was estimated that the
range of hearing for these subjects extended from 100 Hz to 50 kHz. A note
in the paper indicated that later studies found the upper range to be approx-
PE. Nachtigall et al.
stem of the dolphin and recorded from the head surface. The pattern of
brain waves recorded from the dolphin varies as a function of sounds played
to the animal, and the presence of a particular pattern in the animal's electroencephalogram (EEG) can be used as evidence that the animal has
detected the signal. In Popov and Klishin's study, the resulting audiogram
resembled closely the general form obtained using more standard psychophysical measures. One of the reasons that this study is so interesting is
that the animal studied had been found beached and in distress. Its brain
wave activity was being monitored as a health measure. Popov and Klishin
were able to obtain the brain stem response data using the same electrodes
used to monitor the status of the animal and they were able to obtaip an
audiogram without any training. The success of this measure demonstrates
the promise of this procedure for finding out about the hearing of other
cetaceans that may be difficult or impossible to bring into the laboratory.
A related technique used the auditory brain stem response to investigate
the modulation rate transfer function, which is an estimate of the dolphin's
sensitivity to rapidly changing stimuli (Dolphin et al. 1995; Dolphin,
Chapter 7). These electrophysiological measurements promise to add significantly to our ability to measure quickly the hearing capabilities of dolphins and other animals because they require little time to obtain a
threshold and no explicit training. Current work is underway to evaluate
more completely the comparability of the measures obtained using
this technique and those obtained using more traditional psychophysical
measures.
1.4 Early Psychoacoustic Studies with Dolphins
Fraser (1947) published a short article in Nature in which he collected a
number of earlier references and observations regarding the sound production of various cetacean species. He reviewed several reports of
cetaceans emitting high-frequency noises, and recounted his own experience of observing dolphins emitting a fine stream of bubbles while at a
depth of a few fathoms.
Kellogg and Kohler (1952) reported preliminary observations on auditory sensitivity in two dolphin species, Tursiops truncatus and Stenella
plagiodon, housed at the Marine Studios at Marineland, Florida. Based on
earlier field studies reporting high-frequency sound production and sensitivity, and neuroanatomical evidence supporting a highly developed auditory sense, this simple study examined the reactions of dolphins to sounds
over a wide frequency range. The sound stimuli consisted of 2 to 3s bursts
of tonal signals ranging from 20 to 200,000 Hz. The measured behavioral
response was defined as an increase in the rate of locomotion, as judged by
two to four observers. Based on this criterion, it was estimated that the
range of hearing for these subjects extended from 100 Hz to 50 kHz. A note
in the paper indicated that later studies found the upper range to be approx-