8. Psychoacoustic Studies of Dolphins and Whales
359
1988). Results from tonal masking experiments indicate that frequencies in
a relatively narrow band around a tonal signal effectively mask the signal,
and that at moderate noise levels this masking is roughly equivalent for
masking tones both above and below the signal frequency. Critical ratio
results for a number of species indicate good masked hearing capabilities,
and direct critical band measures seem to be summarized well by a constant-Q bandwidth model of auditory filtering.
5. Conclusions and Suggestions for Future Work
Auditory psychophysical work with cetaceans to date has indicated that at
least some small odontocetes possess an extraordinary auditory capability
with much of the system tuned to frequencies that are ultrasonic to humans.
Many animals demonstrate abilities to hear frequencies as high as 100 kHz.
It is generally believed that the high-frequency specialization evolved along
with the ability to echolocate. Most of the work has been conducted on one
species of cetacean, the Atlantic bottlenose dolphin (T. truncatus), and up
until recently most of that work was conducted with single subjects. Work
examining thresholds of a number of bottlenose dolphins indicates that,
not surprisingly, there may well be large individual differences in hearing
abilities within a species.
Most work has been grounded in behavioral psychophysical measures
with well-trained animals providing data based on what is heard or not
heard. Electrophysiological procedures, such as evoked auditory potential
measures in which hearing is measured by recording auditory brain stem
responses taken from the surface of the skin over the brain stem, have been
increasingly used as a more rapid method for evaluating hearing processes
than previous measures. Good comparisons between evoked auditory
potential and behavioral techniques are necessary to examine how well
evoked auditory potential measures match up to what has previously been
established via trained behavioral responses. Electrophysiological measures
will certainly be employed in the acquisition of basic auditory data from
new species in a manner similar to that employed by Popov and Klishin
(1998) on the common dolphin that stranded and was under medical care.
Large cetaceans, such as stranded sperm and mysticete whales, will certainly
be -examined as the procedures are developed and adapted for the large
whales.
Hearing is intimately a part of cetacean echolocation but most cetacean
auditory studies have examined passive hearing independent of the active
echolocation process. Bottlenose dolphins produce short (SOilS) echolocation pulses with amplitudes that may exceed 225 dB re 11lPa followed
immediately by very quiet echoes. Little has been done to examine hearing
during the active echolocation process. The "phantom echo" procedure in
which electronically produced echoes are returned in response to real
359
1988). Results from tonal masking experiments indicate that frequencies in
a relatively narrow band around a tonal signal effectively mask the signal,
and that at moderate noise levels this masking is roughly equivalent for
masking tones both above and below the signal frequency. Critical ratio
results for a number of species indicate good masked hearing capabilities,
and direct critical band measures seem to be summarized well by a constant-Q bandwidth model of auditory filtering.
5. Conclusions and Suggestions for Future Work
Auditory psychophysical work with cetaceans to date has indicated that at
least some small odontocetes possess an extraordinary auditory capability
with much of the system tuned to frequencies that are ultrasonic to humans.
Many animals demonstrate abilities to hear frequencies as high as 100 kHz.
It is generally believed that the high-frequency specialization evolved along
with the ability to echolocate. Most of the work has been conducted on one
species of cetacean, the Atlantic bottlenose dolphin (T. truncatus), and up
until recently most of that work was conducted with single subjects. Work
examining thresholds of a number of bottlenose dolphins indicates that,
not surprisingly, there may well be large individual differences in hearing
abilities within a species.
Most work has been grounded in behavioral psychophysical measures
with well-trained animals providing data based on what is heard or not
heard. Electrophysiological procedures, such as evoked auditory potential
measures in which hearing is measured by recording auditory brain stem
responses taken from the surface of the skin over the brain stem, have been
increasingly used as a more rapid method for evaluating hearing processes
than previous measures. Good comparisons between evoked auditory
potential and behavioral techniques are necessary to examine how well
evoked auditory potential measures match up to what has previously been
established via trained behavioral responses. Electrophysiological measures
will certainly be employed in the acquisition of basic auditory data from
new species in a manner similar to that employed by Popov and Klishin
(1998) on the common dolphin that stranded and was under medical care.
Large cetaceans, such as stranded sperm and mysticete whales, will certainly
be -examined as the procedures are developed and adapted for the large
whales.
Hearing is intimately a part of cetacean echolocation but most cetacean
auditory studies have examined passive hearing independent of the active
echolocation process. Bottlenose dolphins produce short (SOilS) echolocation pulses with amplitudes that may exceed 225 dB re 11lPa followed
immediately by very quiet echoes. Little has been done to examine hearing
during the active echolocation process. The "phantom echo" procedure in
which electronically produced echoes are returned in response to real