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S.H. Ridgway
8.1 Phase Detection by the Auditory System
Mackay (1967) has suggested that dolphins may employ phase information
in sonar detection. Johnson (1967) has elaborated on the advantages of
phase detection for echolocation by dolphins (Evans and Powell 1967; Au
1980). Wever et at. (1971b) suggested that "the representation of detailed
high frequency sounds, and especially their time and phase relations, may
well be the basis for the dolphin's remarkable facility in echolocation."
Since no proof existed, however, that the dolphin auditory system is sensitive to phase at the high frequencies used in echolocation target detection,
we attempted to test the dolphin capability for phase detection using the
ABR (Ridgway et at. 1981). (Note: Humans show a similar phase sensitivity in the ABR at lower frequencies within their auditory range, but cannot
perceptually distinguish clicks of opposite phases.) Reversing the phase
of single sine waves delivered to the hydrophone revealed a small but consistent difference in response between initial compression clicks versus
initial rarefaction clicks, suggesting that the dolphin auditory system might
be capable of encoding phase even at high frequencies.
8.2 Auditory Processing in the Cerebral Cortex:
Lessons from Studies of Bats
Research by Suga (1984) has revealed an intricate organization of auditory
cortex in bats, and he has suggested not only that equally complex organization may be present in other auditory systems of animals but also that
human auditory cortex may show similar arrangements for processing the
complex sounds of speech. In Suga's bats, complex sounds are processed by
neurons tuned to combinations of information-bearing elements or parameters in the sounds. For example, areas of cortex are tuned to particular
echo delays and particular echo amplitudes. Neurons tuned to specific
information-bearing parameters (IBPs) or combinations of IBPs are clustered in areas of the cerebral cortex and systematically arranged along axes
or in coordinate systems for representation of sounds that are biologically
important to the species (Suga 1984).
It seems likely that equally complex sound processing takes place in the
dolphin cortex. If so, the mapping experiments conducted to date (Supin et
al. 1978), thought they showed certain differences of response in three areas
of auditory cortex, may not have been of sufficient acoustic detail to reveal
systematically arranged axes of specific delay-sensitive or amplitudesensitive neurons. Thus the mapping experiments probably do not reveal the
full extent of auditory cortical organization. Possibly, surface segments that
were not responsive in the mapping experiments (Supin et at. 1978) contain
neurons tuned to more specific acoustic parameters than those tested.
Detailed organization of auditory cortex described for bats may be
equally or even more complex in dolphins. If so, the dolphin cortex must
S.H. Ridgway
8.1 Phase Detection by the Auditory System
Mackay (1967) has suggested that dolphins may employ phase information
in sonar detection. Johnson (1967) has elaborated on the advantages of
phase detection for echolocation by dolphins (Evans and Powell 1967; Au
1980). Wever et at. (1971b) suggested that "the representation of detailed
high frequency sounds, and especially their time and phase relations, may
well be the basis for the dolphin's remarkable facility in echolocation."
Since no proof existed, however, that the dolphin auditory system is sensitive to phase at the high frequencies used in echolocation target detection,
we attempted to test the dolphin capability for phase detection using the
ABR (Ridgway et at. 1981). (Note: Humans show a similar phase sensitivity in the ABR at lower frequencies within their auditory range, but cannot
perceptually distinguish clicks of opposite phases.) Reversing the phase
of single sine waves delivered to the hydrophone revealed a small but consistent difference in response between initial compression clicks versus
initial rarefaction clicks, suggesting that the dolphin auditory system might
be capable of encoding phase even at high frequencies.
8.2 Auditory Processing in the Cerebral Cortex:
Lessons from Studies of Bats
Research by Suga (1984) has revealed an intricate organization of auditory
cortex in bats, and he has suggested not only that equally complex organization may be present in other auditory systems of animals but also that
human auditory cortex may show similar arrangements for processing the
complex sounds of speech. In Suga's bats, complex sounds are processed by
neurons tuned to combinations of information-bearing elements or parameters in the sounds. For example, areas of cortex are tuned to particular
echo delays and particular echo amplitudes. Neurons tuned to specific
information-bearing parameters (IBPs) or combinations of IBPs are clustered in areas of the cerebral cortex and systematically arranged along axes
or in coordinate systems for representation of sounds that are biologically
important to the species (Suga 1984).
It seems likely that equally complex sound processing takes place in the
dolphin cortex. If so, the mapping experiments conducted to date (Supin et
al. 1978), thought they showed certain differences of response in three areas
of auditory cortex, may not have been of sufficient acoustic detail to reveal
systematically arranged axes of specific delay-sensitive or amplitudesensitive neurons. Thus the mapping experiments probably do not reveal the
full extent of auditory cortical organization. Possibly, surface segments that
were not responsive in the mapping experiments (Supin et at. 1978) contain
neurons tuned to more specific acoustic parameters than those tested.
Detailed organization of auditory cortex described for bats may be
equally or even more complex in dolphins. If so, the dolphin cortex must