346
P.E. Nachtigall et al.
3. Auditory Localization
The ability to echolocate and capture rapidly swimming fish demands a fine
ability to localize sound direction. The localization of sound in other
mammals is greatly enhanced by a binaural auditory system. While dolphins
and small whales do not have external pinnae, they do have a binaural
system with at least two separate sound pathways to the individual ears
(Norris 1968; Brill et al. 1988; Ketten 1997; Ketten, Chapter 2). The availability of the binaural system allows at least three potential cues for auditory localization: (1) the difference in the time of arrival at the two ears, or
interaural time difference; (2) the difference in the intensity of the sound
at the two ears, or interaural intensity or level difference; and (3) the difference in the phase of the signal as it arrives at the two ears. There is also
some possibility of a frequency difference between the two ears, but this
cue is highly speculative.
3.1 Sound Source Localization
Only a few studies have directly investigated dolphin abilities to localize a
sound source. These studies have been concerned with identifying the
minimum audible angle that can be discriminated. The minimum audible
angle is the angle at the subject between two equidistant sound sources
whose position can just be discriminated. Typically, one of these sound
sources is located directly in front of the animal and the other is moved to
various angles relative to the line between the first sound source and the
animal. The minimum angle when the animal can just discriminate an angularly displaced sound source from the one directly in front is the minimum
audible angle. For comparison purposes, a human's minimum audible angle
is typically about 1° (Mills 1958).
Two studies with harbor porpoise (P. phocoena) found minimum audible
angles of between 3° and 8°. One of these studies (Andersen 1970) was
based on what the author calls a "rather primitive experimental set up"
(p. 262). The porpoise was 50% correct at an angle of 0°6' and 75% correct
at an angle of about 3°. This angle was determined under relatively uncontrolled conditions where the porpoise's location was free to vary as the
animal swam through hoops toward the sound source.
The other experiment with P. phocoena (Dudok van Heel 1962) was not
much better controlled. The porpoise's position at the time the signal
was presented was controlled by having the animal swim through an
arrangement of nets. The sound was played when the porpoise was judged
to be at the starting point. The dolphin then had to swim toward the one of
two hydrophones from which a 6-kHz tone was played. Another net separated the two hydrophones, which allowed the animal's response to be identified clearly. The minimum audible angle in this experiment was found to
be 8°.
P.E. Nachtigall et al.
3. Auditory Localization
The ability to echolocate and capture rapidly swimming fish demands a fine
ability to localize sound direction. The localization of sound in other
mammals is greatly enhanced by a binaural auditory system. While dolphins
and small whales do not have external pinnae, they do have a binaural
system with at least two separate sound pathways to the individual ears
(Norris 1968; Brill et al. 1988; Ketten 1997; Ketten, Chapter 2). The availability of the binaural system allows at least three potential cues for auditory localization: (1) the difference in the time of arrival at the two ears, or
interaural time difference; (2) the difference in the intensity of the sound
at the two ears, or interaural intensity or level difference; and (3) the difference in the phase of the signal as it arrives at the two ears. There is also
some possibility of a frequency difference between the two ears, but this
cue is highly speculative.
3.1 Sound Source Localization
Only a few studies have directly investigated dolphin abilities to localize a
sound source. These studies have been concerned with identifying the
minimum audible angle that can be discriminated. The minimum audible
angle is the angle at the subject between two equidistant sound sources
whose position can just be discriminated. Typically, one of these sound
sources is located directly in front of the animal and the other is moved to
various angles relative to the line between the first sound source and the
animal. The minimum angle when the animal can just discriminate an angularly displaced sound source from the one directly in front is the minimum
audible angle. For comparison purposes, a human's minimum audible angle
is typically about 1° (Mills 1958).
Two studies with harbor porpoise (P. phocoena) found minimum audible
angles of between 3° and 8°. One of these studies (Andersen 1970) was
based on what the author calls a "rather primitive experimental set up"
(p. 262). The porpoise was 50% correct at an angle of 0°6' and 75% correct
at an angle of about 3°. This angle was determined under relatively uncontrolled conditions where the porpoise's location was free to vary as the
animal swam through hoops toward the sound source.
The other experiment with P. phocoena (Dudok van Heel 1962) was not
much better controlled. The porpoise's position at the time the signal
was presented was controlled by having the animal swim through an
arrangement of nets. The sound was played when the porpoise was judged
to be at the starting point. The dolphin then had to swim toward the one of
two hydrophones from which a 6-kHz tone was played. Another net separated the two hydrophones, which allowed the animal's response to be identified clearly. The minimum audible angle in this experiment was found to
be 8°.