126
The horn was activated for 2 min and then cut pieces of earthworms were then
dropped onto the feeding station next to the horn. The trials were then repeated
again approximately every 10 min. After 55 trials over several days, the researchers
failed to observe any fish moving toward the feeding station that was paired with the
sound stimulus even though many of the fish clearly demonstrated a conditioned
arousal response to the acoustic stimulus. Based on their results, von Frisch and
Dijkgraaf concluded that these fish could not localize underwater sound sources and
they postulated that fish in general were incapable of sound source localization due
to two major reasons. First, von Frisch and Dijkgraaf contended that the dominant
view of sound source localization in humans was that the determination of extremely
small ITDs (e.g., on the order of several microseconds) was required for source
localization in the azimuth. Such processing of minute ITDs seemed “hardly imaginable” in fish according to von Frisch and Dijkgraaf (1935) because the inner ear
acoustic end organs are very close together separated by only millimeters in fish
compared to an ear separation greater than 10–15 cm in humans. Furthermore,
because sound travels nearly five times faster underwater than in air the ITDs for
fish underwater would be almost infinitesimal. Second, von Frisch and Dijkgraaf
also pointed out that the European minnow was a pressure-sensitive otophysan fish
that detects sound pressure indirectly via the swim bladder, which vibrates (oscillates) in response to a sound pressure stimulus. The pressure-induced vibrations of
the swim bladder would stimulate both left and right acoustic endorgans equally via
the inner ear projections of the Weberian ossicles, which are a series of specialized
bones that are linked to the swim bladder. The simultaneous stimulation of both ears
by the Weberian ossicles would occur equally regardless of sound source direction.
Thus, von Frisch and Dijkgraaf were left to conclude that fish were unable to detect
sound direction and locate sound sources even though they knew that their conclusions might not satisfy biologists.
2.2 Re-evaluation of Directional Hearing and Source
Localization
The initial negative results of the sound source localization experiments by von
Frisch and Dijkgraaf (1935) came to dominate the expectations that fish were
unable to locate underwater sound sources. However, questions regarding the ability of fish to discriminate sound direction and locate sound sources arose again with
van Bergeijk’s (1964, 1967) influential analysis of directional hearing in fishes. As
did von Frisch and Dijkgraaf (1935), van Bergeijk maintained that fish were unable
to resolve sound direction in the far field due to the fact the fish’s swim bladder acts
as a pressure-to-displacement transformer and therefore in capable of discriminating sound direction. Thus, fish must use some other sensory system to guide directional hearing and sound source localization behavior. Earlier Harris and van
Bergeijk (1962) showed that the mechanosensory lateral line organs of the killifish
(Fundulus heteroclitus) responded in proportion to the near field particle motion
J.A. Sisneros and P.H. Rogers
The horn was activated for 2 min and then cut pieces of earthworms were then
dropped onto the feeding station next to the horn. The trials were then repeated
again approximately every 10 min. After 55 trials over several days, the researchers
failed to observe any fish moving toward the feeding station that was paired with the
sound stimulus even though many of the fish clearly demonstrated a conditioned
arousal response to the acoustic stimulus. Based on their results, von Frisch and
Dijkgraaf concluded that these fish could not localize underwater sound sources and
they postulated that fish in general were incapable of sound source localization due
to two major reasons. First, von Frisch and Dijkgraaf contended that the dominant
view of sound source localization in humans was that the determination of extremely
small ITDs (e.g., on the order of several microseconds) was required for source
localization in the azimuth. Such processing of minute ITDs seemed “hardly imaginable” in fish according to von Frisch and Dijkgraaf (1935) because the inner ear
acoustic end organs are very close together separated by only millimeters in fish
compared to an ear separation greater than 10–15 cm in humans. Furthermore,
because sound travels nearly five times faster underwater than in air the ITDs for
fish underwater would be almost infinitesimal. Second, von Frisch and Dijkgraaf
also pointed out that the European minnow was a pressure-sensitive otophysan fish
that detects sound pressure indirectly via the swim bladder, which vibrates (oscillates) in response to a sound pressure stimulus. The pressure-induced vibrations of
the swim bladder would stimulate both left and right acoustic endorgans equally via
the inner ear projections of the Weberian ossicles, which are a series of specialized
bones that are linked to the swim bladder. The simultaneous stimulation of both ears
by the Weberian ossicles would occur equally regardless of sound source direction.
Thus, von Frisch and Dijkgraaf were left to conclude that fish were unable to detect
sound direction and locate sound sources even though they knew that their conclusions might not satisfy biologists.
2.2 Re-evaluation of Directional Hearing and Source
Localization
The initial negative results of the sound source localization experiments by von
Frisch and Dijkgraaf (1935) came to dominate the expectations that fish were
unable to locate underwater sound sources. However, questions regarding the ability of fish to discriminate sound direction and locate sound sources arose again with
van Bergeijk’s (1964, 1967) influential analysis of directional hearing in fishes. As
did von Frisch and Dijkgraaf (1935), van Bergeijk maintained that fish were unable
to resolve sound direction in the far field due to the fact the fish’s swim bladder acts
as a pressure-to-displacement transformer and therefore in capable of discriminating sound direction. Thus, fish must use some other sensory system to guide directional hearing and sound source localization behavior. Earlier Harris and van
Bergeijk (1962) showed that the mechanosensory lateral line organs of the killifish
(Fundulus heteroclitus) responded in proportion to the near field particle motion
J.A. Sisneros and P.H. Rogers