128
1983). It was a time of experimental innovations and major advancements in the
theories of directional hearing and sound source localization in fishes. Some of the
best early behavioral evidence for directional hearing in fishes comes from a series
of experiments that were initiated by Schuijf et al. (1971), in which they carried out
psychoacoustic conditioning experiments on the Ballan wrasse (Labrus berggylta)
in a deep fjord near Bergen, Norway. In this free-field environment, fish were appetitively conditioned to respond to one of two sound sources separated in the azimuth
by two angles of 10° or 71°. For each angle, the conditioning trial consisted of
switching a train of continuous 115 Hz tone bursts (with a burst duration of 1500 ms)
from one speaker to the other speaker. The positive responses of the fish at both
source angle differences were interpreted by the authors to indicate that the fish
could detect the tone bursts switching from one speaker to the other, which was
assumed to be the result of a perceptional change in sound source direction. As the
authors pointed out, this demonstration essentially shows that fish can be conditioned to discriminate between sources of sound that were spatially separated, but it
represents a weak demonstration of sound source localization because the discrimination may have been made on the basis of cues associated with each source (e.g., a
timbre difference) and may not necessary represent effective cues for source
location.
Schuijf (1975) later demonstrated that the cod (Gadus morhua) could be conditioned to discriminate between two sound sources in the horizontal plane with an
accuracy of 22° and that two ears (each with the saccule and lagena) were required
for this discrimination. The minimum audible angle of 22° in the cod was determined by Schuijf using two- and four-alternative spatial choice experiments that
required the fish to swim toward the active sound source for a food reward. As
Schuijf pointed out, the fish could have solved this task by identifying each sound
source based on differences in timbre cues and not necessarily knowing the locations of the sound sources. Hawkins and Sand (1977) had shown earlier for the cod
the smallest discriminable change in elevation to be approximately 16° and Sand
(1974) also suggested that the two ears were required for directional hearing in the
cod based on saccular potential recordings. Taken together, these experiments are
perhaps the best evidence for directional hearing in fishes.
2.4 Direction-Dependent Masking
Direction-dependent masking has been observed in several species including haddock (Melanogrammus aeglefinus), pollock (Pollachius pollachius), ling (Molva
molva), and cod (Gadus morhua) (Chapman 1973; Chapman and Johnstone 1974;
Hawkins and Sand 1977). Chapman (1973) examined the ability of directional hearing in the haddock, pollock, and ling by investigating directional unmasking, a technique that determines tone detection thresholds in the presence of masking noise.
This technique is similar to measuring the masking level difference studied in
humans (Hirsh 1948). In his experiments, Chapman confined fish in a free-field
J.A. Sisneros and P.H. Rogers
1983). It was a time of experimental innovations and major advancements in the
theories of directional hearing and sound source localization in fishes. Some of the
best early behavioral evidence for directional hearing in fishes comes from a series
of experiments that were initiated by Schuijf et al. (1971), in which they carried out
psychoacoustic conditioning experiments on the Ballan wrasse (Labrus berggylta)
in a deep fjord near Bergen, Norway. In this free-field environment, fish were appetitively conditioned to respond to one of two sound sources separated in the azimuth
by two angles of 10° or 71°. For each angle, the conditioning trial consisted of
switching a train of continuous 115 Hz tone bursts (with a burst duration of 1500 ms)
from one speaker to the other speaker. The positive responses of the fish at both
source angle differences were interpreted by the authors to indicate that the fish
could detect the tone bursts switching from one speaker to the other, which was
assumed to be the result of a perceptional change in sound source direction. As the
authors pointed out, this demonstration essentially shows that fish can be conditioned to discriminate between sources of sound that were spatially separated, but it
represents a weak demonstration of sound source localization because the discrimination may have been made on the basis of cues associated with each source (e.g., a
timbre difference) and may not necessary represent effective cues for source
location.
Schuijf (1975) later demonstrated that the cod (Gadus morhua) could be conditioned to discriminate between two sound sources in the horizontal plane with an
accuracy of 22° and that two ears (each with the saccule and lagena) were required
for this discrimination. The minimum audible angle of 22° in the cod was determined by Schuijf using two- and four-alternative spatial choice experiments that
required the fish to swim toward the active sound source for a food reward. As
Schuijf pointed out, the fish could have solved this task by identifying each sound
source based on differences in timbre cues and not necessarily knowing the locations of the sound sources. Hawkins and Sand (1977) had shown earlier for the cod
the smallest discriminable change in elevation to be approximately 16° and Sand
(1974) also suggested that the two ears were required for directional hearing in the
cod based on saccular potential recordings. Taken together, these experiments are
perhaps the best evidence for directional hearing in fishes.
2.4 Direction-Dependent Masking
Direction-dependent masking has been observed in several species including haddock (Melanogrammus aeglefinus), pollock (Pollachius pollachius), ling (Molva
molva), and cod (Gadus morhua) (Chapman 1973; Chapman and Johnstone 1974;
Hawkins and Sand 1977). Chapman (1973) examined the ability of directional hearing in the haddock, pollock, and ling by investigating directional unmasking, a technique that determines tone detection thresholds in the presence of masking noise.
This technique is similar to measuring the masking level difference studied in
humans (Hirsh 1948). In his experiments, Chapman confined fish in a free-field
J.A. Sisneros and P.H. Rogers