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acoustic testing range in midwater about 21 m deep and cardiac conditioned them to
detect pure tone signals in the presence of masking noise in the horizontal plane.
Chapman found that masking thresholds were highest when the angular separation
of the stimulus tone and noise sources were within 10° of one another in the azimuth, but a significant release of masking (7.7 dB) occurred when the signal and
noise sources were separated by 85° or more.
Directional unmasking was reinvestigated for the haddock and cod by Chapman
and Johnstone (1974) using the same testing range, location and protocol used by
Chapman (1973). In these experiments, Chapman and Johnstone (1974) observed a
significant release of masking (6.4–7.7 dB) for haddock and cod when the signal
and noise source separation was 45° or more. Hawkins and Sand (1977) later demonstrated similar directional unmasking results for the cod in the median vertical
plane. Chapman and Johnstone (1974) also found that haddock and cod could discriminate a change in the direction of a pulsed tone switched between two source
locations when the angular separations in the azimuth were 20° or more. In sum,
these experiments imply that fish are capable of directional hearing in a free field
and that they utilize spatial filtering for signal detection in noise.
2.5 Sound Source Distance Discrimination
Schuijf and Hawkins (1983) investigated the capacity for sound source distance
discrimination in the cod using classical cardiac conditioning. Cod were conditioned to discriminate between two sound sources at two distances with 0° azimuth
and 0° elevation. Evidence suggests that the fish were able to resolve the source
distance using the distance-dependent phase angle between sound pressure and particle motion within the near-field. The authors suggested that source distance discrimination could also be determined by simultaneously comparing the amplitude
ratios between sound pressure and particle motion. In addition, the authors also
suggested that characteristic patterns of amplitude modulation between these two
acoustic components may be generated at different distances as the result of the
reflection of sound by the surface and seafloor, which could provide important distance cues far from a source. The observations by Schuijf and Hawkins (1983) are
consistent with the hypothesis that fish are well able to determine the direction and
distance of low-frequency sound sources, but these experiments along with all the
other previous psychophysical-type experiments on source discrimination did not
definitively demonstrate sound source localization because these experiments did
not actually show that fish could locate sound sources in three-dimensional space.
Directional Hearing and Sound Source Localization in Fishes
acoustic testing range in midwater about 21 m deep and cardiac conditioned them to
detect pure tone signals in the presence of masking noise in the horizontal plane.
Chapman found that masking thresholds were highest when the angular separation
of the stimulus tone and noise sources were within 10° of one another in the azimuth, but a significant release of masking (7.7 dB) occurred when the signal and
noise sources were separated by 85° or more.
Directional unmasking was reinvestigated for the haddock and cod by Chapman
and Johnstone (1974) using the same testing range, location and protocol used by
Chapman (1973). In these experiments, Chapman and Johnstone (1974) observed a
significant release of masking (6.4–7.7 dB) for haddock and cod when the signal
and noise source separation was 45° or more. Hawkins and Sand (1977) later demonstrated similar directional unmasking results for the cod in the median vertical
plane. Chapman and Johnstone (1974) also found that haddock and cod could discriminate a change in the direction of a pulsed tone switched between two source
locations when the angular separations in the azimuth were 20° or more. In sum,
these experiments imply that fish are capable of directional hearing in a free field
and that they utilize spatial filtering for signal detection in noise.
2.5 Sound Source Distance Discrimination
Schuijf and Hawkins (1983) investigated the capacity for sound source distance
discrimination in the cod using classical cardiac conditioning. Cod were conditioned to discriminate between two sound sources at two distances with 0° azimuth
and 0° elevation. Evidence suggests that the fish were able to resolve the source
distance using the distance-dependent phase angle between sound pressure and particle motion within the near-field. The authors suggested that source distance discrimination could also be determined by simultaneously comparing the amplitude
ratios between sound pressure and particle motion. In addition, the authors also
suggested that characteristic patterns of amplitude modulation between these two
acoustic components may be generated at different distances as the result of the
reflection of sound by the surface and seafloor, which could provide important distance cues far from a source. The observations by Schuijf and Hawkins (1983) are
consistent with the hypothesis that fish are well able to determine the direction and
distance of low-frequency sound sources, but these experiments along with all the
other previous psychophysical-type experiments on source discrimination did not
definitively demonstrate sound source localization because these experiments did
not actually show that fish could locate sound sources in three-dimensional space.
Directional Hearing and Sound Source Localization in Fishes