130
2.6 The 180° Ambiguity Problem
Directional hearing in fishes is thought to be dependent on the direct stimulation of
the accelerometer-like otoliths and underlying mechanically tuned hair cells of the
inner ear by acoustic particle motion (de Vries 1950; Dijkgraaf 1960; Fay 1984).
Based on afferent nerve recordings, the saccule is known to be sensitive to low
frequency displacements as small as 0.1 nm rms, which is equivalent to the displacement produced by a 100 Hz sound wave propagating in the far field at 100 dB
re 1 μPa (e.g., Fay 1984; Fay and Edds-Walton 1997). The axis of particle motion
sensitivity of the acoustic end organ varies with the axis of optimal hair cell sensitivity
along the sensory epithelium (Popper and Fay 1993). Although the neural
mechanism(s) is not clearly understood, the fish’s brain is thought to calculate the
sound direction by vector weighing the input from different hair cell epithelia
regions. This process has been called “vector detection” (Schuijf and Buwalda
1975) and is the basis for all current models of directional hearing in fish. However,
there is a problem with the “vector detection” approach for resolving sound source
direction; namely, that the axis of sound propagation does not in itself indicate bearing to the source. For a simple acoustic disturbance, a “particle” of fluid undergoes
a small linear displacement in which its particle motion vector alternately points
towards and away from the acoustic source for equal amounts of time. This particle
motion vector does not indicate which direction a fish should travel to reach the
source because the axis of propagation will cause the hair cells to oscillate both
toward and away from the incident source. The bidirectional information conveyed
by the hair cells and their corresponding auditory afferents will be largely ambiguous, a problem long recognized as the “180° ambiguity” problem.
The 180° ambiguity problem has dominated most of the theoretical and empirical work on directional hearing in fishes since the 1970s and all new experiments on
sound source localization in fish must confront this problem. Schuijf (1975) and
Schuijf and Buwalda (1975) conceived a possible solution to this problem that
entailed processing the phase relations between sound pressure and particle motion
and their solution has become known as the “phase” model (see Sect. 3 in Models
of Directional Hearing and Sound Localization). However, there are potential problems with Schuijf’s solution because it seems to require sinusoidal signals and
because some fish seem to lack the ability to detect the sound pressure cues that are
theoretically necessary for this model to work.
2.7 Phonotaxis Experiments
In many vocal fish species, males often produce advertisement or mate calls to
attract females for courtship and spawning (Fine et al. 1977; Myrberg 1981; Bass
and McKibben 2003; Ladich and Myrberg 2006; Myrberg and Lugli 2006). The use
of these calls in playback studies has been effective in determining the species
response specificity and differential phonotaxis to such calls. Tavolga (1958)
showed that in the goby (Bathygobius soporator) both females and males will
J.A. Sisneros and P.H. Rogers
2.6 The 180° Ambiguity Problem
Directional hearing in fishes is thought to be dependent on the direct stimulation of
the accelerometer-like otoliths and underlying mechanically tuned hair cells of the
inner ear by acoustic particle motion (de Vries 1950; Dijkgraaf 1960; Fay 1984).
Based on afferent nerve recordings, the saccule is known to be sensitive to low
frequency displacements as small as 0.1 nm rms, which is equivalent to the displacement produced by a 100 Hz sound wave propagating in the far field at 100 dB
re 1 μPa (e.g., Fay 1984; Fay and Edds-Walton 1997). The axis of particle motion
sensitivity of the acoustic end organ varies with the axis of optimal hair cell sensitivity
along the sensory epithelium (Popper and Fay 1993). Although the neural
mechanism(s) is not clearly understood, the fish’s brain is thought to calculate the
sound direction by vector weighing the input from different hair cell epithelia
regions. This process has been called “vector detection” (Schuijf and Buwalda
1975) and is the basis for all current models of directional hearing in fish. However,
there is a problem with the “vector detection” approach for resolving sound source
direction; namely, that the axis of sound propagation does not in itself indicate bearing to the source. For a simple acoustic disturbance, a “particle” of fluid undergoes
a small linear displacement in which its particle motion vector alternately points
towards and away from the acoustic source for equal amounts of time. This particle
motion vector does not indicate which direction a fish should travel to reach the
source because the axis of propagation will cause the hair cells to oscillate both
toward and away from the incident source. The bidirectional information conveyed
by the hair cells and their corresponding auditory afferents will be largely ambiguous, a problem long recognized as the “180° ambiguity” problem.
The 180° ambiguity problem has dominated most of the theoretical and empirical work on directional hearing in fishes since the 1970s and all new experiments on
sound source localization in fish must confront this problem. Schuijf (1975) and
Schuijf and Buwalda (1975) conceived a possible solution to this problem that
entailed processing the phase relations between sound pressure and particle motion
and their solution has become known as the “phase” model (see Sect. 3 in Models
of Directional Hearing and Sound Localization). However, there are potential problems with Schuijf’s solution because it seems to require sinusoidal signals and
because some fish seem to lack the ability to detect the sound pressure cues that are
theoretically necessary for this model to work.
2.7 Phonotaxis Experiments
In many vocal fish species, males often produce advertisement or mate calls to
attract females for courtship and spawning (Fine et al. 1977; Myrberg 1981; Bass
and McKibben 2003; Ladich and Myrberg 2006; Myrberg and Lugli 2006). The use
of these calls in playback studies has been effective in determining the species
response specificity and differential phonotaxis to such calls. Tavolga (1958)
showed that in the goby (Bathygobius soporator) both females and males will
J.A. Sisneros and P.H. Rogers