Tone-on-tone masking in the goldfish: The effects of signal phase for signal and masker at 500 Hz

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Abstract
Thresholds for detecting 20-ms, 500-Hz tone bursts (7-ms rise/fall times) in the presence of a 500-Hz masker were measured in goldfish using classical respiratory conditioning with a tracking psychophysical procedure. Thresholds were obtained using a continuous masker, and with the signal temporally centered in a 200-ms gated masker. Thresholds were determined as a function of the phase angle at which the signal was added to the masker. For the continuous masker, the just-detectable sound-pressure increment (at 0-deg phase angle) was 0.15 dB, and the just-detectable decrement (at 180 deg) was −0.22 dB. For the gated masker, the thresholds were 1.75 and −2.4 dB, for the in-phase and phase-inverted signals, respectively. Varying signal phase between 0 and 90 deg resulted in a progressively declining increment threshold, reaching about 0.5 dB at 90 deg. Since the threshold increment is not constant as a function of signal phase, some other cue(s) must underlie detection. Spectral “splatter” is not likely the cue since the increment threshold varied little when signal rise time was reduced to zero, or when the signal was bandpass filtered (96 dB/oct) at 500 Hz. This suggests that goldfish directly detect a transient phase shift occurring in the masker waveform when the out-of-phase signal is added. [Work supported by NINCDS Program Project Grant.]

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