Perception of two-tone complexes by the goldfish (Carassius auratus)

Previous experiments on the sense of hearing in goldfish have used a stimulus generalization paradigm to investigate the perceptual dimensions evoked by spectrally and temporally complex sounds. The present experiments investigated the effects on perception of the frequency separation between two tones. In the first set of experiments, six groups of goldfish were classically conditioned to a single tone and then tested for generalization to two-tone complexes having one frequency component equal to the conditioning tone, and the other differing by 2-256 Hz. Generalization declined with increasing frequency differences up to about 32 Hz, and then increased for wider frequency separations. These functions indicate that a restricted range of beat rates produces a perceptual quality that is quite unlike that of a single tone. The generalization function of frequency separation resembles the inverse of the 'fluctuation strength' and 'roughness' functions for human listeners. The second experiment investigated the effects of spectral location on the perception of a 32 Hz beat rate. Goldfish were conditioned to a two-tone complex (500 and 532 Hz) and then tested for generalization to single tones at various frequencies between 200 and 1200 Hz, and to two-tone complexes having a 32 Hz beat rate but with the lower tone component at various frequencies. For single-tone stimuli, generalization was relatively weak but showed a peak at 500 Hz. For the two-tone stimuli, generalization was more robust, but showed a similarly shaped gradient centered on 500 Hz. Thus, goldfish behaved as if they had acquired information about both temporal modulation and the frequency location of the tone components. These perceptual behaviors appear to be shared with humans and other vertebrates.