Perception of pitch by goldfish.

Classical conditioning and stimulus generalization methods have revealed much about the sense of hearing in non-human animals, and are now used here to investigate how goldfish perceive a variety of complex sounds, including multi-harmonic complexes and rippled noise (RN). In several experiments, animals were conditioned to respond to one type of complex sound, and were then tested for generalization to other sounds differing along one or more acoustic dimensions from the conditioning sounds. Overall, generalization occurred only to the extent that the conditioning and test sounds were essentially similar in spectral range and, in most cases, waveform periodicity. For example, goldfish showed inverted V-shaped generalization gradients to harmonic complexes varying in fundamental frequency after conditioning to complexes having a fundamental frequency of 100 Hz. In several cases, similar gradients were observed whether the fundamental frequency component was present or absent in conditioning and testing complexes, indicating that goldfish, like other vertebrate listeners, do not "miss the fundamental" when it is missing. This generalization pattern tended to disappear when harmonic complexes were used that had random phase relations among the components, or slight mistuning of all components. In a few cases, patterns of generalization were determined by as yet unidentified acoustic features. Goldfish did not generalize to RN or harmonic complexes after conditioning to tones, and vice versa, in spite of the three signal types having fundamental frequency components and periodicity in common. Moreover, goldfish did not generalize robustly to infinitely iterated rippled noise after conditioning to harmonic complexes with a prominent periodic envelope, and vice versa, in spite of the two signal types having similar spectra and pitches as judged by human listeners. These and other results suggest that the pitch of harmonic complexes is prominent in goldfish generalization behavior and that this pitch-like dimension arises primarily from the signal's periodicity. The perceptions of single tones, RNs, and harmonic complexes having the same fundamental frequency are fundamentally different. It is concluded that the different perceptions of these signals arise in part from differences in periodic envelope prominence and spectral envelope, and possibly in the stochastic versus deterministic natures of their respective waveforms.