Perception of Amplitude-Modulated Signals in the Goldfish

Abstract

Judgments of stimulus similarity between AM signals and other simple and complex tones were obtained from goldfish using a stimulus generalization paradigm in combination with a classical respiratory conditioning technique. Several groups of fish were conditioned in inhibit respiration upon the presentation of an AM signal (400 Hz modulated at 100% by 40 Hz), and subsequently tested for generalization to other signals differing in degree of complexity, carrier frequency (fc), and modulating frequency (fm). All stimuli were presented at approximately 30 dB above threshold. Obvious generalization decrements occurred to test signals differing in fc while no decrements occurred to test signals differing only in fm. For subjects trained to AM signals and tested with pure tones, little generalization occurred to any test frequency in the range including fc and fm. Thus, while there is a great perceptual difference between an AM signal and a pure tone at fc, the major sensation evoked in fish by these AM signals is determined primarily by fc, and not by fm. The results suggest that for the fish, whose ear is assumed to analyze primarily in the time domain, envelope periodicity does not generally determine the quality of an AM signal.