Physiological and psychophysical studies of amplitude modulation detection by the goldfish

Abstract

Goldfish were classically conditioned to suppress respiration during a brief period of sinusoidal amplitude modulation (SAM) impressed upon a continuous 800 Hz, 35 dB SL carrier signal. The just-detectable depth of modulation (m) was measured as a function of modulation frequency (fm) between 2.5 and 400 Hz, using a modified method of limits. The sensitivity of modulation detection grows with frequency at about 3 dB/octave up to about 200 Hz where fishes could detect modulation at m = 0.004. This corresponds to a peak-to-trough intensity difference of about 0.07 dB. Similar experiments were performed on single saccular neurons of the goldfish. Modulation period histograms were obtained, and the degree of phase locking and the phase angle of the response were measured as functions of m and fm. Neurons generally showed bandpass frequency response characteristics with best modulation frequencies occurring between 20–300 Hz, and at sensitivities (based on phase-locking criteria) comparable to those measured behaviorally. Each neuron exhibited a characteristic delay between corresponding points on the stimulus envelope and the period histogram. This delay ranges between 2.5–10 ms, and is inversely proportional to the neuron's best modulalion frequency. The extremely high behavioral sensitivity to amplitude modulation appears to depend upon mechanisms leading to temporal contour enhancement in the neural response, and not to spectral processing. [Work supported by the NSF.]