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Journal of Comparative Physiology A

1. 1.

   The spectral properties of the normal electric organ discharge (EOD) ofApteronotus recorded from several different regions of the body are described and related to the known filter properties of the electroreceptors. Amplitude modulations (AMs) of these local EOD waveforms caused by various highly conducting (metal) and non-conducting (plastic) objects are also described. Object size, shape, velocity relative to the animal and object distance from the animal all influence the timecourse as well as the size of these amplitude modulations. Increases in object size result in a longer duration AM whose spectral composition is limited to lower frequencies. A measure of the width of the amplitude spectra of object-caused AMs changes with the −0.49 power of object diameter when metal spheres moving at a given distance lateral to the fish are used as stimuli (Fig. 2A). Increases in object diameter also increase the amplitude of the AM recorded across the skin of the animal. The change in EOD amplitude (μV peak-to-peak) increases with the 1.6 power of sphere diameter (Fig. 10 A). Velocity of the object can have no effect upon the amplitude of the AM but measures of the bandwidth of the AM increases linearly with velocity as is shown in Fig. 1 D for a 6.4 mm metal sphere. Increases in distance between the moving object and the side of the fish cause only minor changes in the bandwidth of the AM, bandwidth decreases as the distance increases (Fig. 2B), however, amplitude of the AM decreases very rapidly with increasing distance (Fig. 8 A and C).

2. 2.

   Receptor responses to a series of electronically produced AMs of the EOD of various amplitudes and frequencies were studied. Knowledge of these receptor properties taken together with the information about the types of AM produced by various moving objects allows predictions to be made of the manner in which variations in object size and rate of movement will influence receptor responses. The relationships between changes in receptor activity and increased EOD amplitude are well fit by power functions having slopes ranging from 0.65 to 0.87 (Fig. 7). The receptors also show different degrees of respon-siveness dependent on the frequency of AM used. The most effective frequency was in the neighborhood of 64 Hz. Single cycles of sinusoidal AM gave similar responses as continuous sinusoidal AM.

3. 3.

   The responses of receptors to various sized objects moving at sequentially greater distances lateral to the fish are described and these are compared with the changes in voltage across the skin caused by the same stimulus (Fig. 8). The receptive fields, or areas through which a given sized object can move and still influence a single receptor's firing frequency, were mapped with a small metal sphere. Changes in firing of at least 40% of the peak change in action potential frequency caused by the object can be evoked when the object is within an area measuring about 61 mm². Changes in frequency above a smaller percentage can be evoked from larger areas and larger changes from smaller areas (Fig. 11B).

4. 4.

   A numerical method for determining the receptive field size, the area of skin that will receive a change in EOD amplitude greater or equal to a specified amount, due to spherical metal objects of various sizes at a given distance lateral to the fish is described in conjunction with Fig. 11 A. These predicted receptive field sizes are compared with actual measurements of receptive field sizes mapped with a 6.4 mm diameter metal sphere.

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AM :

amplitude modulation

ARH :

average response histogram

EOD :

electric organ discharge

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1. Department of Zoology, University of Oklahoma, 73019, Norman, Oklahoma, USA

   Joseph Bastian

I am grateful to Drs. J. Matsubara, W. Heiligenberg, C. Hopkins, L. Maler, T.H. Bullock, and C. Bell for many useful suggestions concerning this work. I also thank P. Barton and J. Sims for help with the manuscript. Supported by NIH Grant RO1NS-12337.

Bastian, J. Electrolocation. J. Comp. Physiol. 144, 465–479 (1981). https://doi.org/10.1007/BF01326832

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• Accepted: 15 July 1981

• Issue Date: December 1981

• DOI: https://doi.org/10.1007/BF01326832

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