Summary
Undisturbed specimens of Gymnotus carapo discharge their electric organs at a rate of approximately 50 Hz, with a standard deviation of 3 Hz; stimuli of several sensory modalities (touch, vibration, light, electric fields) are capable of eliciting transient increases in discharge frequency rising to a maximum near 30 Hz above resting frequency and returning exponentially over several seconds. Such frequency transients may be evoked by sudden increases or decreases in the electrical resistance of the environment caused by shunting recording electrodes on either end of the fish. Threshold responses are seen to resistance changes of 10 to 20 ohms, which create changes of approximately 50 μV/cm in the amplitude of the electric discharge measured along the length of the fish.
Gymnotus proves remarkably sensitive to electrical stimuli occuring in phase with the discharge train of its own electric organs, with a threshold near 50 μV/cm; out-of-phase stimuli ten times this amplitude fail to elicit frequency transients. Responses are also seen to small changes in latency between stimulus and discharge trains within the time span of an individual organ discharge.
Phase-sensitivity is apparently a consequence of the sigmoid stimulus/response curve common to most receptors; a large “activating” pulse, normally generated by the electric organs, raises electroreceptor activity into the receptor's dynamic range, after which a relatively small stimulus is capable of causing a significant change in output. Such a mechanism probably serves as a peripheral filter, eliminating low-level interference from other electric fish. The frequency transients themselves are interpreted as arousal responses, momentarily increasing the fish's sensory resolution in the face of potential hazards or prey.
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Supported by NIH Grant NS 05423, NTH Biomedical Sciences Support Grant 5 S 05 FB-07091-04, and NIH Training Grant 2 T 01 GM-00836-07.
The authors particularly wish to thank Mr. Gary F. Shelton, whose skill as an aquarist kept our specimens in good health, and Dr. Selden L. Stewart, who introduced one of us (JAM) to the techniques and philosophy of modern data analysis. We also wish to acknowledge the comments and suggestions of Drs. T. H. Bullock, S. Hagiwara and D. Junge, who read an early version of the manuscript.
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Macdonald, J.A., Larimer, J.L. Phase-sensitivity of Gymnotus carapo to low-amplitude electrical stimuli. Z. Vergl. Physiol. 70, 322–334 (1970). https://doi.org/10.1007/BF00297752
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DOI: https://doi.org/10.1007/BF00297752