ELECTRORECEPTORS OF A WEAKLY ELECTRIC FISH .1. CHARACTERIZATION OF TUBEROUS RECEPTOR ORGAN TUNING

Stimulus filtering by tuberous electroreceptors of the weakly electric fish Eigenmannia virescens was investigated at the level of the afferent discharge by employing a computer to set the stimulus parameters and to assess the minimum threshold response to sinusoidal electric fields. With the fishes' electric organ discharge blocked pharmacologically, all tuberous receptors were found to be sharply tuned to sinusoidal stimulus frequencies within the species' range of electric organ discharge frequency. On the average, the frequency vs threshold curves had lowest thresholds of 0.5 mV/cm (ca. 100-200 μV transepithelial voltage), cutoff slopes of -41 dB/decade and 55 dB/decade for low and high frequencies respectively, and a q10db of 0.9. Receptors in individual fish were tuned approximately to the individual's own discharge frequency, but the average best frequency was generally less than the electric organ discharge frequency, suggesting that the receptor filter may be matched to the frequency components of single electric organ discharges rather than the waveform produced by temporal summation of successive discharges. When stimulated with frequency modulated sinewaves the afferent response is independent of the direction of frequency sweep. Many receptors showed increased responsiveness to twice the best frequency when FM rather than pure tones were presented. The frequency threshold curves were evaluated in terms of best-frequency threshold, high and low frequency cutoff slopes, and frequency selectivity. Each of these parameters was distributed approximately as would be expected for a single class of receptors. Pairwise analysis of tuning curve characteristics did not suggest receptor subclasses. Receptor subclasses were not detected even when stimulus-response criterion previously reported to discriminate two subclasses were applied. © 1979 Springer-Verlag.