Variations in the frequency response of electroreceptors dependent on receptor location in weakly electric fish (Gymnotoidei) with a pulse discharge

Abstract

Variations in electric organ discharge (EOD) pulse shape and variations in electroreceptor frequency response characteristics at different locations along the body were studied in two unidentified species of Gymnotid fish (Hypopomus). The electric organ discharges of the two species, recorded in a head to tail fashion, were quite different. That of one species was biphasic and of shorter duration (< 1 ms) than the monophasic pulse (2 ms duration) of the other. The waveforms of the “local” electric organ discharges (estimated changes in transepidermal voltage due to an EOD) were found to vary in shape and spectral composition along the length of the fish having a biphasic EOD (Fig. 2). The local EOD's of the other species did not show these large variations as judged by the similarity of the amplitude spectra of these local EOD's (Fig. 3). One major class of electroreceptor, burst duration coder, from the species with the biphasic EOD had frequency response characteristics that also varied as a function of the receptor's location on the body, and the nature of these variations suggests that these receptors are specialized to respond optimally to the waveform of the EOD peculiar to that location. Burst duration coders of the second species did not show this sort of variation in frequency response characteristics in accord with the lack of variation of the local EOD's of this species. Purely resistive objects are shown to cause simultaneous increases in some frequency components along with attenuation of other frequency components of local EOD's from certain regions of the body in the species with the biphasic EOD. The variety of receptor types present in these same regions of the body suggests that the animals may be able to utilize information about the electrical environment coded as changes in selected spectral components of the EOD as well as that coded as changes in the over-all amplitude of the EOD.