The Change of Discharge Frequency by A.c. Stimulus in a Weak Electric Fish

A South American gymnotid, Eigenmannia, is known to be a kind of weak electric fish, producing continuously the electric organ discharge with an intensity of about 1 V. and a constant frequency of about 300 cyc./sec. at 25 C. (Coates, Altamirano & Grundfest, 1954; Lissmann, 1958). The frequency is a function of temperature, with a Q10 of about 1*5. At a fixed temperature, however, the discharge frequency is extremely constant. Furthermore, the constancy is not affected by several kinds of artificial stimuli, including the ordinary electrical stimulus (cf. Grundfest, 1957). On the other hand the animal is apparently highly sensitive to the change of the electric field surrounding it; it escapes from a metallic conductor but is indifferent when the conductor is insulated (Grundfest, 1957). The exact measurement of the threshold has, however, not yet been made. This paper is concerned with a peculiar response of the fish to an externally applied electrical current. The effective stimulus was an alternating current with a frequency very close to the discharge frequency of the fish, to which the latter changed slightly but significantly. It will be shown that the threshold of this response is very low (around 10 ^.V./cm. in some fish), suggesting that some 'electric receptor' receiving the stimulus signal is at work.