The jamming avoidance response in gymnotoid pulse-species: A mechanism to minimize the probability of pulse-train coincidence

Abstract

Gymnotoid electric fish with pulse-type electric organ discharges (EODs) shorten (lengthen) their EOD intervals as pulses of a slightly slower (faster) train scan their EODs (Figs. 1, 2). They thus minimize the chance of pulse coincidence by transient accelerations (decelerations) of their EOD rate. Studies in curarized preparations demonstrate that this Jamming Avoidance Response (JAR) is controlled by electroreceptive input alone and without reference to an internal electric organ pacemaker-related signal (Fig. 8). A sufficient stimulus input consists of a train of strong, EOD-like stimulus pulses (S1), which mimic the animal's experience of its own EOD, and a train of small pulses (S2) of slightly different repetition rate, which mimic EODs of a neighbor. Correct behavioral responses require S1 pulses of sufficient intensity to recruit pulse-markertype receptors; also spatial and temporal patterns must closely resemble those of the animal's EOD. These features are of little significance for S2 pulses which, while scanning S1 pulses, only provide a small perturbation of electroreceptive feedback from S1 pulses. Inappropriate S1 stimulation impairs and sometimes reverses (Fig. 7) the behavioral discrimination of scan directions. The JAR is explained in terms of excitatory and inhibitory processes (Fig. 3) which are triggered by S2 stimulation, at specific phases within the S1 cycle (Figs. 4–6). The JAR in pulse species strongly resembles the JAR in wave-species (Bullock et al., 1972) and could be considered an evolutionary ancestor of the latter. It is a response to a particular novelty in electroreceptive feedback.