BEHAVIORAL DETECTION OF ELECTRIC SIGNAL WAVE-FORM DISTORTION IN THE WEAKLY ELECTRIC FISH, GNATHONEMUS-PETERSII

The ''novelty response'' of weakly electric mormyrids is a transient a cceleration of the rate of electric organ discharges (EOD) elicited by a change in stimulus input. In this study, we used it as a tool to te st whether Gnathonemus petersii can perceive minute waveform distortio ns of its EOD that are caused by capacitive objects, as would occur du ring electrolocation. Four predictions of a hypothesis concerning the mechanism of capacitance detection were tested and confirmed: (1) G. p etersii exhibited a strong novelty response to computer-generated (syn thetic) electric stimuli that mimic both the waveform and frequency sh ifts of the EOD caused by natural capacitive objects (Fig. 3). (2) Sim ilar responses were elicited by synthetic stimuli in which only the wa veform distortion due to phase shifting the EOD frequency components w as present (Fig. 4). (3) Novelty responses could reliably be evoked by a constant amplitude phase shifted EOD that effects the entire body o f the fish evenly, i.e., a phase difference across the body surface wa s lacking (Figs. 3, 4). (4) Local presentation of a phase-shifted EOD mimic that stimulated only a small number of electroreceptor organs at a single location was also effective in eliciting a behavioral respon se (Fig. 5). Our results indicate that waveform distortions due to pha se shifts alone, i.e. independent of amplitude or frequency cues, are sufficient for the detection of capacitive, animate objects. Mormyrids perceive even minute waveform changes of their own EODs by centrally comparing the input of the two types of receptor cells within a single mormyromast electroreceptor organ. Thus, no comparison of differentia lly affected body regions is necessary. This shows that G. petersii in deed uses a unique mechanism for signal analysis, which is different f rom the one employed by gymnotiform wavefish.