Eigenmannia lineata is a tropical South American gymnotiform fish that is both electrogenic and electroreceptive. Its electric organ generates a constant-frequency wave discharge (EOD) that is sexually dimorphic in waveform and harmonic content. Eigenmannia discriminates digitally synthesized, natural male and female EOD waveforms of identical frequency and amplitude. Experiments were devised to investigate behaviourally the sensory mechanism of this discrimination.

Both the content in higher harmonics (as seen in an amplitude spectrum) and the waveform (phase spectrum) could provide the cues Eigenmannia uses for discrimination. Five Eigenmannia were trained to discriminate artificially generated stimuli composed of the fundamental frequency, f1, and its harmonic, f2, of a frequency twice that of f1 and of weaker intensity (−13 to −3dB re f1; a similar variation is found among the EODs of different Eigenmannia individuals). The rewarded stimulus, S+ (with a strong f2 intensity of −3dB re f1), remained constant throughout the experiments. The family of negative (S-) stimuli all had a phase difference of 0°, that is, no phase difference, between their harmonics, while the S+ stimulus had a phase difference of 90°. Therefore, all S- stimuli differed from the S+ stimulus in waveform, while one S- stimulus had an amplitude spectrum identical to that of the S+ stimulus.

All fish discriminated any S- signal tested from the S+ stimulus, including the S- stimulus with an identical amplitude spectrum, both when the test signals had identical energy contents (that is, slightly different peak-to-peak amplitudes) and when they had identical peak-to-peak amplitudes (hence, slightly different energy contents).

These results arc evidence for a true waveform (time domain) sensitivity of Eigenmannia. A sensory mechanism is proposed that enables Eigenmannia to assess the P/N ratio of a wave signal, that is, the duration ratio of positive (P) and negative (N) half-waves between zero-crossings of a stimulus cycle (the S+ and all S- stimuli, as well as female and male EODs, differ in this regard).

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