The electric organ discharge (EOD) of most species of
the freshwater knifefishes (Gymnotiformes) of South
America is of the wave, not the pulse, type. Wave EODs are
usually of constant frequency and amplitude, and show a
bewildering multitude of species-characteristic waveforms.
The EOD of Eigenmannia is sexually dimorphic in
waveform and in the intensity of its higher harmonics. In
a go/no go paradigm, trained food-rewarded fish
discriminated between these waveforms, and naive
(untrained) fish showed a significant preference. To
determine whether spectral or waveform (time) cues are
used by the fish, artificial stimuli of identical amplitude
spectrum were synthesized that differed only in phase
relationship between their harmonics, i.e. waveform, and
the fish discriminated even among these stimulus
waveforms (i.e. spectral cues are not required). Our
sensory model predicts that, for successful waveform
detection, a minimum frequency difference is required
between the stimulus and the EOD. As expected, trained
fish confused test stimuli of different waveform that were
frequency-clamped and phase-locked to the EOD
(frequency difference 0 Hz). Opening the electronic
feedback loop immediately restored discrimination
performance on an on/off basis, and a strong jamming
avoidance response (JAR; a frequency shift away from the
stimulus) accompanied every behavioural decision (to go
for a food reward). The strong habituation of the JAR that
occurs in response to stimuli of no behavioural
consequence (the usual test situation) was not seen in the
present experiments. The proposed sensory model (which
is based on time-marking T electroreceptors) is supported
by these experiments, and a biological function for the JAR
– subserving EOD waveform discrimination is shown to be
useful in a social context.