Electrosensory phase sensitivity in the weakly electric fish Eigenmannia in the detection of signals similar to its own

Abstract

The electric organ discharge (EOD) of the
South American knifefish Eigenmannia sp. is a permanently
present wave signal of usually constant amplitude
and frequency (similar to a sine wave). A fish
perceives discharges of other fish as a modulation of its
own. At frequency identity (AF = 0 Hz) the phase difference
between a fish's own electric discharge and that of another fish affects the superimposed waveform. It
was unclear whether or not the electrosensory stimulus-
intensity threshold as behaviourally determined depends
on the phase difference between a fish's own EOD and a sine-wave stimulus (at AF = 0 Hz). Also the strength of the jamming avoidance response (JAR), a discharge frequency shift away from a stimulus that is
sufficiently close to the EOD frequency, as a function of
phase difference was studied. Sine-wave stimuli were
both frequency-clamped and phase-locked to a fish's
discharge frequency (AF = 0Hz). In food-rewarded
fish, the electrosensory stimulus-intensity threshold depended significantly on the phase difference between
a fish's discharge and the stimulus. Stimulus-intensity
thresholds were low (down to 3 gV/cm, peak-to-peak)
when the superimposed complex wave changed such
that the shift in zero-crossings times relative to
the original EOD was large but amplitude change
minimal; stimulus-intensity thresholds were high (up to
16.9 gV/cm, peak-to-peak) when the shift in zero-crossings
times was small but amplitude change maximal.
Similar results were obtained for the non-conditioned
JAR: at constant supra-threshold stimulus intensities
and AF -- 0 Hz, the phase difference significantly affected
the strength of the JAR, although variability between
individuals was higher than that observed in the
conditioned experiments.