Discrimination of the sign of frequency differences bySternopygus,an electric fish without a jamming avoidance response

Several species of weakly electric fish reflexively change their frequency of electric organ discharge (EOD) in response to sensing signals of similar frequency from conspecifics; that is, they exhibit jamming avoidance responses (JAR).Eigenmannia increases its EOD frequency if jammed by a signal of lower frequency and decreases its EOD frequency if jammed by a signal of higher frequency. This discrimination is based on an analysis of the patterns of amplitude modulations and phase differences resulting from signal interference. Fish of the closely related genus,Sternopygus, however, do not exhibit a JAR. Here we show that despite lacking this behavior,Sternopygus shares many sensory processing capacities withEigenmannia:

1.	Fish could be conditioned to discriminate the sign of the frequency difference (Df) between an exogenous sinusoidal signal and its own EODs with as few as 300 training trials.
2.	Fish can discriminate the sign of Df for jamming signals with an amplitude as low as 2 V/cm (p-p); which is approximately 40 dB below the amplitude of the fish's EOD, as measured lateral to the operculum.
3.	Fish appear to discriminate the sign of Df by evaluating modulations in signal amplitude and differences in the timing of signals received by different areas of the body surface.
4.	Sternopygus processes electrosensory information through band-pass modulation filters. Tuning to modulation rates over at least 4–16 Hz exists.