Electrosensory stimulus-intensity thresholds in the weakly electric knifefish Eigenmannia: reduced sensitivity at harmonics of its own organ discharge

Abstract

The South American knifefish Eigenmannia sp. can
detect the electric organ discharges (EODs; 250–600 Hz)
of conspecifics when they are superimposed over its own
EOD. This study investigates the minimum frequency
difference necessary for such signal perception, using the
application of sine-wave stimuli. Electrosensory stimulusintensity thresholds were determined for trained fish using stimuli associated with food rewards. These sinewave stimuli were ‘clamped’ to the EOD frequency of the
fish. Electrosensory thresholds were also determined for
the spontaneous jamming avoidance response (JAR; a
change in EOD frequency evoked by a stimulus of
sufficiently similar frequency), in this case using
unclamped stimuli.
Over the wide frequency range investigated (0.3–3.01
times EOD frequency), the lowest stimulus-intensity
thresholds of 0.6mVcm21 (peak-to-peak) (0 dB) at a water
conductivity of 100mScm21 were found close to (but not
exactly at) the EOD fundamental frequency. At exact
frequency identity between the EOD and the stimulus, the
stimulus-intensity response threshold rose abruptly by
more than 10 dB compared with slightly higher or lower
stimulus frequencies. A similar ‘needle-like’ threshold
increase was found at exactly two and three times the EOD
frequency, but neither at harmonic ratios between
stimulus and EOD frequency that represent fractions (e.g.
at 5:4=1.25, 4:3=1.33, 3:2=1.5 or 5:3=1.67 times EOD
frequency) nor at subharmonics such as half or two-thirds
of the EOD frequency. The steepest increase of stimulusintensity response threshold was in the range 0.998–1.002 times EOD frequency, corresponding to a threshold change, or electrosensory filter slope, of 5000 dB per octave.
For the spontaneous JAR, a similar stimulus-intensity
threshold increase was observed when EOD frequency
equalled stimulus frequency. Because of the longer rise
time for the stimulus amplitude (400 ms rather than 35 ms)
the stimulus intensity threshold was higher (up to 32 dB;
mean, 20 dB) than in the other experiments (up to 15 dB;
mean, 13 dB).
A difference in frequency between the EOD and the
applied stimulus as small as 1 Hz (that is, 0.2 % of the EOD frequency) was sufficient for good signal perception in
Eigenmannia sp. The JAR appears to be useful in avoiding
insensitivity at exact integer harmonics of the EOD
frequency.