Behavioral thresholds to electric signals in high frequency electric fish

Summary1) The behavioral thresholds ofApteronotus andEigenmannia to imposed sine wave electric fields were measured as a function of stimulus frequency using operant conditioning techniques. 2)Apteronotus responds to a broad range of frequencies. Its threshold—frequency curve is complex, showing three distinct threshold dips at: <5–15 Hz, 0.6–0.9 μV/cm; 200–300 Hz, 0.9 μV/cm; and 700–2200 Hz to 0.2 μV/cm. The high frequency dip itself is bimodal, with a primary minimum at the fish's electric organ discharge (EOD) fundamental, and a secondary minimum at its second harmonic. 3) The threshold—frequency curve forEigenmannia differs in several respects, being unimodal and sharpely tuned to the fish's EOD frequency with a sensitivity of 0.2–0.3 μV/cm.Eigenmannia shows a lack of keen low frequency perception, and no marked sensitivity increase at the second harmonic of its EOD. 4) The EOD wave forms ofApteronotus andEigenmannia were Fourier analysed:Eigenmannia's EOD approximates a sine wave, whileApteronotus' EOD contains considerable power at harmonic frequencies. 5) It is proposed that behavioral thresholds to frequencies within ca. 50–100 Hz of the fish's EOD fundamental or second harmonic are improved by the fish's extraction of, and preference for, low frequency beats of its EOD with the stimulus. This is suggested by the sharp, EOD specific tuning demonstrated by bothEigenmannia andApteronotus and the second harmonic sensitivity peak ofApteronotus, which are not predicted by known receptor response characteristics. Such a beat extraction capability might provide a mechanism for improving the resolution of conspecific electrocommunication. 6) The influence of water resistivity on the behavioral thresholds ofApteronotus was measured. High frequency sensitivity increases as a power function with decreasing water resistivity down to 1 k Ohm · cm, below which it remains constant. Low frequency sensitivity increases progressively from 100 k Ohm · cm down to 10 k Ohm · cm, but decreases again below this value. Thus low frequency eleotroreceptors are maximally sensitive in 10 k Ohm · cm water, while high frequency receptors function best in water below 1 k Ohm · cm.