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Electroreception in G. carapo: detection of changes in waveform of the electrosensory signals

Pedro A. Aguilera and Angel A. Caputi^*

Departamento de Neurofisiología Comparada, Instituto de Investigaciones Biológicas Clemente Estable, Unidad Asociada a Facultad de Ciencias, Universidad de la República, Av. Italia 3318, Montevideo, Uruguay

View larger version (36K): [in a new window]   Fig. 1. Modulation of the sLEOD by loading the stimulus-object with resistors. (A) The diagram illustrates the methodology employed. LEOD of Gymnotus carapo was recorded between an electrode adjacent to the skin and the closest base of a cylindrical object placed 2 mm away from the skin. The electrode was a 100 µm bare-tip insulated wire; the object consisted of a plastic tube 2 mm diameter () and 10 mm long with a carbon plug electrode in each opening. An external variable impedance z0 was connected to the carbon plugs to modify the object longitudinal impedance. To evaluate impedance discrimination, a second impedance was alternatively connected using a timed switch. Changes in object longitudinal resistance resulted in marked changes in image contrast as shown by recorded sLEOD waveforms corresponding to open circuit (black), 22 k (red), 68 k (blue) and short circuit (gray; same color code throughout the figure). To align the traces we used as a time reference a far-field recording of the EOD that was not modified by the presence of our small stimulus-object. (B) sLEOD corresponding to the three first loads as a function of the sLEOD corresponding to open circuit. Note the small phase shift. (C) Spectral density of the same signals. The ordinate corresponds to the energy dissipated locally during eacy EOD. Note the hump in the high frequency shoulder of all spectra (arrow).

View larger version (31K): [in a new window]   Fig. 2. Modulation of the sLEOD by loading the stimulus-object with capacitors. (A) sLEOD waveforms corresponding to open circuit (black), 16 nF (blue), 10 nF (green), 5 nF (red) and short circuit (grey; same color code throughout the figure). (B) Normalised waveforms (with respect to sV3) to compare the relative values of sV1, sV4 and the phase advance of the slope sV3-sV4. (C) sLEOD corresponding to the four first loads as a function of the sLEOD corresponding to open circuit. Note the large phase shift. (D) Spectral density of the same signals. Ordinate corresponds to the energy dissipated locally during each EOD. Note that the relative amplitude of the humps in the high frequency shoulder (arrow) increases for intermediate loads, becoming the absolute maximum for 10 nF and 5 nF (red and green traces, respectively).

View larger version (26K): [in a new window]   Fig. 3. Amplitude of every wave component as a function of impedance. (A—C) Resistive loads. Superimposed traces from five fish of different lengths (key in D) were normalised to their respective value in the absence of the object for each wave component. Note the differences in modulation of the different waves. Arrows indicate the resistance causing a 50% modulation of the peak of each wave component. (D—F) Capacitive loads. Superimposed traces from five fish of different lengths (key in D) were normalised to the open- and short-circuit respective values for each wave component. Note that the 50% point occurs at 20 nF for sV1, at 7 nF for sV3 and 2 nF for sV4. Note also that the curve corresponding to sV4 has a maximum at 10 nF.

View larger version (25K): [in a new window]   Fig. 4. Stimulus domain as defined by time-waveform parameters. (A) sV4/sV3 as a function of sV3. (B) sV1/sV3 as a function of sV3. Points obtained using resistive (red symbols) and capacitive (blue symbols) loads bound a domain in which are contained resistance-capacitance (RC) combinations (black filled symbols: in parallel; open symbols: in series). Note the different shape of plots A and B; for an explanation, see text.

View larger version (19K): [in a new window]   Fig. 5. Stimulus domain as defined by power spectrum parameters. (A) Relative power at the high frequency flank (where the hump occurs) as a function of rms value. (B) Relative power at the rising edge at 100 Hz as a function of rms value. Note the flat curves obtained using resistive loads (red symbols). These curves and those obtained using capacitive loads (blue symbols) bound a domain in which are contained resistance—capacitance combinations (black filled symbols: in parallel). Note the different shape of plots A and B; for an explanation, see text. Data presented in Figs 4 and 5 were obtained from different animals.

View larger version (14K): [in a new window]   Fig. 6. Energy of the sLEOD as a function of sV3. sLEOD rms value is a linear function of sV3 independently of the impedance of the stimulus object. Data were obtained from the same fish with resistive loads (red symbols), capacitive loads (blue symbols) and resistive and capacitive loads connected in parallel (black symbols).

View larger version (18K): [in a new window]   Fig. 7. Amplitude of the novelty response as a function of the change in sLEOD parameters. In these experiments, the object load was changed from open circuit to a given resistor value (red symbols) or a given capacitor value (blue symbols). (A) Amplitude of novelty response (interval shortening as a percentage of the basal interval) as a function of energy (as the change in rms value) of the sLEOD. Linear regression analysis was performed for experiments in which the change in sLEOD was obtained by loading the object with resistors: amplitude of the novelty response = 0.15xlog(rms/0.35); r2=0.81, N=15, P<0.0001; the regression line is drawn in red. Changes from open circuit to capacitive loads evoked larger responses than changes from open circuit to resistive loads, which generated sLEODs with the same rms value, except at the extremes of the range. (B) Difference between the predicted amplitude of the novelty response minus the measured data as a function of the sV1/sV3 ratio. The correlation was statistically significant only for experiments performed with capacitors (r2=0.75, N=30, P<0.001). (C) Difference between the predicted amplitude of the novelty response minus the measured data as a function of the sV4/sV3 ratio. The correlation was statistically significant only for experiments performed with capacitors (r2=0.75, N=30, P<0.001).

View larger version (24K): [in a new window]   Fig. 8. Waveform changes are detected by G. carapo. To test if waveform changes are detected independently of changes in rms value, the load of the stimulus-object was chosen in such way that the rms values of the resulting sLEODs were the same. This was achieved by substituting a resistor (red traces) by a capacitor (blue traces) or vice versa. The plots on the right show that novelty responses were evoked when the impedance change was from resistance to capacitance (filled symbols) but not when the change was from capacitance to resistance (dotted line indicates the baseline interval). (A—C) Three examples from the same fish.

View larger version (23K): [in a new window]   Fig. 9. Changes in waveform and amplitude have additive effects. (A) Four loads, open circuit (generating the minimum rms value, black trace), short circuit (generating the maximum rms, 15 , grey trace) and two different impedances (generating the same intermediate rms value but very different waveform, a resistor of 21 k, red trace, and a capacitor of 10 nF, blue trace) were selected to test whether amplitude and waveform are independently evaluated by the fish. Right: the differences in amplitude and waveform of the signals compared by the fish. The differences in amplitude are shown by the main slope of the loop and the differences in waveform by its deviation from a straight line. (B—E) Each panel represents paired experiments in which the stimulus-object impedance was changed in either direction between open circuit and resistance (B), short circuit and resistance (C), open circuit and capacitance (D) and short circuit and capacitance (E). When resistive load were used, waveform changes were small (B,C) and novelty responses were only elicited by increases in sLEOD rms value. Consistently with the experiment shown in Fig. 7, the amplitude of the elicited novelty responses was graded with the increase in rms value. When large waveform changes (caused by a capacitive load, D and E) were associated with the same changes in rms value, the amplitude of the elicited novelty responses varied as if the rms value and the waveform were independently evaluated. The amplitude of the novelty response was relatively increased by a waveform change consisting of a reduction of the early slow-negative wave plus an increase and advance of phase of the late sharp-negative wave (compare the responses marked with filled symbols in D and E). When a similar waveform change was associated with a decrease in the rms value it provoked a small novelty response (compare E, open symbols, with C, open symbols). Finally, when increases in rms value were associated with opposite changes in waveform (reduction of the late negative wave and increase of the positive-negative slope), novelty responses were not elicited (E, filled symbols)