<p><b>A</b>. Visual responses to an optimally oriented moving light bar of a 4RSN recorded under 1 mM QX314 while injecting different steady currents. The black, continuous line are the recorded V<sub>m</sub> values, whereas the blue, dashed trace shows the reconstructed V<sub>m</sub> values obtained by inserting back the estimated g<sub>E</sub> and g<sub>I</sub> values into the fundamental membrane equation. The instantaneous total synaptic conductance is calculated based on the instantaneous slope of the current-voltage relation (G<sub>tot</sub>, blue). The time-dependent excitatory (g<sub>E</sub>, green) and inhibitory (g<sub>I</sub>, red) conductances are plotted below. Gray traces represent the 95% confidence intervals obtained by bootstrapping of the data (see Methods). Conductance measurements began after the response to the injected current was at steady state (after 200 ms). Resting conductances were calculated in absence of visual stimulation (dashed line: stimulus end). <b>B</b>. Visually-driven PSPs vary linearly with the injected current. Plot showing the linearity of the relationship between the amplitude of the visually-driven PSP response and the value of the injected current (r = −0.97) for a 4RSN (this plot refers to the example shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082044#pone-0082044-g002" target="_blank">Figure 2</a> of the Main Text). Means ± standard erros are shown. The median of the correlation coefficients for all the recorded neurons was −0.94 (25<sup>th</sup>–75<sup>th</sup> percentiles: −0.88 − −0.99). <b>C</b>. Plot of the recorded <i>vs</i> reconstructed V<sub>m</sub> values obtained by inserting back the estimated g<sub>E</sub> and g<sub>I</sub> into the membrane equation. The linearity of the cell and the accuracy of the V<sub>m</sub> reconstruction is shown by the fact that data points align along the line of steepness 1 and intercept 0 in the plot. <b>D</b>. Temporal intervals between the peaks of g<sub>E</sub> (green) and g<sub>I</sub> (red) and that of the V<sub>m</sub> response. For each cell, values for both contralateral and ipislateral responses are plotted. Note that in the vast majority of cases the conductance values have been obtained in close proximity of the V<sub>m</sub> peak response (within 200 ms, dashed lines). <b>E</b>. Example of a voltage clamp recording (see Methods) following visual stimulation with a moving bar in the preferred direction. By clamping the cell at the reverse potential for inhibition (−80 mV when considering a liquid junction potential of approximately 14 mV) only excitatory currents can be seen (green: average response overlapped to single trials, shown in gray). Conversely, clamping the cell at the reversal potential for inhibition (+14 mV when considering the liquid junction potential) reveals the presence of inhibitory currents (red: average response overlapped to single trials, shown in gray).</p
