Finding the optimal homeostatic conductance. A neuron with only a leak conductance (reversal at -70 mV) and spike mechanism was simulated using NEURON software. A. The method of measuring distance from optimality. Top, the neuron received two EPSGs of equal amplitude (30 nS) separated by a 5 ms interval (thick black). At the time of each real EPSG, test ESPGs (thin black, shown only for the second) of varying amplitudes were applied to find the “threshold EPSG” (thick gray) for which the EPSP peak (bottom) is precisely at spike threshold. The “residual” is the difference in peak amplitude of the real and threshold EPSG, and it measures the distance of excitability from optimality. B. EPSPs generated by the real EPSGs in ‘A,‘ but with leak conductances of 10, 30, and 50 nS. C. Threshold EPSGs for the same three leak conductances. By comparing to the real EPSGs, it can be seen that although the 10 nS conductance best minimized the residual for the first EPSG, the sum of the two squared residuals is less for the 30 nS conductance. D. The sum of squared residuals was minimized by leak conductances of 30 and 25 nS in the case of 5 and 10 ms inter-EPSG intervals, respectively.