Figure 1. The variance-mean analysis technique.

A, a series of simulated EPSC peak amplitudes were plotted against event number. The model assumed release probability (P_r) was uniform across 50 independent release sites and average quantal amplitude (Q_av) was 50 pA at each site. P_r was set to three different values during three separate epochs, each of 190 events: Low P_r, 0.02; Mod P_r, 0.5; and High P_r, 1.0. B, the variance of the EPSC amplitudes was plotted against the mean amplitude for each epoch. The variance-mean plot was then fitted with a parabola. The initial slope of the fitted curve provides an estimate of Q_av, and the parabolic function estimates P_r and the number of release sites (N).