Figure 2. The variance-mean plot can identify the locus of synaptic modulation when applied to simulated data.

A, simulated EPSC amplitudes plotted against event number. A model of a compound synaptic contact was used to simulate EPSC amplitudes under control conditions and following each of 3 types of modulation: postsynaptic modulation that increased Q_av, presynaptic modulation that increased P_av, and modulation that increased the number of active terminals. Under each condition, 50 events were generated before and 50 after the simulated application of Cd²⁺. B, the average variance-mean plot under control conditions (○), and following each type of modulation: postsynaptic (⋄), presynaptic (□), and increased number of terminals (▵). Error bars indicate ±s.e.m. (n = 20). C, average value of Q_av estimated from the slope of individual variance-mean plots under control conditions and following each type of modulation. Error bars indicate +s.e.m. (n = 20). The dashed line indicates the expected value of Q_av under control conditions. Only postsynaptic modulation produced a significant change in Q_av.