Dependence of synaptic modifications on the initial synaptic strength. The percentage change in the EPSC amplitude after the repetitive stimulation (1 Hz for 60 sec) was plotted against the initial mean amplitude of EPSCs. Open circlesrepresent data from synapses exposed to repetitive presynaptic stimulation with positively correlated postsynaptic spiking (data set includes those shown in Fig. 2C). Filled circles represent data from synapses exposed to repetitive presynaptic stimulation with negatively correlated postsynaptic spiking (data set includes those shown in Fig. 3C). Percentage changes were calculated from the average EPSC amplitude 20–30 min after the repetitive stimulation. Lines represent best fits with linear regression between the percentage change and the logarithm of initial EPSC amplitudes for positively correlated (r = −0.72, p = 0.00017) and negatively correlated (r = 0.037,p = 0.89) spiking, respectively.