Spike transmission probability depends on the presynaptic ISI and differs between synapses. A, Cross-correlograms between presynaptic and postsynaptic spiking at three different synapses show an increase in the postsynaptic spike count (or probability) after a short latency, indicative of a monosynaptic connection. The efficacy (Eff.) for each synapse is calculated as the ratio between the number postsynaptic spikes that are above baseline in the transmission interval (denoted by the horizontal bar) and the number of presynaptic spikes. B, ISI distributions (log-scale) for the presynaptic neurons. The distributions are color-coded into five quantiles with equal numbers of presynaptic spikes. C, We calculate a separate cross-correlogram using the subset of presynaptic spikes where the preceding spike fell within each ISI range. Colors correspond to B going from shorter presynaptic ISIs (left) to longer ISIs (right). Note that both the baseline firing rate and the synaptic peak for each connection change as a function of presynaptic ISI.