*A,* Schematic depicting two possible mechanisms underlying correlated IPSCs. In the first scenario, depicted to the left, “synchronous firing”, correlated IPSCs would be caused by two or more interneurons firing simultaneously. In this case, each IPSC would be the sum of the spiking of several interneurons. In the second scenario, “shared input”, in a system where every interneuron has highly divergent axons and contacts many postsynaptic PCs, each time an interneuron fires a spike, an IPSC would be recorded from all of its downstream postsynaptic targets nearly simultaneously. *B,* Connection probabilities for PC→PC pairs pvGFP→PC pairs and sGFP→PC pairs showing significantly higher connection probability for interneuron→PC than for PC→PC (p = 0.001 Kruskal Wallis; *p* < 0.001 for PC vs pvGFP and PC vs sGFP; *p* > 0.05 for pvGFP vs sGFP, Dunn’s multiple-comparison test), ** signifies significance p < 0.01) *C,* Distance vs. correlations coefficients of IPSCs (blue circles) and EPSCs (red crosses) were plotted for all cell pairs. Both EPSC and IPSC correlations drop off with distance with slopes that were not significantly different from one another (analysis of covariance p = 0.52). *D,* Normalized distribution of conductances for IPSCs recorded during thalamically triggered activations (top, red), and synaptic conductances measured from pvGFP→ PC pairs (blue) or sGFP→ PC pairs (green). The mean of these distributions did not differ from one another (p = 0.442, Mann-Whitney).

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