In several neocortical regions, around 1 in 7 of nearby PC pairs are connected, but the disynaptic PC–MC–PC motif that mediates FDDI is more than twice as prevalent (Berger et al. 2009). Whereas PC–PC connections are depressing (red trace), the PC–MC pathway is facilitating (blue trace), so PCs must fire at high frequency to drive MCs and generate FDDI (Silberberg & Markram, 2007). Dashed lines: axons; continuous lines: dendrites; scalebars: 1 mV, 50 ms.