...
In discussion, Patrick brought up Li et al (2004), a paper from Mu-ming Poo's lab showing that those presynaptic neurons that spike early, and that consequently (via STDP) have their synaptic weights upon a postsynaptic neuron strengthened, are also themselves rendered more excitable. That is, by this mechanism STDP can "work on the presynaptic neuron" as well as on its output synapse. The net effect of this can be to durably group a set of early-firing neurons together into a fully synchronous ensemble evoking activity in that postsynaptic cell.
...