We have discovered it, and understand it quite well. It's fairly simple, at the level of the synapse there are ultimately three ways to evoke increased firing in the downstream neuron: (1) the upstream neuron's axon can release more neurotransmitter, (2) the upstream neuron can establish more axon collateral connections with the downstream neuron, or (3) the downstream postsynaptic membrane can express a greater number of neurotransmitter receptors.