Figure 1. Illustration of polychronous neuronal groups and associative short-term plasticity.

(A) Synaptic connections between neurons n1, n2, …, n7 have different axonal conduction delays arranged such that the network forms two functional subnetworks, red and black, corresponding to two distinct PNGs, consisting of the same neurons. Firing of neurons n1 and n2 can trigger the whole red or black PNG: (B) If neuron n1 fires followed by neuron n2 10 ms later, then the spiking activity will start propagating along the red subnetwork, resulting in the precisely timed, i.e., polychronous, firing sequence of neurons n3,n4,n5,n6,n7, and in the short-term potentiation of the red synapses. (C) If neurons n2 and n1 fire in reverse order with the appropriate timings, activity will propagate along the black subnetwork making the same set of neurons fire but in a different order: n7,n5,n3,n6,n4, which temporarily strengthens the black synapses. Readout: post-synaptic neurons that receive weak connections from neurons n3, n4, and n5 with long delays and from neurons n6 and n7 with shorter delays (or, alternatively, briefly excited by the activity of the former and slowly inhibited by the latter) will fire selectively when the red polychronous pattern is activated, and hence could serve as an appropriate readout of the red subnetwork. A similar readout mechanism is illustrated in [53].