 While spike-timing-dependent plasticity is an essential ingredient for didactic organisation, other features of neuronal activity appear to be required for didactic organisation to occur "in vivo".One of these features is that activity propagated through a network needs to have a 'causal' character.For example, chain of reciprocally connected neurons with this ‘causal activity’ characteristic would be capable of propagating a wave of spikes along its length, rather than the wave disintegrating into a cascade of spikes ‘bouncing’ back and forth between neurons in the chain.