The ocular dominance columns look like Turing patterns, which can be formed by modified Hebbian mechanisms. In a normal Hebbian model, if two neurons are connected to a neuron and fire together, they increase the strength of the synapses, "moving" the axon terminals closer together. The model must be modified to incorporate incoming activity that is locally excitatory and long range inhibitory, because if this is not done then the column width will only be dependent on the width of the axonal arbor, and also segregation will often fail in the presence of inter eye correlation. This basic model has since been extended to be more physiologically plausible with the addition of long term potentiation and depression, synaptic normalization, neurotrophin release, reuptake, and spike-timing-dependent plasticity.