The three processes of adaptive synaptogenesis: Random synaptogenesis, bi-directional associative modification of existing synapses, and synaptic shedding.

<p>(Top) Synaptogenesis and positive associative modification (LTP). (Left) Receptivity for new innervation is below the cutoff firing-rate and three sites for a new synapse are indicated by the broken circles. (Center) Of the three locations, one new synapse is randomly generated (small complete circle), the uppermost available synapse. (Right) The new synapse increases strength due to the associative modification equation and positive correlation with enough of the other synapses on this neuron. Because of the increased excitation due to the new synapse, the post-synaptic neuron increases its average firing-rate above the prescribed design value; as a result of exceeding this value, receptivity for new innervation goes to zero (the broken circles disappear). (Bottom) Negative associative modification (LTD) and shedding. (Left) Having randomly acquired a certain set of inputs, one positively correlated subset (shown in shades of blue) dominates the excitation of the postsynaptic neuron while another input (shown in red) is negatively correlated with this subset. (Center) Associative synaptic modification decreases the weight of the uncorrelated input as indicated by the smaller circle. (Right) Associative synaptic modification further decreases the weight of the uncorrelated input, and because the weight value falls below the threshold, the synapse is shed.