In Part I we looked at a variety of synaptic weight mechanisms involving presynaptic plasticity. Postsynaptic mechanisms differ not only in their biological mechanisms but also in a number of ways that have a direct effect on network functionality. In presynaptic plasticity the most common case is one in which the synaptic changes are generic and non-associative. Generic means the weight change tends to take place at every synapse made by the presynaptic neuron. Non-associative means that the changes are independent of whatever may be going on at synapses made to the same postsynaptic cell by other presynaptic neurons. In contrast, most (but not all) postsynaptic mechanisms are both specific and associative. Specific means the weight change can be confined to the particular synapse. Associative means the weight change mechanism can involve the interaction with signaling going on at other synapses connecting to the postsynaptic neuron or with the output state of the postsynaptic neuron (i.e. the change can depend on whether or not the postsynaptic cell fires an action potential).
