Synaptic Plasticity

A neuron will do just nothing without synaptic stimulation. But furthermore, a neural networks capability of learning and self-organization lies in its synaptic plasticity, i.e. the ability to change its synapses' weights. In the FACETS hardware, every synaptic circuit implements a biologically realistic learning rule called Spike Timing Dependent Plasticity (STDP).

An STDP synapse connecting neuron A with neuron B will get stronger, if both neuron A and neuron B fired and the time difference Δt between the spike times is such that the correlation between the spikes could have been causal. if only an acausal correlation is possible, the synapse will be weakend. A hardware STDP curve showing the dependence of that strengthening respectively weakening on Δt can be seen below.