A stochastic approach to STDP

We present a digital implementation of the Spike Timing Dependent Plasticity (STDP) learning rule. The proposed digital implementation consists of an exponential decay (exp-decay) generator array and a STDP adaptor array. The weight values are stored in a digital memory, and the STDP adaptor w ill send these values to the exp-decay generator using a digital spike of which the duration is modulated according to these values. The exp-decay generator will then generate an exponential decay, which will be used by the STDP adaptor for performing the weight adaption. The exponential decay, which is computational expensive, is efficiently implemented by using a novel stochastic approach. This stochastic approach was fully analysed and characterised. We use a time multiplexing approach to achieve 8192 (8k) virtual STDP adaptors and exp-decay generators with only one physical adaptor and exp-decay generator respectively. We have validated our stochastic STDP approach with measurement results of a balanced excitation experiment. In that experiment, the competition (induced by STDP) between the synapses can establish a bimodal distribution of the synaptic weights: either towards zero (weak) or the maximum (strong) values. Our stochastic approach is therefore ideal for implementing the STDP learning rule in large-scale spiking neural networks running in real time.