Figure 1. Single neuron equipped with STDP-plastic synapses.

A: Single neuron excited by input spike trains. The synaptic strength of synapse is denoted by . B: Potentiation (LTP) and depression (LTD) curves with in (4). Darker curves indicate stronger values for the weight : (light blue), (medium blue), and (dark blue) in (6). In the top left quadrant for LTP, the two curves in lighter blue are superimposed, since potentiation is quasi-constant for small weights. C: Functions for LTP and for LTD in log-STDP (blue solid curve) in (6) with , and ; mlt-STDP similar to van Rossum et al.'s model [24] (pink dashed line); and add-STDP similar to Song et al.'s model [1] (gray dashed-dotted curve for depression and pink dashed curve for potentiation). D: Weight change (in percent of the original weight) resulting from 20 successive modifications induced by log-STDP with random pairing of pre- and postsynaptic spikes (within the range ms). In qualitative agreement with experimental measurements [19], smaller weights experience large fluctuations whereas larger weights exhibit less variability. The mean expected modification (blue solid curve) and is indicated by the vertical arrow.