Ezequiel Di Paolo (2002) "Spike-Timing Dependent Plasticity for Evolved Robots" Adaptive Behavior Vol. 10,  p.243
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"3.2.1 STDP
The properties of plastic windows (Figure. 1) are evolved for each synapse in the neural network controller. Following (Song et al., 2000), synaptic change is implemented using two recording functions per synapse $P^{-}(t)$ and $P^{+}(t)$"

     - Song, S., Miller, K. D., & Abbott, L. F. (2000). Competitive Hebbian learning through spike-timing-dependent synaptic plasticity. Nature Neuroscience, 3, pp. 919-926.