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following (Song et al., 2000), synaptic change is implemented using two recording function per synapse P^{-}(t) and P^{+}(t)

c 2000 Nature America Inc. . http://neurosci.nature.com
Sen Song1, Kenneth D. Miller2 and L. F. Abbott1
Competitive Hebbian learning
through spike-timing-dependent
synaptic plasticity

=> 

(parameters take from ref. 37).

37. Troyer, T. W. & Miller, K. D. 
Physiological gain leads to high ISI variability in a simple model of a cortical regular spiking cell. 

Neural Comp. 9, 971.983 (1997).