I think for you it might be helpful to learn more about the physical mechanisms that control synaptic plasticity.
The calcium level in the spine determines the direction and magnitude of the change. High concentrations increase the weight, prolonged periods of low calcium concentration decrease the weight, and very low concentrations have no effect on the weight.
Calcium enters the spine head by two ways: NMDA receptors and voltage controlled calcium channels. Both of these channels are effected by the voltage so nearby synapses also control the plasticity.
I think that the model introduced here Pyramidal cell model predicting plasticity in neocortex gives a good overview of how the whole thing works.