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===Discrete or continuous synapses===

Most models describe the synaptic weight as a continuous variable, although it is quite conceivable that weights are coded in discrete jumps. In fact, certain benefits would arise from such discrete synaptic weights. For example, bistability of individual synapses  would help to assure the long-term stability of synapses over weeks or  years in the presence of molecular turnover (Lisman 1985). While this is a strong argument in favor of discrete synapses it does not preclude that, on a shorter time scale, synaptic weights undergo continuous synaptic depression or facilitation which would be overlaid on the discrete long-term dynamics.

Since in the typical STDP experiment, results are averaged across several synapses, the question of whether single synapses respond to plasticity protocols with discrete or continuous changes cannot readily be answered. There are at least two studies suggesting that synaptic weights onto CA1 pyramidal neurons in the hippocampus are altered in discrete steps (Petersen et al, 1998; O'Connor et al, 2005). Other studies, however, appear to be in disagreement with this view. For example, recent glutamate uncaging experiments suggest that weights change continuously (Tanaka et al, 2008).

Finally, even if weights are discrete, it is difficult to provide conclusive experimental evidence showing stepwise changes in plasticity. The stochastic nature of neurotransmitter release, for example, hampers such experiments, by adding noise to the point that stepwise changes might be masked, although glutamate uncaging would help address this problem. Furthermore, the situation might be complex, and plasticity may be for example be discrete postsynaptically and continuous presynaptically. Last but not least, synapses are at different distances from the soma and their corresponding postsynaptic potentials are therefore subjected to different amounts of dendritic filtering as they propagate toward the soma. Since most connections in the brain are made up of more than one synaptic contact, which are made onto different dendritic compartments at different electrotonic distances from the soma, the net result is that any discrete steps that might exist would be exceedingly difficult to find conclusive evidence for experimentally. Even if weights were discrete, synaptic weight distributions would seem continuous and discrete plasticity would appear continuous.

Whether synapses typically are discrete or continuous thus remains an open but very intriguing question.