The Relationship Between Spike-Timing-Dependent Plas- ticity (STDP) and Sliding Threshold (BCM) Synaptic Modification

There has been an increased interest in a novel type of synaptic plasticity spike-timingdependent plasticity (STDP), in which potentiation or depression occurs depending on the temporal order of the pre and postsynaptic spikes; see Fig. 1a. However, it is not clear how STDP relates to the best studied form of synaptic plasticity – classical LTP and LTD. The two kinds of plasticity must be somehow related, since it is believed that they are based on the same biophysical mechanism. Here we consider different implementations of STDP and compare them with a standard LTP/LTD implementation called the Cooper or BCM (Bienenstock-Cooper-Munro) synapse. In the BCM formulation, one considers instantaneous firing rates rather than individual spikes. Synaptic input that drives postsynaptic firing to high levels results in an increase in synaptic strength, whereas input that produces only low levels of postsynaptic firing results in a decrease (Fig. 1b). The threshold firing rate, the crossover point between potentiation and depression, is itself a slow function of postsynaptic activity, moving so as to make potentiation more likely when average activity is low and less likely when it is high. Considerable experimental evidence for this kind of plasticity has been obtained in neocortex, at some of the same synapses at which evidence for STDP