While spikes are generally regarded as the common currency of neuronal communication, experimental and theoretical studies over the past decade have accumulated compelling evidence that short-term synaptic plasticity can also serve related functions (Abbott and Regehr, 2004; Abbott et al., 1997; Maass and Markram, 2002; Mongillo et al., 2008; Sussillo et al., 2007; von der Malsburg, 2004; Zucker and Regehr, 2002). Connectivity in the cortex is characterized by a large range of variation of synaptic weights (Gloveli et al., 1997; Holmgren et al., 2003; Markram et al. 1998; Reyes et al., 1998; Wang et al., 2006), which can change dynamically by both presynaptic and postsynaptic mechanisms (Chung et al., 2002; Deisz and Prince, 1989; Gupta et al., 2000; Markram et al., 1998; Thomson et al., 2002). The fraction of potentiating and depressing synapses is approximately the same in the intact neocortex (Fujisawa et al., 2009; Markram et al. 1998). Indeed, a balance between depressing and potentiating synapses in model networks is needed for stability. At the same time, networks with dynamic synapses can respond robustly to external inputs yet return
