Fig. 1.

Prediction of a calcium-based model of spike-timing–dependent plasticity. Cartoon showing qualitatively calcium transients induced by pairing a presynaptic spike with a postsynaptic spike with a delay ∆t, for three extracellular calcium concentrations (high on top, low on the bottom). Synaptic changes depend on two plasticity thresholds, one for LTP (blue) and one for LTD (red). The resulting 'STDP curves' (change in synaptic stength ∆w as a function of ∆t) are shown on the right. At high extracellular calcium, the calcium transient exceeds LTP threshold in a range of positive ∆ts, and the STDP curves has a LTP window surrounded by two LTD windows. Decreasing extracellular calcium leads to a decrease in the amplitude of the calcium transient, which no longer cross the LTP threshold, resulting in a STDP curve with only LTD. Finally, a further reduction in extracellular calcium leads to no threshold crossing, and consequently no synaptic changes.