(A – B) Traces schematize a model of bidirectional BTSP that depends on (1) presynaptic spike timing, (2) plateau potential timing and duration, and (3) the current synaptic weight of an input before an evoked plateau. (A) Presynaptic spikes (first row, black) result in local Vm depolarization of a postsynaptic spine (second row, gray), which generates a long duration plasticity ‘eligibility trace’ (ET) (third row, green) that marks the synapse as eligible for later synaptic potentiation or depression. The large Vm depolarization associated with the dendritic plateau potential (first row, brown) is assumed to effectively propagate to all synaptic sites (second row, gray), which generates a separate long duration ‘instructive signal’ (IS) (third row, yellow) that is also required for plasticity. Both potentiation and depression are saturable processes that depend on the temporal overlap (product) of ET and IS (fourth row, purple). (B) Equation defines the rate of change in synaptic weight in terms of a potentiation process that decreases with increasing initial weight , and a depression process that increases with increasing initial weight . Plot shows the relationship between and the signal overlap under conditions of low (red), intermediate (purple), or high (blue) initial weight. (C – E) Heatmaps of changes in synaptic weight in terms of time delay between presynaptic spike and postsynaptic plateau, and initial synaptic weight for three variants of the weight-dependent model of BTSP. Dashed traces mark the equilibrium initial synaptic weight at each time delay where potentiation and depression are balanced and additional pairings of presynaptic spikes and postsynaptic plateaus result in zero further change in synaptic weight. (C) Model in which potentiation () and depression () processes are nonlinear (sigmoidal) functions of signal overlap (). (D) Model in which are potentiation () and depression () processes are linear functions of signal overlap (). (E) Model in which the durations of the and instructive signal () are constrained to a short (100 ms) timescale, similar to intracellular calcium.