Figure 4.

Estimation performance of the presynaptic membrane potential. (a) Performance as a function of the determinism of presynaptic spiking, β (Online Methods), in the optimal estimator (red), an optimally fitted dynamic synapse (blue) and an optimally fitted static synapse without short-term plasticity (green). When β = 0 (entirely stochastic spiking), spikes are generated independently of the membrane potential and, as a consequence, all models fail to track the membrane potential. As β becomes larger (more deterministic spiking), the dynamic synapse model matches the optimal estimator in performance and substantially outperforms the static synapse. Realistic values for βσ_OU (here σ_OU = 1 mV) have been found to be between 2 and 3 in L5 pyramidal cells of somatosensory cortex⁴⁹. (b) Estimation performance with stochastic vesicle release as a function of the number of synaptic release sites, N. The dynamic synapse (blue) tracked the performance of the optimal estimator (red) well and outperformed the static synapse (green) at all values of N. The performance of all estimators decreases only when the number of independent release sites becomes very low (N = 1 or 2 ). When N is large (N → ∞), synaptic transmission becomes deterministic, even though spike generation itself remains stochastic (with parameter β = 2 shown by the arrow in a).