(a) Comparison of GECI model's peak ΔF/F0 (red line) for 1, 2, 3, 4, 5, 10, and 40AP trains to individual G-CaMP2 responses (thin lines) and the mean G-CaMP2 response (black circles). Increasing model kon and koff 5-fold (blue dotted line) increases response. (a, inset) Model fluorescence time course. (b) Predicted peak change in fluorescence over baseline to AP trains (1, 2, 3, 4, 5, 10, 40) for a range of KA values. (b, inset) 1AP (solid) and 40AP (dotted) trains normalized to optimal response. (c) Optimal affinity for 1AP response shifts between n = 2.5 (solid) and n = 1 (dotted). (d) Model response to 1AP increases with kinetic rates kon = (0.8, 1.6, 2.4, 4, and 8) × 106. (e) The dependence of effective dynamic range on total dynamic range is reduced as fractional saturation of model GECI at [Ca2+]0 increases from 0 to 0.32. (e, inset) Increasing total dynamic range (Fmax/Fmin = 2, 3, 5, 10, 15, 20) shifts the optimal KA for single AP detection from 130 to 260 nM. (f) Responses to 1AP when time course of calcium transient decay is decreased to 70 ms. n = 2.5, kon = 8 × 105 M−1 s−1, Fmax/Fmin = 5 (black), kon = (8,16, 32, 40, and 80) × 105 M−1 s−1, Fmax/Fmin = 15 (colors).