Figure 10.

Trade-offs between VSFP S/N and adverse effects of VSFP sensing capacitance analyzed for the case of action potential detection in the layer 5 (L5) pyramidal neuron model. (A) Electrical (top) and fluorescence response (bottom) of the somatic membrane in the L5 neuron model including 200 units/μm² of a generic VSFP with 1.2 sensing charges (z), −40 mV half activation (V_1/2), 5% per 100 mV voltage sensitivity (S_1/2) and a sensing time constant (at V_1/2) of 2 ms. The stimulus is realized by activation of a distal synaptic conductance (time constant rise: 0.3 ms; decay: 3 ms) with peak conductance 50% above action potential threshold. Onset of the stimulus is indicated by arrow. Also shown are the electrical control response (top; no VSFP included; gray dashed line) and the fluorescence response including simulated photon shot noise (bottom; green) corresponding to 1.5 kHz sampling from a spherical membrane of 25 μm diameter. (B) Fluorescence S/N (color scale given at the right) and VSFP-induced shift of spike latency (in % versus control; black lines) of the first action potential in simulation traces analogous to A as function of the sensing time constant and VSFP density (50 × 40 parameter grid). (C) Values of fluorescence S/N ratio produced by the first action potential in traces analogous to A as function of sensing time constant and synaptic peak conductance (as % deviation from the threshold of action potential generation in absence of VSFP) for 200 (left), 500 (middle), and 1000 (right) VSFP units/μm². The number of elicited spikes is indicated by white contour lines. Gray areas delimit the range of subthreshold potentials. (D1) Shift of latency of the first spike (in % versus control) in simulations analogous to A with 500 VSFP units/μm² and a synaptic peak conductance 100% above control spike threshold as function of the voltage of VSFP half activation (V_1/2) and for values of gating valence between 0.8 (bottom curve) and 2.0 (top curve) as labeled in the figure. The VSFP sensing time constant was 2 ms. (D2) Fluorescence responses ΔF/F₀ produced by the first spike in the same set of simulations as in (D1) assuming a maximum fluorescence dynamic range ΔF_max/F₀ of 5%. (D3) Normalized shift of first spike latency (black dots) and fluorescence spike response (green dots) as function of the sensing valence z evaluated from the peak values in D1 and D2. (E1) S/N for spike detection as a function of VSFP dynamic range ΔF_max/F₀ in the generic model (2 ms time constant; V_1/2 equal −40 mV; 200 VSDs/μm⁻²). (E2) S/N for spike detection as a function of VSFP voltage sensitivity (S_1/2) and VSFP density using the generic model (like in E1; z = 1.2) mapped to the color code to the right. Vertical contour lines represent VSFP-induced shifts of spike latency as in B.