Figure 4. Current responses of different ChR variants to light steps of increasing intensities.

In each panel, we show the light responses of a certain variant of ChR to 8 different 1-sec light steps of increasing intensity (top). At the beginning of each light step, the model was set to its basic state (C ₁ = 1, O ₁ = C ₂ = O ₂ = 0). The traces show the current flowing into the cell through the two open states, according to Ohm's law: I = −70 mV (g ₁ O ₁+g ₂ O ₂). A: wild-type ChR-2. B-D: By changing one parameter at a time (indicated in bold) we achieve Variant A of ChR, which has no steady state current at high intensities, while remaining such currents at medium intensities. E: Variant B corresponds to wild-type ChR-2 with 100-fold decreased light sensitivity.