Figure 1.

Classes of voltage indicators. (a) In a real-time voltage reporter, the population in a fluorescent state, F, is a function of membrane voltage, regardless of illumination. (b) In a sample and hold voltage sensor, voltage establishes a D₁ ⇌ D₂ equilibrium and a write pulse establishes a rapid D₂ ⇌ F equilibrium. Thus the population of F tracks the membrane voltage. The population of F is frozen at the end of the write pulse. (c) In a light-gated voltage integrator, voltage establishes a D₁ ⇌ D₂ equilibrium, and a write pulse drives the unidirectional D₂ → F transition. Thus the population of F accumulates in a voltage-dependent manner. The population of F is frozen at the end of the write pulse. In both types of flash memory sensor, the population of F persists in the dark and is later probed via a read pulse that elicits fluorescence.