Genetically-encoded voltage indicator (GEVI)

A protein sensor that reports membrane potential directly and almost instantaneously, resolving individual action potentials and subthreshold dynamics that calcium indicators cannot. Families include voltage-sensitive-domain fusions (the ASAP series), microbial-rhodopsin-based sensors (Arch-derived, Ace), and chemigenetic hybrids (Voltron / Positron, which bind a bright synthetic dye). Voltage imaging is the frontier of optical read-out because it captures the actual electrical signal at the speed of the spike.

The engineering is brutal: the voltage-dependent fluorescence change occurs in under a millisecond, demanding kilohertz frame rates that yield few photons per frame, so signal-to-noise is low and photobleaching is fast; the signal is confined to the thin membrane rather than filling the cell body. These constraints have kept GEVI imaging to small fields, shallow depths and short recordings, though it is advancing rapidly. Voltage imaging is also generally incompatible with two-photon holographic stimulation of the same cells because of speed limits and spectral crosstalk.