Fig. 1

Historical overview of genetic voltage indicators. Sensors fall into three distinct families based on voltage sensing domains (VSD; left), microbial rhodopsins (middle), or chemogenetic probes (right) and are arranged chronologically according to year of first report. Color of the box refers to the activation wavelength reported in the paper or inferred from the spectrum of the fluorescent protein. Black stars denote reported two-photon measurements. Note that HAPI-Nile Red and Voltron are also rhodopsin-based. See text for references