Adam Cohen Depts. of Chemistry and Chemical Biology and of Physics Harvard University Abstract: To probe neural function, one would like to monitor the simultaneous electrical activity in a large number of neurons, with high resolution in space and time. Genetically encoded optical indicators of membrane potential are a promising approach to this challenge. But thus far these molecules have lacked adequate sensitivity and speed. We developed a voltage indicator based on a mutant of green proteorhodopsin. In the wild, bacteria use this light-driven proton pump to convert sunlight into electrochemical energy. We engineered the protein to run backward: to convert changes in membrane potential into a measureable optical signal. Bacteria expressing this protein show periodic flashes of fluorescence, indicating spontaneous electrical spiking. I will discuss possible mechanisms and biological significance for this spiking behavior.