Our mental state is encoded in a set of electrical spikes that propagate through our neurons. These spikes are about 100 mV tall, last about 1 ms, and travel at a few meters per second. One can record these spikes with an electrode, but an electrode only reports voltage at a single point, while a human brain has ~1011neurons. For decades neuroscientists have sought an optical tool to convert neuronal spikes into light, with the goal of visualizing activity in large networks of neurons.
We identified a protein from a Dead Sea microorganism that achieves this goal. In the wild the protein serves as a light-driven proton pump: it absorbs sunlight and generates a transmembrane voltage which its host uses as a source of metabolic energy. We found a way to run this protein "in reverse", to convert changes in membrane voltage into light. Upon expressing this gene in neurons, we acquired the first movies showing electrical impulses propagating through neurons.
