Cohen, now a chemistry professor at Harvard, is still fascinated with electrical signals—in particular, with learning how they propagate in the brain. Researchers traditionally implant electrodes in order to measure neuronal activity, but that approach can Adam Cohen
Harvard Universityread only a few cells at a time. Cohen wanted to engineer entire neurons to glow when they fire. To do this, he turned to a protein that a microbe from the Dead Sea uses to convert sunlight into electrons. Maybe, he thought, it could work in reverse, turning an electrical impulse into a flash of light. One of his postdocs inserted the gene into a virus and infected cultured rat neurons to make them produce the protein. Late one night last year, their efforts paid off: Zapped with electrodes to induce a nerve impulse, the isolated brain cells blinked like tiny Christmas lights.