Vol. 330 no. 6011 pp. 1612-1613 DOI: 10.1126/science.330.6011.1612 News IntroductionTen years ago, Karl Deisseroth was stuck. A psychiatrist and neuroscientist, he wanted to learn how different brain circuits affect behavior—and what went awry in the brains of his patients with schizophrenia and depression. But the tools of his trade were too crude: Electrodes inserted into the brain would stimulate too many cells in their vicinity. So in 2004, Deisseroth and his students invented a new tool. They inserted a gene for a light-activated algal protein into mice brains, where it entered nerve cells. By stimulating those cells with a laser,...