This article is one in a series of 10 stories we're running this week covering today's most significant emerging technologies. It's part of our annual "10 Emerging Technologies" report, which appears in the March/April print issue of Technology Review.

In his psychiatry practice at the Stanford Medical Center, Karl Deisseroth­ sometimes treats patients who are so severely depressed that they can't walk, talk, or eat. Intensive treatments, such as electro­convulsive therapy, can literally save such patients' lives, but often at the cost of memory loss, headaches, and other serious side effects. Deisseroth, who is both a physician and a bioengineer, thinks he has a better way: an elegant new method for controlling neural cells with flashes of light. The technology could one day lead to precisely targeted treatments for psychiatric and neurological dis­orders; that precision could mean greater effectiveness and fewer side effects.