But perhaps the newest and most exciting instrument in the neurologist's tool kit is optogenetics, which was once considered science fiction. Like a magic wand, it allows you to activate certain pathways controlling behavior by shining a light beam on the brain. Incredibly, a light-sensitive gene that causes a cell to fire can be inserted, with surgical precision, directly into a neuron. Then, by turning on a light beam, the neuron is activated. More importantly, this allows scientists to excite these pathways, so that you can turn on and off certain behaviors by flicking a switch. Although this technology is only a decade old, optogenetics has already proven successful in controlling certain animal behaviors. By turning on a light switch, it is possible to make fruit flies suddenly fly off, worms stop wiggling, and mice run around madly in circles. Monkey trials are now beginning, and even human trials are in discussion. There is great hope that this technology will have a direct application in treating disorders like Parkinson's and depression.