The Nobel Prize in medicine has been awarded to three scientists whose work led to a tool that uses light to help unravel how the brain works. Karl Deisseroth, Peter Hegemann, and Georg Nagel have researched light-gated ion channels and optogenetics. They are based in Germany and the United States. The Nobel Assembly praised their method, noting its global use in revealing brain mysteries. The prize was announced Monday at the Karolinska Institute in Stockholm. This year’s prize money is 12 million Swedish kronor, or about $1.2 million. The physics prize follows on Tuesday, with chemistry and literature later in the week.