A technique that uses light to control brain cells enables researchers to turn behaviors on and off in lab mice to better understand the human brain and disorders including autism, depression, eating disorders, and more. Karl Deisseroth, M.D., Ph.D, professor of Bioengineering, Psychiatry and Behavioral Sciences at Stanford University's Howard Hughes Medical Institute, developed optogenetics and explains his research and how his findings impact what he sees clinically working with human patients.