the_newsbeagle writes: With optogenetics, scientists can tag neurons with light-responsive proteins, and then trigger those neurons to "turn on" with the pulse of a light. In the latest application, MIT researchers used light to turn on certain neurons in male mice's hippocampi that were associated with a happy memory (coming into contact with female mice!), and then tested whether that artificially activated memory changed the mice's reactions to a stressful situation (being hung by their tails). Mice who got jolted with the happy memory struggled to get free for longer than the control mice. This tail-suspension test was developed to screen potential antidepressant drugs: If a rodent struggles longer before giving up, it's considered less depressed.