LA JOLLA—Say you’re reaching for the fruit cup at a buffet, but at the last second you switch gears and grab a cupcake instead. Emotionally, your decision is a complex stew of guilt and mouth-watering anticipation. But physically it’s a simple shift: instead of moving left, your hand went right. Such split-second changes interest neuroscientists because they play a major role in diseases that involve problems with selecting an action, like Parkinson’s and drug addiction.