When the brain goes awry, it can reveal to us some clues to how it works in all of us. In my latest “Matter” column for the New York Times, I look at a rare but fascinating disorder that causes people to hallucinate music. How someone could imagine that a piano was playing nearby–or a marching band or church choir–may tell us something about how our brains make sense of the world by making predictions about what comes next. Check it out.