In children with amblyopia or "lazy eye," one eye is weaker than the other. As a result, the brain starts to favor input from the stronger eye, causing the weaker eye to lose vision. If amblyopia is caught early enough, putting a patch over the dominant eye teaches the brain to pay attention to the weaker eye, strengthening its vision. Unfortunately, this strategy works best until about age 5 or 6. After that, the "critical period" when the brain can rewire its visual circuits begins to close, and vision loss is hard to reverse. But it turns out certain drugs...