Gary Marcus critiques the notion "that you can’t do certain things—like learn a language, or learn an instrument—unless you start early in life." A medical example:
Amblyopia is a visual disorder in which the two eyes don’t properly align; sometimes it’s called “lazy eye.” The standard medical advice is to treat your child early, by getting them to wear an eye patch over the good eye (in order to strengthen the weak one). If you don’t treat the problem early, you can just forget about ever fixing it. Just after my book went to press, however, Dennis Levi, the dean of the School of Optometry at Berkeley, conducted a brilliantly simple study that was easy to conduct, yet would have seemed like a waste of time to anybody steeped in critical-period dogma. Levi and his collaborator stuck eye patches on the good eye of adult amblyopics, aged fifteen to sixty-one, whom everyone else had written off on the presumption that they could not learn anything new. He then set his subjects down at a video game—a first person shooter called Medal of Honor: Pacific Assault, to be exact—and told them to have fun. Levi found that his subjects got better at virtually every aspect of visual perception he could measure. It wasn’t that it was too late for adults to overcome amblyopia, it was that the myth of critical periods had kept people from trying.