"T.N., as the blind man is known, suffered a stroke in 2003 which destroyed an area at the back of his brain that processes visual information: the primary visual cortex. The stroke affected only one hemisphere of his brain. What places TN in a category of his own, at least as far as the annals of science are concerned, is that about a month later he suffered a second stroke which wiped out the primary visual cortex on the other side of his brain. Suddenly, though his eyes were healthy, he became blind. TN’s blindness is unusual, however, because he can still see in some situations, although he is unaware that he does so—a phenomenon known as blindsight."