It had been more than a decade since B.T." had last seen anything. After suffering a traumatic accident as a young woman, doctors diagnosed her with cortical blindness, caused by damage to the visual processing centers in her brain. So she got a seeing eye dog to guide her and grew accustomed to the darkness. Besides, B.T. had other health problems to cope with - namely, more than 10 wildly different personalities that competed for control of her body. It was while seeking treatment for her dissociative identity disorder that the ability to see suddenly returned. Not to B.T., a 37-year-old German woman. But to a teenage boy she sometimes became. With therapy, over the course of months, all but two of B.T.'s identities regained their sight. And as B.T. oscillated between identities, her vision flicked on and off like a light switch in her mind. The world would appear, then go dark. Writing in PsyCh Journal, B.T.'s doctors say that her blindness wasn't caused by brain damage, her original diagnosis. It was instead something more akin to a brain directive, a psychological problem rather than a physiological one. B.T.'s strange case reveals a lot about the mind's extraordinary power - how it can control what we see and who we are. B.T. exhibited more than 10 personalities, each of them varying in age, gender, habits and temperament. They even spoke different languages: some communicated only in English, others only in German, some in both.