Daniel Rios is 24 years old, with wavy black hair, a thick mustache and a glassy stare that seems to look both at you and through you. One day almost four years ago, while he was taking a shower, a blood vessel ruptured in his brain, and he collapsed on the bathroom floor. After emergency surgery, he lay in a coma for three weeks. When he finally opened his eyes, he could not speak or move his body; his head simply lolled. In the months that followed, the doctors monitoring him at the Center for Head Injuries at the J.F.K....