The video my colleagues and I shot is amazing. A blind man is making his way down a long corridor strewn with boxes, chairs and other office paraphernalia. The man, known to the medical world as TN, has no idea the obstacles are there. And yet he avoids them all, here sidling carefully between a wastepaper basket and the wall, there going around a camera tripod, all without knowing he has made any special maneuvers. TN may be blind, but he has “blindsight”—the remarkable ability to respond to what his eyes can detect without knowing he can see anything at all. [To see the film of the experiment, go to www.ScientificAmerican.com/may2010/blindsight.]
TN’s blindness is of an extremely rare type, caused by two strokes he suffered in 2003. The strokes injured an area at the back of his brain called the primary visual cortex, first on his left hemisphere and five weeks later on the right. His eyes remained perfectly healthy, but with his visual cortex no longer receiving the incoming signals he became completely blind.