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Is it possible to see something without knowing you can see it? Maybe that's not so hard to imagine if you think of subliminal images flashed for a frame or two on a movie screen—too quickly for you to see consciously but perhaps long enough to add a frisson of fear. (Those frames in The Exorcist don't count—if you can notice them, they're not subliminal.)

But what about seeing something when you think you are totally blind? What about navigating around obstacles that you cannot see and aren't even expecting?

That's a feat which was performed by a man known to the medical world as patient TN. In 2003, TN lost use of a brain region called the primary visual cortex, which is responsible for processing the visual information that goes on to form our conscious sight. TN's injury was an extraordinary piece of bad luck, taking two successive strokes to knock out the region in both his left and right hemispheres.

After his strokes, ordinary tests of TN's sight turned up nothing, not even an ability to detect large objects moving right in front of his perfectly healthy eyes. But researchers had hints that TN might exhibit blindsight, a weird ability to respond to visual information despite having no conscious knowledge of seeing anything. They realized TN's total cortical blindness presented a rare opportunity, and in 2008 they decided to try out a bold but breathtakingly simple experiment.

They took him to a hallway and asked him to walk along it without his white cane. TN was reluctant, but they finally persuaded him to try. After all, how bad could it be, even being blind, to walk slowly down an empty corridor?

The man right behind TN in the video is Lawrence Weiskrantz of Oxford University. Weiskrantz coined the word "blindsight," and he has been a leading figure in researching the phenomenon for over 40 years—early on, in the face of widespread skepticism. This experiment with TN was inspired by one that Weiskrantz and his then student Nicholas Humphrey carried out in the early 1970s with a monkey with no primary visual cortex. Some of that experiment is also on video.

You can read more about blindsight in the May issue of Scientific American, in the article, "Uncanny Sight in the Blind" by Beatrice de Gelder, who was the lead researcher on the TN navigation experiment. De Gelder has also explored how blindsight can detect the emotion expressed on a person's face (but, curiously, not a person's identity or even their gender) and which neural pathways in the brain are responsible for blindsight (it involves the subcortex, which is more primitive than the cortex in evolutionary terms).

Beatrice M. L. de Gelder is a cognitive neuroscientist and neuropsychologist. She is professor of Cognitive Neuroscience and director of the Cognitive and Affective Neuroscience Laboratory at the Tilburg University, and was senior scientist at the Martinos Center for Biomedical Imaging, Harvard Medical School,

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