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Seeing without Sight

July 7, 2010

Brit Trogen

Science in Seconds blog brit trogen

 

Two years ago, a patient identified only as "TN" made headlines for a feat of superhuman ability: TN could see. 

 

What sounds mundane was in fact quite astonishing. After having been rendered clinically blind by a pair of strokes that selectively damaged the visual cortex of his brain, TN was able to successfully navigate a complex obstacle course without once colliding with an obstacle. What's more, his brain would respond to facial expressions with the appropriate amygdalal response. But most bizarre was that all of this occurred without the conscious awareness of the subject: TN didn't realize he was stepping around an object, or responding to a smiling face, and spent the rest of his life living like a blind man, cane in hand. He was one of the first mainstream cases of blindsight.

 

Blindsight is exactly what it sounds like: the response to visual stimuli by those who would normally be classified as "blind," often due to damage in the visual system of the brain. But even after years of knowing about the existence of this phenomenon in humans and animals, until very recently there's been little evidence for its physical basis. But for the first time, a real explanation of this mysterious occurrence has been revealed. 

 

The answer lies in the brain. One popular theory for a mechanism of blindsight has been that while conscious vision passes through the visual cortex, an additional unconscious visual pathway exists through an alternative route, but no one knew where that might be. Using fMRI, this new study has now shown that one area is more likely than any other to be the region responsible for this strange event. Called the lateral geniculate nucleus (LGN), it was initially discovered for being activated in monkeys who were unconsciously using blindsight in recognizing visual stimuli. The researchers then found that by inhibiting this region with a chemical called THIP, blindsight responses were selectively lost.

 

While this is only one study, the implications are pretty cool. How much of our perception could rely on regions like this that we have yet to identify? When it comes to vision, we're quick to believe what we see. But maybe discoveries like these will help us to realize that what's right before our eyes could be much more than it appears.

 

doi:10.1038/nature09179

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