A man left totally blind by a massive stroke navigated a complex maze of boxes, chairs and other objects without stumbling or colliding into any of the obstacles.

Brain scans showed that after suffering two consecutive strokes, the man, 56, lost all function in his visual cortex, the brain's primary vision-processing center. But despite the loss, an international research team (from the U.S. and five other countries) reports in the journal Current Biology that "he could successfully navigate down the extent of a long corridor in which various barriers were placed." 

Neuroscientists call this ability blindsight.  People with blindsight, "usually tell you that they cannot see a thing…. They cannot consciously see but they have some type of awareness," says Susana Martinez-Conde, a neuroscientist at the Barrow Neurological Institute in Phoenix, Ariz.  For instance, she notes, they will correctly guess the number on flashcards more than 50 percent of the time even though their eyesight is shot.

In a person with normal vision, information is passed from the retina (light sensitive area at the back of the eyes) to the visual cortex (the brain's vision center), which relays it to other brain processing areas such as the posterior parietal cortex. In this man's case, the retinas worked perfectly well, but the information highway to the brain was blocked at the visual cortex.

This means that the man must have been using alternative pathways (that bypassed the visual cortex) to connect to the other brain processing regions, Martinez-Conde says, noting that most people likely have these alternate routes but don't rely on them because the dominant visual cortex pathway functions properly.

This study is not the first to document blightsightness, but it is first to describe the phenomenon in a patient who had suffered destruction of the visual cortex in both hemispheres of the brain, according to Nature News.

Over the past several years, scientists have identified brain circuits that may serve as alternative routes, but have yet to pinpoint which ones enable blindsightness, and exactly how they function. But Martinez-Conde says that a combo of electroencephalography (EEG), which measures electrical activity in the brain over time, and functional magnetic resonance imaging (fMRI), which shows blood flow to areas of the brain that are active, may shed light on these circuits.

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