The video above seems completely unremarkable at first – man walks down a corridor, navigating his way around easily visible and conspicuous obstacles. But it’s far from an easy task; in fact, it should be nigh-impossible. The man, known only as TN, is totally blind.

His inability to see stems from a failure in his brain rather than his eyes. Those work normally, but his visual cortex – the part of the brain that processes visual information – is inactive. As a result, TN is completely unaware of the ability to see and in his everyday life, he behaves like a blind person, using a stick to find his way around. Nevertheless, he can clearly make his way through a gauntlet of obstacles without making a single mistake.

TN was a doctor before two successive strokes destroyed his ability to see. The first one severely damaged the occipital lobe on the left side of his brain, which contains the visual cortex. About a month later, a second stroke took out the equivalent area on the right hemisphere. TN is one-of-a-kind, the only known patient with damage like this in the entire medical literature. The fibres that connect the occipital lobes on the right and left halves of the brain have also been severely damaged and tests reveal that no blood flows between these disconnected areas.

Alan Pegna from the University of Bangor in Wales was the first to study TN’s abilities after he was recovering from this second stroke in a Swiss hospital. Pegna was the first to discover TN has an ability called blindsight, that allows him to unconsciously detect things in his environment without any awareness of doing so. He could correctly guess the emotions playing across the faces of other people. And as he did so, his right amygdala – an area of the brain involved in processing emotions – became active.

His visual cortex is a different story. This area, which normally lights up like a beacon when people view the world around them, failed to do so in TN’s brain. With his consent, Beatrice de Gelder from Tilburg University put him through a series of visual tasks and scanned his brain using several different techniques. To a one, all of these tests failed to show any trace of activity in the visual cortex. For example, he repeatedly failed to spot bright circles of static or flickering light, displayed against a dark screen.

But not all the tests were letdowns. When de Gelder showed him a metre-long rod attached to a wall, he was able to tell whether it was horizontal, vertical or diagonal with 100% accuracy. He could discriminate between any two orientations of the rod, as long as they were about 25 degrees apart. It was a small victory, but an encouraging one and, actually, another famous blindsight patient called DB also did well on the same test.

In light of these results, de Gelder decided to see how TN would fare in a more realistic setting. Her team created the obstacle course of bins, boxes and tripods arranged randomly along a long corridor. To everyone’s astonishment, TN negotiated it perfectly, never once colliding with an object despite lacking both guide and walking-stick. An experimenter followed him at all times in case he fell, but it never happened. The first time he completed the course, the assembled researchers burst into applause.

How does he do it? Some scientists have suggested that blind or blindfolded people can use sound to guide their movements, using an extremely crude version of the sophisticated echolocation wielded by bats. But de Gelder says that it’s a very remote possibility – during TN’s walk down the corridor, neither he nor the experimenter behind him made any noises loud enough to produce helpful echoes. And while echoes may be useful for large objects or spaces, humans aren’t sensitive enough to them to be able to avoid smaller objects, as TN clearly does in the video.

TN’s ability is unique among humans, but not among all animals. In 1974, Nick Humphrey at Cambridge University found that a monkey called Helen, who also had lesions in both her visual cortices, could also avoid obstacles in an open space. In Helen’s case, Humphrey thought that her skill might stem from a small undamaged part of her left visual cortex.

But TN’s more comprehensive damage rules that explanation out for him. Instead, de Gelder thinks that other parts of the brain outside of the visual cortex may be responsible. When de Gelder asked TN to look at pictures of bodies and faces and say if they were the right way up or upside-down, he only answered correctly at levels expected by chance. However, as he was looking at the images, electrodes recorded some activity in the left part of his brain, in areas near, but not within, the visual cortex.

This time round, TN failed to guess the expression on happy or fearful faces put in front of him, as he did in Segna’s study. The researchers said that he seemed less willing to make rapid, spontaneous guesses and instead gave answers after much time and effort. But again, electrodes detected activity in the left half of the brain, near the back. Some mental cogs were indeed whirring to life.

The identities of these other pathways, which allow TN to be aware of his surroundings without any conscious sight, remain a mystery. But, perhaps, not for much longer.

Reference: Current Biology in press