Published online 22 December 2008 | Nature | doi:10.1038/news.2008.1328

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Blind man walking

Man navigates obstacles he can't consciously see.

blind man'Blindsight' can allow people to be aware of their surroundings without consciously seeing.Punchstock

A man with brain damage that makes him clinically blind can navigate an obstacle course, seemingly by using a primitive part of his brain to perceive the objects in his path.

This remarkable ability, discovered through a chance observation, is shedding light on a curious phenomenon known as blindsight.

The man, known as patient TN, was studied by a multinational team led by Beatrice de Gelder at Tilburg University in The Netherlands and Alan Pegna of the Geneva University Hospitals in Switzerland.

The researchers tested TN extensively to confirm that he was completely blind. They used brain imaging to show that there was no activity in his visual cortex, the part of the brain that processes most of the information coming from the retina.

They then persuaded TN to set his stick aside and walk down a corridor strewn with lab equipment. He was able to do so flawlessly, despite being unable to consciously see any of the obstacles. Head down and hands loose by his side, he twisted his body to slalom slowly but surely between a camera tripod and a swingbin, and neatly stepped around a random series of smaller items.

"At first he was nervous," says de Gelder. "He said he wouldn't be able to do it because he was blind." The scientists broke into spontaneous cheers when he succeeded. The results are reported today in Current Biology1.

Subconscious sight

The 56-year-old patient lives in Burundi. He had been a doctor for the World Health Organization before suffering two consecutive strokes in 2003, which, in an unusual coincidence, sequentially wiped out the visual cortex in each of his brain's hemispheres. He had no significant damage to other parts of his brain.

Blindsight requires that a patient's eyes are not damaged. Most studies of blindsight in humans have been conducted in people with damage to the visual cortex in just one hemisphere, making them blind on only one side of their visual field. Tests of blindsight usually involve presenting pictures to individuals' blind side and asking them to guess aspects of the pictures' content. Without being aware of it, they are usually able to discriminate, to a limited extent, characteristics such as colour, the orientation of a rod and emotional expressions on faces.

Scientists believe this happens because a small part of the visual input to the brain from the retina is diverted to more primitive parts of the brain that operate below the level of consciousness.

But such patients seem not to rely on blindsight for navigation, bumping into objects on their blind side. TN is the first patient reported in the literature to have symmetrical elimination of the visual cortex on both sides of the brain.

Pegna had previously shown that TN is able to recognize whether the expressions on faces he can't consciously see are happy or sad2, and wanted to extend the study. De Gelder wanted to test her idea that body language may also be detectable by blindsight. So in May 2007, the team of scientists flew TN over to the Netherlands for a week-long series of experiments. Originally, navigation was not among them. "But we noticed he seemed to be avoiding objects in his path, so we added another simple experiment to find out if he really was sensing them," says Pegna.

Unknown signals

Some blind people can use their hearing to assist in navigation, sensing the reflection of sound waves to help them locate obstacles. "We don't think this is likely to be the answer [in TN] because echolocation is not so efficient for small objects, and he could avoid them with precision," says de Gelder.

Blindsight pioneer Alan Cowey, a neuropsychologist at the University of Oxford, UK, points out that TN was not blindfolded to block out all visual input at any stage of the test, "so it can't be ruled out that he was using the echo of the sound of his own breathing or footsteps".

But if echolocation is not the answer, then TN's blindsight ability shows something very interesting about our brains, Cowey says. "It seems that in the real world, we may be able to use signals in the brain of which we are completely unaware."

The scientists are planning to bring TN back to Europe for further testing. "Being a doctor, he is very aware of the interest his case presents for basic scientists, and wants to help," says Pegna. Other experiments will include looking in more detail at how emotional stimuli are processed at the subconscious level, he adds. 

  • References

    1. de Gelder, B. et al. Curr. Biol. 18, 24 (2008).
    2. Pegna, A. J., Khateb, A., Lazeyras, F. & Seghier, M. L. Nature Neurosci. 8, 24–25 (2004).
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