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3-D effects may require one eye only

Peering through a peephole can bring flat images to life

3D VISION  Photos take on three dimensions when viewed through a small hole with one eye. The effect is conventionally thought to require two eyes.

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Rather than donning special glasses and paying extra to see a movie in 3-D, try sitting through a regular showing with one eye closed. A new study suggests that viewers can see flat images in three dimensions by peering through a small hole with just one eye, though its methods have drawn criticism.

Researchers have long thought that the immersive effects of a 3-D movie result from binocular vision. As far back as the 1830s, scientists knew that a picture could spring to life when presented as two separate images, one for each eye. The prevailing view is that this 3-D effect occurs when the brain merges the two slightly disparate images into one. Modern 3-D movies continue to capitalize on this phenomenon.

Scientists had previously reported 3-D vision using only one eye, but those findings were largely dismissed partly because the researchers themselves were the test subjects who saw the 3-D effect, says Dhanraj Vishwanath of the University of St. Andrews in Scotland.

To better test whether monocular vision can yield a 3-D experience, Vishwanath and St. Andrews colleague Paul Hibbard presented 23 people with photographs depicting different objects or scenes. Twenty participants reported seeing a photo in three dimensions when peering through a pea-sized hole with only one eye, the researchers reportin the September Psychological Science.

Participants saw in 3-D only when the hole prevented them from seeing the edges of the picture. Vishwanath speculates that when viewing an entire picture with both eyes, the brain combines the eyes’ images to compute the distance between the observer and the flat surface of the picture. But when the eye can see only objects within the picture, the brain might instead try to figure out the distance between the pictured objects and the observer. This could create a 3-D effect, Vishwanath says.

Not everyone is convinced. “All of the experiments asked only subjective questions about perceived depth,” says Richard Born, a neuroscientist at Harvard Medical School who studies the visual cortex.

Lloyd Kaufman, a cognitive scientist who studies visual perception at New York University, agrees. He says quantitative measurements are needed to convince him a one-eyed 3-D effect is as strong as that achieved from two eyes.

But Vishwanath says that the 3-D effect is itself a qualitative phenomenon, and so it could not have been assessed in any other way. He says that researchers’ reluctance to ask observers what they see has hampered understanding of the phenomenon. Vishwanath and colleagues are now testing the monocular 3-D effect in people with a condition called strabismus in which the eyes don’t align properly and thus don’t form binocular 3-D images.

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