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New angle on vision


We judge distance from the ground up.
© Photodisc

Our brains use angular measurements to decide how far away objects are.

Even if trigonometry wasn’t your strong suit in school, your brain uses it constantly. You judge distance by measuring the angle between the ground and your line of sight to an object, a new study shows. The finding could improve the design of robots and artificial vision systems1.

Volunteers who looked through prisms that increased this angle thought objects were closer than they really were, missing them when throwing beanbags or trying to walk to them blindfolded.

Some prism-wearing participants even leaned forward, imagining that the ground was tilted away from them. "They tried to adjust their body perpendicular to the perceived ground surface," says Teng Leng Ooi of the Southern College of Optometry in Memphis, Tennessee, a member of the research team.

After just 20 minutes, volunteers adjusted to the distortion and judged distances correctly. This suggests that the brain possesses a ’plastic’ mechanism to tune its vision system to a constantly changing environment, the researchers say. When volunteers took the prisms off, they temporarily went to the opposite extreme, overestimating distances.

The experiments take a more "ecological" approach to vision processing than a lot of previous research, says Hal Sedgwick, who studies visual perception at the SUNY College of Optometry in New York City. "Rather than thinking of perception as occurring through an empty, abstract space, this looks at it from the point of view of an organism living in an environment, locating objects relative to the ground."

The long view

The idea that humans use the angle with the ground to measure distance is an old one. Ancient Chinese artists drew distant objects higher in the field of view, unlike European artists who generally relied on perspective, in which lines meet at infinity. The eleventh-century Arabic scholar Alhazen, whom some credit with having invented the scientific method, also hypothesized that humans use angles with the ground to judge distances.

Alhazen’s idea faded from attention over the years, and was resurrected only in the middle of the twentieth century, when psychologist James Gibson independently reached the same conclusion while helping to train pilots during World War II. Since then, however, the theory has lacked direct evidence.

For this reason, "this new study is quite important work," says Sedgwick. Ooi and colleagues have, he believes, produced "convincing evidence supporting the ground theory".

Understanding how humans process vision could help engineers to design more realistic virtual-reality systems and build robots that can navigate their environment better, Ooi suggests. It could even help people suffering from brain damage that interferes with their distance estimation, she says. "Research to elucidate space vision should help us predict the problems encountered by brain-injured patients, and to fix their problems through rehabilitation or compensatory robotic devices."


  1. Ooi, T. L. et al. Distance determined by the angular declination below the horizon. Nature, 414, 197 - 200, (2001).

ERICA KLARREICH | © Nature News Service
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