Human eyes evolved for x-ray vision

New York, Aug 29 : The use of both eyes to view the world around us has long been associated with a 3-D vision. But a new study by Rensselaer Polytechnic Institute has uncovered an eye-popping advantage to binocular vision: our ability to see through things.

Most creatures like fish, insects, reptiles, birds, rabbits, and horses, for example, exist in uncluttered environments like fields or plains, and they have eyes located on either side of their head.

These sideways-facing eyes allow an animal to see in front and behind, an ability also known as panoramic vision.

Humans and other large mammals - primates and large carnivores like tigers, for example - exist in cluttered environments like forests or jungles, and their eyes have evolved to point in the same direction.

While animals with forward looking eyes lose the ability to see what's behind them, they gain x-ray vision, according to Mark Changizi, assistant professor of cognitive science at Rensselaer.

He said eyes facing the same direction have been selected for maximising our ability to see in leafy environments like forests.

All animals have a binocular region - parts of the world that both eyes can see simultaneously - which allows x-ray vision and grows as eyes become more forward facing.

Demonstrating our x-ray ability is fairly simple: hold a pen vertically and look at something far beyond it. If you first close one eye, and then the other, you'll see that in each case the pen blocks your view. If you open both eyes, however, you can see through the pen to the world behind it.

"As long as the separation between our eyes is wider than the width of the objects causing clutter - as is the case with our fingers, or would be the case with the leaves in the forest - then we can tend to see through it," Changizi said.

To identify which animals have this impressive power, he studied 319 species across 17 mammalian orders and discovered that eye position depends on two variables: the clutter, or lack thereof in an animal's environment, and the animal's body size relative to the objects creating the clutter.

Changizi discovered that animals in non-cluttered environments - which he described as either "non-leafy surroundings, or surroundings where the cluttering objects are bigger in size than the separation between the animal's eyes" (think a tiny mouse trying to see through six-inch wide leaves in the forest) - tended to have sideways-facing eyes.

Most animals have sideways-facing eyes that allow for a panoramic view of nearly all that's around them, both in front and behind.

"Animals outside of leafy environments do not have to deal with clutter no matter how big or small they are, so there is never any x-ray advantage to forward-facing eyes for them," said Changizi.

"Because binocular vision does not help them see any better than monocular vision, they are able to survey a much greater region with sideways-facing eyes."

--IANS