The advantage of using two eyes to see the world around us has long been associated
solely with our capacity to see in 3-D. Now, a new study from a scientist at
Rensselaer Polytechnic Institute
has uncovered a truly eye-opening advantage to binocular vision: our ability
to see through things.
Most animals have sideways-facing eyes that allow for a panoramic view of nearly all that's around them, both in front and behind. Credit: Rensselaer/Changizi
Most animals — fish, insects, reptiles, birds, rabbits, and horses, for
example — exist in non-cluttered 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 of and behind itself, 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-facing 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, who says eyes facing the same direction have been selected for
maximizing 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 for 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.
To demonstrate how our eyes allow us to see through clutter, hold up all of
your fingers in random directions, and note how much of the world you can see
beyond them when only one eye is open compared to both. You miss out on a lot
with only one eye open, but can see nearly everything behind the clutter with
"Our binocular region is a kind of 'spotlight' shining through the clutter,
allowing us to visually sweep out a cluttered region to recognize the objects
beyond it," says Changizi, who is principal investigator on the project.
"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
To identify which animals have this impressive power, Changizi 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 6-inch wide leaves in the
forest) — tended to have sideways-facing eyes.
"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," says 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."
However, in cluttered environments — which Changizi defined as leafy
surroundings where the cluttering objects are smaller than the separation between
an animal's eyes — animals tend to have a wide field of binocular vision,
and thus forward-facing eyes, in order to see past leaf walls.
"This X-ray vision makes it possible for animals with forward-facing eyes
to visually survey a much greater region around themselves than sideways-facing
eyes would allow," says Changizi. "Additionally, the larger the animal
in a cluttered environment, the more forward facing its eyes will be to allow
for the greatest X-ray vision possible, in order to aid in hunting, running
from predators, and maneuvering through dense forest or jungle."
Changizi says human eyes have evolved to be forward facing, but that we now
live in a non-cluttered environment where we might actually benefit more from
"In today's world, humans have more in common visually with tiny mice
in a forest than with a large animal in the jungle. We aren't faced with a great
deal of small clutter, and the things that do clutter our visual field —
cars and skyscrapers — are much wider than the separation between our
eyes, so we can't use our X-ray power to see through them," Changizi says.
"If we froze ourselves today and woke up a million years from now, it's
possible that it might be difficult for us to look the new human population
in the eyes, because by then they might be facing sideways."