by Katie Oliver

Beliefs about the way visual perception works have undergone some fairly radical changes throughout history. In ancient Greece, for example, it was thought that beams of light emanate from our eyes and illuminate the objects we look at. This "emission theory" ["a href="" target="_blank">PDF] of vision was endorsed by most of the great thinkers of the age including Plato, Euclid, and Ptolemy. It gained so much credence that it dominated Western thought for the next thousand years. Of course, now we know better. (Or at least some of us do: There’s evidence that a worryingly large proportion of American college students think we do actually shoot beams of light from our eyes, possibly as a side effect of reading too many Superman comics.)

The model of vision as we now know it first appeared in the 16th century, when Felix Platter proposed that the eye functions as an optic and the retina as a receptor. Light from an external source enters through the cornea and is refracted by the lens, forming an image on the retina—the light-sensitive membrane located in the back of the eye. The retina detects photons of light and responds by firing neural impulses along the optic nerve to the brain.

There’s an unlikely sounding quirk to this set-up, which is that mechanically speaking, our eyes see everything upside down. That’s because the process of refraction through a convex lens causes the image to be flipped, so when the image hits your retina, it’s completely inverted. Réné Descartes proved this in the 17th century by setting a screen in place of the retina in a bull’s excised eyeball. The image that appeared on the screen was a smaller, inverted copy of the scene in front of the bull’s eye.

So why doesn’t the world look upside down to us? The answer lies in the power of the brain to adapt the sensory information it receives and make it fit with what it already knows. Essentially, your brain takes the raw, inverted data and turns it into a coherent, right-side-up image. If you’re in any doubt as to the truth of this, try gently pressing the bottom right side of your eyeball through your bottom eyelid—you should see a black spot appear at the top left side of your vision, proving the image has been flipped.

In the 1890s, psychologist George Stratton carried out a series of experiments [PDF] to test the mind’s ability to normalize sensory data. In one experiment he wore a set of reversing glasses that flipped his vision upside down for eight days. For the first four days of the experiment, his vision remained inverted, but by day five, it had spontaneously turned right side up, as his perception had adapted to the new information.

That’s not the only clever trick your brain has up its sleeve. The image that hits each of your retinas is a flat, 2D projection. Your brain has to overlay these two images to form one seamless 3D image in your mind—giving you depth perception that’s accurate enough to catch a ball, shoot baskets, or hit a distant target.

Your brain is also tasked with filling in the blanks where visual data is missing. The optic disc, or blind spot, is an area on the retina where the blood vessels and optic nerve are attached, so it has no visual receptor cells. But unless you use tricks to locate this blank hole in your vision, you’d never even notice it was there, simply because your brain is so good at joining the dots.

Another example is color perception; most of the 6 to 7 million cone photoreceptor cells in the eye that detect color are crowded within the fovea centralis at the center of the retina. At the periphery of your vision, you pretty much only see in black and white. Yet we perceive a continuous, full-color image from edge to edge because the brain is able to extrapolate from the information it already has.

This power of the mind to piece together incomplete data using assumptions based on previous experience has been labeled "unconscious inference" by scientists. As it draws on our past experiences, it’s not a skill we are born with; we have to learn it. It’s believed that for the first few days of life babies see the world upside down, as their brains just haven’t learned to flip the raw visual data yet. So don’t be alarmed if a newborn looks confused when you smile—they’re probably just trying to work out which way up your head is.