Buzz
by Katie Oliver
Throughout history, our understanding of how visual perception works has drastically evolved. In ancient Greece, for instance, it was believed that beams of light radiated from our eyes, illuminating the objects we observed. This "emission theory" ["a href="https://web.archive.org/web/20111008073354/http://conference.nie.edu.sg/paper/Converted%20Pdf/ab00368.pdf" target="_blank">PDF] of vision was widely accepted by great philosophers like Plato, Euclid, and Ptolemy. It was so influential that it shaped Western thinking for over a millennium. Today, however, we know better. (Or at least some of us do: There’s evidence suggesting that a disturbing number of American college students still believe we shoot beams of light from our eyes, possibly influenced by reading too many Superman comics.)
The modern understanding of vision as we know it began to take shape in the 16th century when Felix Platter proposed that the eye functions as an optical device, with the retina serving as a receptor. Light enters through the cornea and is bent by the lens, creating an image on the retina—the light-sensitive layer at the back of the eye. The retina detects light photons and sends neural signals via the optic nerve to the brain.
An interesting oddity about our visual system is that, from a mechanical perspective, our eyes actually see everything upside down. This is due to the process of refraction through the convex lens, which flips the image. By the time the image reaches the retina, it is entirely inverted. Réné Descartes proved this in the 17th century by replacing the retina with a screen in a bull’s excised eyeball. The image that appeared on the screen was a smaller, upside-down version of the scene in front of the bull’s eye.
So why doesn’t the world appear upside down to us? The answer lies in the brain’s ability to adjust sensory input and align it with prior knowledge. Essentially, your brain takes the inverted information and processes it into a correct, right-side-up image. If you’re uncertain about this, try gently pressing on the bottom right side of your eyeball through your eyelid. You should see a black spot appear in the top left corner of your vision, showing that the image has been flipped.
In the 1890s, psychologist George Stratton conducted a series of experiments [PDF] to explore the mind’s ability to normalize sensory information. In one experiment, he wore glasses that reversed his vision for eight days. During the first four days, his vision remained upside down, but by day five, it spontaneously corrected itself as his brain adapted to the new sensory input.
That’s not the only fascinating ability your brain possesses. The image that strikes each of your retinas is a flat, 2D representation. Your brain must merge these two images into a single, unified 3D image, providing you with accurate depth perception that allows you to catch a ball, sink a basket, or hit a distant target.
Your brain also works to fill in gaps where visual information is absent. The optic disc, or blind spot, is a region on the retina where the blood vessels and optic nerve are connected, and it lacks visual receptor cells. However, unless you actively search for this empty spot, you would never notice it because your brain is so adept at completing the picture.
Another example is color perception; the majority of the 6 to 7 million cone photoreceptor cells responsible for detecting color are concentrated in the fovea centralis, located at the center of the retina. At the edges of your vision, you primarily see in black and white. However, we experience a continuous, full-color view from one edge to the other because the brain can extrapolate from the existing information it receives.
This mental ability to piece together incomplete information using assumptions derived from past experiences is called "unconscious inference" by scientists. Since it relies on our previous encounters, it’s not an innate ability; we must learn it. It is believed that during the first few days of life, babies perceive the world upside down because their brains haven’t yet learned to flip the raw visual data. So don’t be surprised if a newborn looks perplexed when you smile—they are likely trying to figure out which way your head is oriented.
