Optical illusions are interesting. Visually, they’re a piece of sheer confusion. (It’s rarely the case our brain literally can’t make sense of what we’re seeing.) But more than that, what makes them so compelling is the fact that even when you know why the illusion works, it still works.

Think about it. Few forms of entertainment based on deception have such staying power. Stage magic, while exciting and perplexing, depends upon the audience being in the dark. Once you learn the secret, the magic (again, quite literally) is gone. Optical illusions are different, because there’s actually no deceit at play. Our brains are fooling themselves.

Seeing is Believing

You may not know them by name, but nearly everyone’s seen a version of the Ponzo illusion and the Ebbinghaus illusion, two versions of effectively the same design: Two objects or lines appear different sizes because the context surrounding each item distorts its relative size. There’s also the raft of illusions based on some trickery of light. Consider the Checker shadow illusion, which suggests two gray squares are different shades because they sit in varying degrees of shadow. In reality, the shades of gray are identical.

In both cases — and among all optical illusions — the basic effect pits your brain’s expectations against what it actually sees. One of the most elegant demonstrations of this has been the emergence of 3D tattoos, which fool our brains even further as they take the pixels of a digitized illusion and extract them for real-life displays of disbelief. The spider on a man’s hand doesn’t just look gruesome; it looks like it could crawl up his arm at any second. The “gaping wound” looks like it needs real medical attention.

3D tattoos rely on contrast and shadow to make the image look as if it could literally jump off the skin. Photo courtesy of Vitaliy Arcofam. Vitaliy Arcofam, CC BY-NC-ND 2.0

"I think 3D tattoos are so new to us that they are still jarring," Rocky Rakovic, the editor of Inked magazine, told Newsweek. "Our brains are accustomed to process the idea of art on skin, but not yet for art jumping off skin."

A wolf claws right through a man's arm in this 3D tattoo. At least, that's the most logical story your brain can come up with. Photo courtesy of Tattoo_Lover. Tattoo_Lover, CC BY 2.0

The Secrets of the Effect

What makes optical illusions so powerful? Psychologists speculate the answers lies with color — and the details that mattered to our species thousands of years ago. Black and white renderings of our environment conceal a wealth of information about where predators lie. Shadows mask danger, and the total absence of light threatens us even more. Dr. R. Beau Lotto, a neuroscience researcher at the Institute of Ophthalmology in London, says our sense of color evolved to keep us safe.

“The light that falls onto our eyes is determined by multiple things in the world, not only the color of objects, but also the color of their illumination and the color of the space between us and those objects,” Lotto explains. “You vary any one of those parameters and you change the color of the light that falls onto your eye.”

The implication here is that any image could be the result of a number of real-world sources. Lotto offers the example of two surfaces, one yellow and one orange, that when viewed at different angles, through different media, with varying light sources, hit your retinas as the same color. What you see, in other words, is meaningless, because the source of that image could be anything.

Optical illusions are the silly, yet profound, result of that understanding. A three-pronged fork whose tines are impossibly drawn registers as a fork even when the details are lost in translation. Two gray squares register as different shades when they’re put in a context we’re familiar with, and one which tells us to necessarily change their appearance. The simple knowledge that both squares are the same color does nothing to change the millennia of hard-wiring telling us otherwise.

What Seeing is Good for

In reality, the potency of optical illusions reveals at once how fearful the brain is of getting things wrong and how easily it’s duped. Past experiences dictate what it should anticipate, so it adjusts its settings to be overly cautious — it may be smart, but not smart enough to know a checker square from a jungle cat.

To further demonstrate this, Lotto presents the checker illusion in a different setting: two identical gray squares, each set against either a black or white border. “What’s significant is not simply the light and dark surrounds. It’s what those light and dark surrounds meant for your behavior in the past.”

The reason we like optical illusions, and 3D tattoos that both make our skin crawl and pique our most curious sense of intrigue, is that our greatest intuitions — despite giant leaps in our abilities to produce thought — still can’t tell two colors apart. And this, on its most playful level, feels violating. Yet we can’t help but be drawn to it, and Lotto argues there’s a reason.

“The brain didn’t actually evolve to see the world the way it is,” he submits, “We can’t. Instead, the brain evolved to see the world in the way that it was useful to see. And how we see is by continually redefining normality” — whether that’s to keep us safe in the wild or simply to discriminate between light and dark squares, which, to the great frustration of your brain, happen to be the same.