10 Illusions that Hijack your Brain

10 years ago I wrote a book for O’Reilly that remains one of my favorites to this day. Your Brain: The Missing Manual is a fun romp through the tangled passageways of your brain. Here I’ve excerpted some of its discussion about how optical illusions work, including a homemade color illusion that makes the infamous blue dress look tame.

Your brain is a reality-construction machine. It takes the oceans of information that flood your senses, and transforms them into a subjective inner world.

This inner world has a few things in common with outside reality, but less than you’d think. It’s run by a processing system that’s quick to jump to conclusions, confidently ignorant of its mistakes, and easily fooled. This processing system sees what it expects to see, hears what it expects to hear, and petulantly refuses to be corrected on even the simplest point. You may enjoy this world or you may not. However, you’ll never get a chance to step out of your head and take a clear look at what’s really happening outside.

The Doors of Perception

It’s tempting to divide the brain’s information processing into two neat categories: conscious (what you know you see and hear) and subconscious (what your brain deals with automatically, behind the scenes). After all, you don’t consciously perceive the inner ear signals that ensure you stay balanced while navigating an intricate dance routine, but you are acutely aware of the crushing heel that your dance partner just placed on your big toe.

However, if you dig a little deeper into the brain’s jelly-like matter you’ll quickly find that it’s a little bit like sharing an apartment with a group of freewheeling friends — there’s a lot more going on then you realize (and a fair bit more than you’d probably agree to). Basic avenues of perception that you take for granted, like seeing, hearing, and touch, are actually colored by layers and layers of the brain’s automatic preprocessing. In other words, your brain expects the world to behave in certain ways, and it subtly shapes your perception to fit.

This automatic processing sounds a bit suspicious, but you’d be ill advised to turn it off (and short of heavy quantities of illegal pharmaceuticals, there’s no way you could). Most people don’t want to spend minutes thinking about shapes, illuminations, and perspective just to watch their favorite television show. Similarly, they don’t want to go through a painstaking process of logical deduction to determine if the object they’re looking is a person and, furthermore, if it is in fact their spouse (as memorably described in Oliver Sacks’ The Man Who Mistook his Wife for a Hat).

Optical Illusions

One of the most fascinating ways to size up the workings of the brain is by exploring optical illusions, the strange images that aren’t quite what they seem to be. To a certain extent, all optical illusions work by exploiting a chink in the brain’s visual processing systems — an automatic assumption that doesn’t always hold true. However, there’s an amazing diversity in the way these illusions work. You can easily line up a dozen different optical illusions and find that each one relies on a different trick to short-circuit the brain.

Some of the simplest illusions work by overstimulating some part of the brain’s visual processing system. Conceptually, their effects are like the afterimage you get when you stare into the sun (against your mother’s advice).

One example of this phenomenon is found in the twinkling lights that you’ll see at the corners of the grid shown below. The corners are occupied with white circles, but as your eyes move around you’ll see black circles flash into existence.

As with many optical illusions, it’s difficult to pinpoint exactly what goes wrong in your brain when you look at the grid. However, part of the brain’s strategy when picking out shapes involves emphasizing edges and contrasts. In high-contrast images like this grid and the slanted lines shown below, the effect can be pumped up to dizzying proportions.

Your Shifty Eyes

Some of the most captivating optical illusions are those that involve imaginary motion, like the pattern of dots shown below.

To really understand this illusion, you need to realize that your eye has a dirty secret — it’s only able to see fine detail in a small fragment of its visual field. The pinpoint-sized part of your eye that sees sharply is called the fovea. If you look at a person an arm’s length away, the fovea gives you a sharply detailed region that’s about the size of a dime.

Your brain uses a crafty trick called saccades to compensate for this weakness. Saccades are quick, automatic eye movements. They’re keenly important for reading articles like this one, and they’re equally indispensable for taking in the full detail of a visual scene. On average, your eye performs two or three saccades each second, ricocheting about your visual field without you even realizing it, each time capturing the fine detail of another tiny region. Inside your brain, these separate dime-sized pictures are pasted together to create a single, seamless whole.

With this in mind, the drifting dots you saw earlier are easier to understand. As your eyes jump from one circle to the next, trying to stitch together the complete picture, your brain is confused by the alternate shading. After each saccade, the previously viewed dots aren’t quite where your brain expects them to be, and so it assumes that they’ve shifted ever-so-slightly to the side. This creates the impression of motion.

One of the most famous illusions to take advantage of our shifty-eyed nature is the rotating snakes illusion, created by Akyioshi Kitaoka. The effect is strongest out of your peripheral vision.

If you’re severely drunk, your saccades slow down, and you start to see the world the way your eye really perceives — a patch of sharpness surrounded by a blurry field.

Distortions and Mismeasurements

Many of the most familiar optical illusions are distortions. They take advantage of the brain’s assumptions to skew the way you perceive contours, lengths, colors, and shading.

For example, the long diagonal lines in the following picture are perfectly parallel. However, the pattern of cross marks in the line fools your brain into thinking they lean toward one another.

Here, your brain is confused by the angles that aren’t quite what it expects. It’s as if your brain expects the hatch marks to cross each line at a right angle. You can almost feel your brain mentally twisting the lines to make them fit its expectation.

The next image shows a series of concentric circles.

Here your brain is locked into a different interpretation, and insists on seeing a spiral. (Trace your finger around one of the circles if you don’t believe it’s concentric.)

The remarkable part of both these illusions isn’t the reason your brain is fooled — after all, its mistaken logic is reasonable and (more importantly) it’s blindingly fast. The amazing part is that even if you carefully measure the angle of the slanted lines or trace out the circles, thereby proving the illusion, you still can’t convince your brain that it’s made a mistake. In fact, no amount of pleading can convince your brain to alter its wonky interpretation.

Color Confusion

Your brain can easily make similar mistakes when comparing brightness or colors. In the carefully set up illusion below, my daughter holds a Rubik’s cube under two types of lighting. In the center of the front face of the cube is a square that appears to be either yellow (in the left picture) or blue (in the picture on the right). However, the color is actually identical in both — it’s the dull shade of gray that’s shown in the bar bellow.

In this illusion, the brain isn’t exactly wrong — it’s simply compensating for what it believes is a difference in lighting. It concludes that a square that appears gray under blue light is probably yellow, and a square that appears gray under a yellow light is probably blue. In other words, your brain’s perception has a built-in routine for evaluating lighting conditions. This is the reason you can see quite normally at home in the evening, even though the artificial lights you’re using cast a yellow-red shade of light that’s dramatically different than the blue-tinted radiance of the sun at noon.

Seeing Things

One of the hardest challenges for the brain’s visual systems is picking out shapes. It’s an extraordinarily difficult task. Not only can shapes be moved, rotated, resized, distorted, and obscured, but they can also exist in an endless number of variations.

The brain deals with this problem using a toolkit of assumptions. And the brain deals with it very well — it can easily best computerized shape-spotters when scanning pictures, faces, and moving scenes. However, the brain’s eagerness to find shapes also leads it to find shapes when none are present, as with the white triangle of the forefront of the following picture.

When confronted with this picture, your brain doesn’t need to conjure up a white triangle. There’s a reasonable alternate explanation — that the image contains three pacman-like circles with wedges cut out of them, and the wedges are lined up with the gaps between the blue triangles inside. However, a just-so arrangement like this would be unlikely in the natural world, so your brain quickly dismisses that possibility. In essence, your brain picks up on a few clues and performs a rapid analysis to determine the most likely explanation. However, you don’t merely think about that most likely explanation, you also see it.

If you rotate the pacmen around, the illusion disappears, and the image reverts to a collection of harmless shapes.

This hints at one of the key limitations of vision. Our brains are tuned to see what’s mostly likely in the ordinary, natural world. However, we haven’t caught up with the way that man-made products can deliberately hijack these assumptions. In other words, our natural-born visual senses set us up to be the perfect dupes in a world filled with man-made objects.

The brain’s obsessive pattern matching isn’t limited to shapes. It happens with faces (which we see in unlikely places like house fronts) and speech sounds (for example, if parts of a word are beeped out in a recording, we “hear” the full word based on what makes sense in the context of a sentence). It happens when an over-enthusiastic home cook finds an image of Jesus in a burnt grilled cheese sandwich.

The brain is also primed to identify letters and spot words. Can you read this sentence?

If you said I LIKE IUMRING TQ GQNGIUSIQNS, you have a perverse sense of humor. You’re also correct.

Other Types of Assumptions

You’ve no doubt seen illusions that use ambiguous pictures, which can be interpreted in different ways. One legendary example is the two faces and vase, shown below.

This sort of automatic interpretation is obvious with contours and shapes, but it can also apply to more complex meanings that aren’t specifically related to vision.

For example, consider the following figure, which was the subject of a cross-cultural study. If you were asked to express this scene in a couple of sentences, how would you describe it?

Most Westerners describe this scene pretty plainly. There’s a group of people gathered in discussion (possibly a family), there appears to be a window on the left above one of the women, and the shading of the floor and corner of the wall make it clear that everyone is gathered indoors. But these obvious “facts” aren’t quite as obvious to a different sort of people, with different experiences and hence different assumptions engraved in their brains.

When researches showed this picture to East Africans, nearly all of them said the woman on the left was balancing a box on her head. The corner of the room in the back was interpreted as a tree, under which the family is sitting. Now if you look at the figure again, you’ll probably agree that this interpretation makes just as much sense as your own. As a Westerner who has spent much of your life indoors, your brain is used to interpreting scenes using the boxlike shapes and angular cues of modern architecture (like windows and the corners of walls). Rural East Africans have a different store of experience to bring to bear on new scenes. All this shows that a surprising amount of higher-level reasoning can leak into processes like hearing and seeing, and color the results without you even realizing what’s taking place.

Have a favorite illusion? Share it in the comments below! Or, check out the full book, which tackles the brain science of love, aging, emotions, memory, and creative thinking, at https://www.amzn.com/0596517785