Science Facts

What Causes Optical Illusions? – Images & Facts

Optical Illusions

Optical illusions are some of the fascinating visual experiences of humans. Whether natural or artificial, they take biological assumptions that we use every day to survive and turn them upside down. People have been squinting and staring at optical illusions for centuries, but scientists still don’t fully understand why certain combinations of shapes and lines mess with our minds.

What they do know is that these distortions probably come from how brains interpret the images rather than how eyes see them. Each eye projects a slightly different view of the world that the brain then combines to create a 3D perception. These are optical illusions, images that somehow trick our brains into perceiving something different from what exists in reality.

What causes optical illusions?

Sight is simply a perception of reality that assume to be true because practically it works. Every vision is actually reflected light waves, flipped upside down, reversed, then converted into electrical impulses, reinterpreted, and finally projected as visual consciousness. This whole process takes a little less than a tenth of a second. Now, that tenth of a second delay is way too long. So to make all of its processing duties easier, the brain makes predictions about where things in the field of vision will be. And sometimes, it just makes things up all on its own. That’s where optical illusions come in.

How do optical illusions work in psychology? Optical illusions are hacking eye-to-brain connections by predicting where the brain will screw up and send false information. There are effectively three types of optical illusions:

  • Literal illusion.
  • Physiological illusion.
  • Cognitive illusion.

Literal illusion

A literal illusion is the simplest and can be demonstrated in American illustrator Charles Allan Gilbert’s drawing called All is Vanity.

Literal illusion
Literal illusion

At first glance, the image appears to be of a skull, but as eyes adjust to the details, it is, in fact, a woman sitting at her vanity mirror. There are no skulls in the image whatsoever, but nearly all viewers see a skull. Why? It is because the brain does everything, and it can bridge the tenth-of-a-second delay. And one of these methods is by filling in details based on pattern recognition.

In this instance, the image has enough “skull-like” patterns that the brain initially reads as a skull, saving it the energy to process every shape, color, and angle. Literal illusions work by approximating a different object or setting close enough to fool your brain for a moment before all the details settle in.

Physiological illusion

The next type is a little trickier and is called a physiological illusion. Take a look at this grid and try to count the grey dots at the intersections. What’s that? Do they keep disappearing?

Physiological illusion
Physiological illusion

This illusion, which was discovered by Ludimar Hermann in 1870 and is thus called the “Hermann grid,” is not based on the brain’s mistakes. But the limitations of the retina. Specifically, it is believed to be based on something called “lateral inhibition.” The light-receiving neurons in your eye photoreceptors are arranged in rows. When one of them is stimulated by light, the surrounding neurons are inhibited and respond less strongly.

How do optical illusions fool your brain? Your eye does this to filter out a lot of light noise, keeping the most dominant neurons brightly lit and creating contrast with less dominant light sources. In the Hermann grid, your receptors are flooded with white light from four sides at the intersections.

It creates a lot of lateral inhibition, and the receptors in those areas respond less strongly, making the intersection appear darker. But, if you look directly at anyone’s intersection, the grey disappears. It is because the center of vision has a significantly higher concentration of photoreceptors. And it creates a more precise picture than anywhere else in the eye.

Cognitive illusion

Finally, there are cognitive illusions. These are the coolest because they take place far away from your eye or the purely visual processing center of the brain. These illusions happen in the understanding of the image itself and existing knowledge of the world.

Look at this famous staircase, the Penrose stairs, which directly influenced famous optical illusion maker MC Escher.

Cognitive illusion
Cognitive illusion

The trick with both Penrose and Escher is that the image is physically consistent with how a staircase works but practically impossible. You’d be going down or up forever. According to the father and son team which created the staircase, each part of the structure is acceptable as representing a flight of steps. But the connexions are such that the picture, as a whole, is inconsistent.

The image doesn’t make any sense while having no actual flaws. Our brain doesn’t know what to do with the conflicting information, so it accepts a paradoxical image. Ultimately, the brain and eyes take many shortcuts for efficiency, but at the expense of accuracy.

Geometrical illusion

A geometrical illusion arises from a geometrical figure, a straight line, square, or circle, onto a background of other, often parallel, lines. In the middle of the 20th century, neuroscientist Bela Julesz invented a kind of illusion called the random-dot stereogram. It is similar to the Magic Eye illusions. These illusions are made up of two images of random dots.

Geometrical illusion
Geometrical illusion

If you unfocus your eyes just right or use a stereoscope, they come together to form a picture. People could still see the distortions when Julesz converted geometrical illusions like the Poggendorff illusion into a random-dot stereogram. That means that the illusion occurs in the brain once the images have been combined, not in the eyes.

One explanation has to do with a phenomenon called lateral inhibition. In the visual cortex, some neurons specifically respond to lines oriented in different directions. Individual nerve cells respond to the vertical, and different nerve cells respond to the tilted ones. And these tend to inhibit or turn each other off. This means that the perceived direction of the two lines will diverge so acute angles will appear to get bigger.” That could also explain the Hering illusion.

Hering illusion

The long red lines seem to bend, but they’re parallel. The thing is, lateral inhibition can’t explain every geometrical illusion. In the Poggendorff illusion, a line passes behind a big vertical rectangle, and it looks like the two ends don’t align but are really continuous. If this illusion occurs because lateral inhibition makes the acute angles look larger, then the illusion should disappear. But you can still see it, and when you strip it down to just the acute angles, the illusion’s gone.

Hering illusion
Hering illusion

Another theory is that brains try to process these 2D images like 3D objects, adding the illusion of depth and perspective where there is none. That especially seems to be the case for the Ponzo illusion. Even though the top line looks longer, it’s the same length as the bottom one. This could be because of a process called size constancy. The two lines take up the same amount of space in visual fields, but minds interpret it as larger because the top line looks farther away.

Another explanation could be that minds judge an object’s size by what’s next to it. So because the top line intersects with the lines next to it, and the bottom one is surrounded with white space, the top line looks longer. When the 3D world projects 2D information onto our retinas, the brain then has to turn back into a 3D perception, sometimes minds cut corners.

Checker shadow illusion

Human eye movements are pretty jolty, and it’s those small, usually unnoticeable jolts along with the lag in the brain’s visual processing. That makes the image appear to move.

Checker shadow illusion
Checker shadow illusion

Not every illusion works the same way, but nearly everyone’s reverse engineers are biological presets. First, human brains are trained to recognize patterns, and due to the checkerboard, we think the square should naturally alternate colors.

Secondly, brains inherently understand shadows. In the case of images, shadows darken the color of the image behind them. These two visual predispositions programmed into the brain were fooled into interpreting the image like we would any 3d object. But in actuality, the squares are the same color.

Ebbinghaus illusion

The optical illusion is a whole draw on a few biological problems. The visual processing only updates about every one-tenth of a second. It is incredibly slow. So brains have developed methods of stitching visual senses together to create a flowing image. Brains do this so that we can kill prey for food or keep ourselves from getting hurt. But visual illusions exploit this time deficiency.

Diverging from physical problems, the brain has also trained itself to recognize patterns, estimate light, and fill in the gaps to what eyes miss. These skills work to benefit everyday life, but illusions take these biological predispositions and reverse-engineer them to trip brains up.

Ebbinghaus illusion
Ebbinghaus illusion

To the enjoyment, of course, take this. It’s called the Ebbinghaus illusion. Those orange circles they’re the same size. Brains do this because they have developed a method to estimate the relative size based on objects’ references. It helps us recognize scale at great distances, but our brain gets a little confused when thrown into a 2d image. Look at the circle of pink dots and stare at the cross in the center.

After a few seconds, you’ll find that it starts to appear that as a pink dot is removed, a green one is put in its place. This all has to do with something called the truck slurs effect. As the dots move in peripherals, they become blurred, and the brain isn’t focused on processing things outside of focus as fast.


Sources:

Gregory, Richard, “Putting illusions in their place.”
Bach, Michael; Poloschek. “Optical Illusions” (PDF). Adv. Clin. Neurosci. Rehabil. 6 (2): 20–21.
Gregory, Richard L., “Visual illusions classified” (PDF). Trends in Cognitive Sciences.
DeCastro, Thiago Gomes; Gomes, William Barbosa, “Rubber Hand Illusion: Evidence for a multisensory integration of proprioception.” Chouinard, Philippe A.; Chouinard, Virginie-Anne; Sperandio, “A review of abnormalities in the perception of visual illusions in schizophrenia.”

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