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 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. Sometimes, it 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.
A literal illusion is the simplest and can be demonstrated in American illustrator Charles Allan Gilbert’s drawing All is Vanity.
At first glance, the image appears to be of a skull, but as eyes adjust to the details, it is a woman sitting at her vanity mirror. There are no skulls in the image, but nearly all viewers see a skull. Why? The brain does everything, and it can bridge the tenth-of-a-second delay. 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.
The next type is a little trickier and is called a physiological illusion. Take a look at this grid and count the grey dots at the intersections. What’s that? Do they keep disappearing?
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 “lateral inhibition.” The light-receiving neurons in your eye photoreceptors are arranged in rows. When 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. The center of vision has a significantly higher concentration of photoreceptors. Also, it creates a more precise picture than anywhere else in the eye.
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 understanding the image itself and existing world knowledge.
Look at this famous staircase, the Penrose stairs, which directly influenced famous optical illusion maker MC Escher.
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.
A geometrical illusion arises from a geometrical figure, a straight line, square, or circle, onto a background of other, often parallel, lines. In the 20th century, neuroscientist Bela Julesz invented the random-dot stereogram illusion. It is similar to the Magic Eye illusions. These illusions are made up of two images of random dots.
If you unfocus your eyes 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. Some neurons respond to lines oriented in different directions in the visual cortex. Individual nerve cells respond to the vertical, and different nerve cells respond to the tilted ones. 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.
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 continuous. If this illusion occurs because lateral inhibition makes the acute angles look larger, the illusion should disappear. But you can still see it, and when you strip it down to the acute angles, the illusion’s gone.
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 turns 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.
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 image’s color behind them. These two visual predispositions programmed into the brain were fooled into interpreting the image like any 3d object. But in actuality, the squares are the same color.
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 kill prey for food or keep themselves 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.
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 comparative size method 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.
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.”