Why Optical Illusions Fool Our Brains

Do you see what we see?

If you thought vision had everything to do with your eyes, you’re wrong.


The lens of the eyeball focuses light back onto the retina, where photoreceptive rods and cones are affected by the wavelength of the light. Information about the light entering the eye travels through the optic nerve, where it is then interpreted by the brain. The brain is responsible for taking raw data about light wavelengths and untangling the patterns, using memory in order to make sense of the images that the brain ultimately "sees."

While you definitely can't see without your eyes, nothing would make sense without input from the brain.

Our eyes transmit a tremendous amount of information back to the brain, and it requires too much brain power to process all of it. In order to make the job easier, the brain has devised shortcuts to understand what it is seeing. Shadows, perspective, and color are all clues the brain uses in order to make decisions about what it is looking at.

This image isn’t a .gif, so why does it look like it’s moving?

iStock / myistock88

These circles appear to be moving due to illusory motion. While it isn't entirely well-understood how the brain perceives movement from these images, a 2012 study from St. Joseph's Hospital and Medical Center in Phoenix found that small, rapid eye movements are responsible for aiding the illusion. If you focus your gaze very intently on one spot of the image, the motion will stop. 

Research is ongoing to determine how the brain is tricked by these images and understanding how perception of these images differ could provide a wealth of data about the origins of motion perception itself.

Which box is darker: A or B?

While box A seems noticeably darker, it probably wouldn't be an illusion if the answer were that simple. 

In fact, the two boxes are actually the exact same color.

See for yourself:

This illusion is able to trick our brains for a couple of different reasons, according to researchers at MIT. Because of the checkerboard pattern — which typically uses a dark color and light color — our memory tells us that A and B should be different. However, the cylinder casts a "shadow" on the board, significantly darkening the boxes surrounding B. B itself also becomes darker than expected, matching the shade of A exactly.

If the rectangle connecting A and B appears to have a gradient and A still appears darker, use your fingers to cover up the edge of the rectangle, blocking out the rest of the checkerboard. It makes it easy to see the color completely unchanged between the two boxes.

Does this image look like a shelf full of potpourri warmers, or a line of yellow faces who cannot believe what they’re seeing?

If you see the faces that look completely aghast, you are experiencing the neurological phenomenon known as pareidolia. This effect causes people to see "faces" in inanimate objects, like surprised faces in electrical outlets, or the Virgin Mary in toast. As The Atlantic reports, it's extremely common to see faces in everyday objects. This ability likely came about as a way to quickly recognize fellow human beings, even when visibility was low.

Where the heck are these black dots coming from?

iStock / GluckKMB

The image above is known as a scintillating grid illusion, and was first described in the journal Vision Research. As the eye makes tiny, rapid movements, black dots appear to "scintillate" in the intersections of the white lines. Just as with the illusory motion image, the effect only works when the eyeball is moving slightly, scanning the image.

This is an updated version of the Hermann grid illusion, which induces an effect called lateral inhibition. Due to the sharp contrast of color coming in the peripheral vision, the photoreceptive cells in the retina of the eyes become confused, and create the illusion of dots that don't exist.

Stare at the black cross in the middle of this circle:

In your peripheral vision, you'll notice one lilac dot is removed at a time, and it appears that a green one is put into its place. Once you look directly at the disappearing dot, it becomes obvious that there isn't actually a green dot. 

The reason this occurs is due to Troxler's effect. As Nature explains, this effect causes things in peripheral vision to fade and disappear. Thus, because the lilac dots aren't the center of your focus, your vision more or less takes them for granted. This particular illusion goes one step further. Your brain becomes accustomed to seeing the purple color, and when it momentarily blots out of your vision, the spot is replaced by the complimentary color. Purple and green are across from one another on a color wheel, and that is why green takes over when the lilac goes away.

Cover image via iStock / Tramper2.


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