More than Meets the Eye: How Optical Illusions Stump Our Brains

Author: Haley Amemiya

Editors: Kevin Boehnke, Zuleirys Rodriguez, Patricia Garay, and Scott Barolo

ninio
Figure 1. Ninio’s Extinction Illusion

There are twelve black spots in scientist Jacques Ninio’s Extinction Illusion. Can you see them all at once?

This design is a variant of a classic optical illusion: the scintillating grid. When Japanese psychology professor Akiyoshi Kitaoka posted Ninio’s illusion on Facebook, it went viral. People around the world were struck with one simple fact: We are not in control of our brains. Despite knowing the truth of this illusion, we still fall victim to its tricks.

We experience the world with our senses, which are governed by networks in the brain. Although we have our differences, we share almost identical genetic information. The invisible similarities in our wiring make experiences like optical illusion universal. Therefore, understanding why we are incapable of beating this illusion and our awe in optical illusions can illuminate the common human experience.

What Are Optical Illusions?

Humans see optical illusions when the visual system (eyes and brain) attempts to interpret an image that evokes a perception that deviates from reality. Your brain displays an image that makes the most “sense,” but it is not always what is actually in front of our eyes. For example, Figure 2, square “A” looks darker than square “B” to our eyes, but they are actually the same shade of gray.

 

checker-shadow
Figure 2. Checker Shadow Illusion

 

 

Optical illusions have their limits, however—they can’t always fool us. There are tricks to let our brain to interpret the image correctly. In this case, if you directly compare the two squares with a gray bar, you can see that they are the same color, as shown in Figure 3.

 

fig-3
Figure 3. Squares “A” and “B” are the same shade of gray.

 

It is not clear why certain optical illusions occur, but investigating our visual system can help us understand why we cannot see all twelve dots at the same time in Figure 1, at least. To explore the scientific explanation of Ninio’s Extinction Illusion, we will first observe two classic examples of optical illusions that preceded Ninio’s Extinction Illusion: the Hermann grid (Figure 4) crafted in 1870 by Ludimar Hermann, a German physiologist, and the scintillating grid (Figure 5) published in 1994 by E. Lingelbach.

 

hermann
Figure 4. The Hermann Grid
scintillating
Figure 5. Scintillating Grid

 

The Hermann grid tricks the eyes into seeing gray spots in between the line junctions. The scintillating grid similarly tricks the eyes into seeing dark spots, but they seem to “scintillate” as you try to fix your eye on a single dark spot. While still not fully understood, the Hermann grid, the scintillating grid, and Ninio’s Extinction Illusion have been theorized to occur because of lateral inhibition in our visual system.

Lateral Inhibition May Explain Some Optical Illusions  

Lateral inhibition occurs in skin, eyes, and ears as a way to enhance sensory perception. It occurs when an activated neuron dampens the activity of its neighbors, thus keeping the responses to a local stimulus. This process enhances sensory precision, making images sharper and sounds clearer. For instance, if someone pokes you on the arm, that region is activated while the surrounding region is inhibited. As a result, you can pinpoint where you are being touched.

Lateral inhibition in the visual system occurs in a part of the eye called the retina before visual signals even reach the visual cortex (see Figure 6)—the part of the brain that processes visual information. When light passes through the eye, the message is relayed into retinal ganglion cells (a type of neuron located near the surface of the retina) to be further processed in the brain.

 

human-visual-system
Figure 6. The human visual system

 

When the eye processes an image like the Hermann grid, it must decipher the contrast between the white lines and black boxes. When the eyes focus on one intersection of the bright white lines to make the lines sharper and clearer, a ganglion cell focusing on the intersection is activated. That ganglion cell silences its neighbors through lateral inhibition, causing black spots to appear at the white intersections in our peripheral vision.

Why does our visual circuitry have this bug? Lateral inhibition allows for a sharper visual response and contrast. Inhibiting activation of all neurons allows for sharper visual interpretations. Similarly, some of us are better at executing on one task, rather than multiple tasks at once; our visual system focuses on one task at a time via lateral inhibition.

However, in its attempt to make an image clearer by focusing on one task and damping the rest, the brain displays an optical illusion under very specific and rare conditions—like in the Hermann grid.

Optical Illusions Are Remnants of Evolution

In a world where we constantly label and analyze every pathway in nature so we can learn about the ways to control them, it might be uncomfortable for some to accept that we have no control over optical illusions. The reality that our body dictates our daily experience rather than the other way around might be not something that we can easily digest or fascinate us.

However, while this quirk in our visual systems may be a random fascination, it might be an essential tool to experience our environment that survived many years of evolution. In 1956, Haldan Hartline found that horseshoe crabs also exhibit lateral inhibition in their sensory system—who would have thought that we have something common with horseshoe crabs? So the next time you are experiencing optical illusions, think of it as experiencing a trait that is a product of evolution!

 

About the author

haleyHaley Amemiya is a first-year PhD student at the University of Michigan in the Program in Biomedical Sciences exploring research in MCDB and Bioinformatics. Haley graduated from the University of Washington in Seattle with degrees in Molecular, Cellular, and Developmental Biology (MCDB) and Biochemistry in 2016. She recently reformed the Association for Women in Science at the University of Michigan and serves as their president. Her love of science is shared among love of cooking, animals, Netflix, and outdoor activities. Follow Haley on Instagram or find her on LinkedIn.

 

Read all posts by Haley here.

 

Image Sources:

Figure 1: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.294.6770&rep=rep1&type=pdf

Figure 2: https://en.wikipedia.org/wiki/Optical_illusion#/media/File:Grey_square_optical_illusion.svg

Figure 3: https://en.wikipedia.org/wiki/Optical_illusion#/media/File:Grey_square_optical_illusion_proof2.svg

Figure 4: https://en.wikipedia.org/wiki/Grid_illusion#/media/File:HermannGrid.gif

Figure 5: https://en.wikipedia.org/wiki/Grid_illusion#/media/File:Grid_illusion.svg

Figure 6: https://upload.wikimedia.org/wikipedia/commons/thumb/b/bf/Human_visual_pathway.svg/1280px-Human_visual_pathway.svg.png

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s