November 1st, 2022
Written by: Catrina Hacker
If there’s anything we can rely on to tell us the truth, it’s our own eyes… right? Maybe not. Our visual system isn’t like a camera, faithfully capturing the photons of light around us to take a picture. Starting with the neurons in our eyes, our brains use quick tricks to shape how we see the world. To demonstrate what tricks the brain is using to make sense of the world, scientists can design optical illusions that take advantage of these shortcuts. Here, I’ll highlight three kinds of optical illusions and what they teach us about our brains.
#1: Aftereffects
Aftereffects occur when we fixate on one image for a while and then look away. They rely on the fact that neurons that initially respond strongly to something (like the color green, or an object moving to the left) tend to decrease their response over time. This is a phenomenon known as adaptation and it has been observed in neurons across the brain and nervous system1. One reason neuroscientists think neurons might adapt is to prioritize things in our environment that are new or different from what we’ve been seeing. When looking into the branches of a nearby tree, you’ll see lots of green leaves, and the neurons that respond to green leaves will adapt. When you suddenly see a bright red apple, those neural signals are now stronger than the ones telling you you’re seeing green leaves, which lets you know there’s something new worth exploring.
Aftereffect illusions rely on this adaptation by causing certain neurons to adapt as the viewer looks at an image. When the image is removed from the screen, it takes a while for these neurons to return to their normal responsiveness. Until the adapted neurons return to their usual level of activity, we tend to see the “opposite” of what we were just looking at. Scientists utilize this adaptation to create aftereffects that can trick us into seeing things like motion and color.
Motion
The motion aftereffect is one of the most famous aftereffect illusions. To try it, follow the instructions in this video. The illusion is most likely to work if you put the video in full screen.
Did Van Gogh’s Starry Night come to life? You should have seen the sky moving for several seconds after the black rotating image disappeared from the screen. The video starts by having you watch motion in one direction for a short period of time (30-60 seconds). This causes neurons that respond to motion in that direction to adapt, or decrease their activity. When the moving image is removed from the screen, these neurons continue to have low activity compared to other neurons that didn’t adapt. This causes our brains to think that we are seeing motion in the opposite direction until those adapted neurons fully recover several seconds later2.
Color
Color aftereffects can happen even more quickly and strongly than motion aftereffects. To see yourself, watch the video below. This time, focus on the cross in the middle of the screen and watch the pink dots. After a little while, look away from the cross and follow the pink dots around the edge of the circle with your eyes. This illusion is also best viewed in full screen.
If the illusion worked, you should have seen a green dot rotating around the circle when you focused on the cross. When you looked away from the cross you should have noticed that there were never any green dots, only pink dots disappearing. This aftereffect is due to cells in a part of our eye called the retina that detect the color of what we’re looking at. When you focus on the center of the ring of dots you adapt the cells that respond to pink. When a pink dot is removed, those neurons haven’t fully recovered from adaptation, so you see the “opposite” of that color in its place: green3. When you look away from the cross and around the screen these cells are no longer adapted, so you don’t see green anymore.
Combining adaptation to lots of colors can produce even more stunning afterimages, like this castle illusion. The science is the same, but in this case the viewer adapts many different colors at different places on the image to produce a more complex afterimage.
#2: Gestalt Principles: Closure
Visual illusions go beyond features like color and motion. In the early 20th century, a group of psychologists noticed several interesting things about how we tend to interpret what we see. Their observations, called Gestalt principles, describe ways in which we tend to see an image as more than just a sum of its parts4. Like the other shortcuts described in this post, these assumptions usually help us interpret the things we see in our messy visual worlds by helping us make a best guess at what we’re looking at based on what is typically true.
One of these principles is closure. This is the idea that even when there are large gaps in the outline of an object, we tend to see the object as a whole rather than many smaller parts4. Consider the example on the left of Figure 1. Do you see a white square placed over four black circles? Most people do, but there aren’t any squares or circles in the image. Rather than interpreting this image as four Pacmans, our brains prefer to assume that there is a white square over the circles. When we rotate the same Pacmans so that there is no rectangle to close, our brains see them as four individual Pacmans instead (Figure 1, middle). Another famous example of closure is the World Wide Fund for Nature logo. Even though there aren’t any continuous lines outlining the panda, we easily fill in the gaps to see it as one panda and not many smaller patches of black and white.
To learn more about other Gestalt principles, watch this video from Khan academy.
#3: Color constancy
You can see that your favorite bathing suit is blue whether you’re lounging in the shade of an umbrella or laying out in the bright afternoon sun. This ability to determine an object’s color in many different lighting conditions is called color constancy5. While we take this for granted, our brains face a big challenge in keeping our perception of colors constant across different lighting conditions. The color of objects is determined by the wavelength of light that is reflected off them and back to our eyes. However, the wavelength of light that the same object reflects changes depending on what kind of light it is sitting in even though the object itself hasn’t changed. Our brains must determine what kind of light is shining on the object and use that to determine the actual color of the object.
Take the image on the left of Figure 2 as an example. We see square A as darker than square B, because our brain assumes that the cylinder is casting a shadow on square B and not on square A. This changes the assumed underlying color of square A compared to square B. However, when we put two bars of the same color as the squares lined up next to them (Figure 2, right) it’s obvious that the two squares are in fact the same color. This illusion is so powerful that it’s still impossible to unsee after we know that the two boxes are actually the same color.
“The Dress”
How our brain solves color constancy is at the heart of the internet-famous real-world illusion of “the dress”. Some people see the dress as black and blue, while others see it as white and gold. This image happens to produce a set of wavelengths that are ambiguous, or unclear, to our brains. Figure 3 illustrates how a black and blue dress under orange light appears to be the same color as a white and gold dress under blue light. Because the colors in the image could be interpreted either way, and each of our brains are slightly different from the next, some people see the dress as black and blue, while others see it as white and gold. The dress is really black and blue6.
These are only a small sample of the many optical illusions scientists have designed to reveal the tricks our brains use to make sense of the world. You might feel betrayed by the lies our eyes tell us. After all, what can we rely on if not our own eyes? But they should inspire amazement and appreciation for how much our brains do to help us make sense of the world. While optical illusions force our brain to make the wrong assumption about what it’s seeing, these quick tricks are an important part of our ability to make sense of the world.
References
1. Benda, J. Neural adaptation. Curr. Biol. 31, R110–R116 (2021).
2. Mather, G., Pavan, A., Campana, G. & Casco, C. The motion aftereffect reloaded. Trends Cogn. Sci. 12, 481–487 (2008).
3. Zaidi, Q., Ennis, R., Cao, D. & Lee, B. Neural Locus of Color Afterimages. Curr. Biol. 22, 220–224 (2012).
4. Todorovic, D. Gestalt principles. Scholarpedia 3, 5345 (2008).
5. Foster, D. H. Color constancy. Vision Res. 51, 674–700 (2011).
6. Gegenfurtner, K. R., Bloj, M. & Toscani, M. The many colours of ‘the dress’. Curr. Biol. 25, R543–R544 (2015).
Cover photo by CDD20 on Pixabay
Figure 1 generated in Adobe Illustrator by Catrina Hacker.
Figure 2 is from Edward H. Adelson and uploaded to Wikimedia commons by Tó Campos.
Figure 3 image is from Wikimedia commons. Figure design by Kasuga~jawiki; vectorization by Editor at Large; “The dress” modification by Jahobr.
PennNeuroKnow 
