Short Wave - Why Illusions Are A Brain Feature, Not A Bug

Episode Date: July 30, 2025

A grayscale ballerina who appears to be moving. A human who can fit in a doll box. A black-and-white prism that appears to change shape when viewed from three different directions. Those are the top winners of the 2024 Best Illusion of the Year Contest, open to illusion makers around the world and co-created by neuroscientist Susana Martinez-Conde. Today on the show, we get lost in the magic and science of visual illusions. Have a neuroscience question? Email us at [e-mail]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. When Susanna Martinez-Gonde was around 10, she was on a school trip near her hometown of Acorunio, Spain, when she found herself by a river just staring at the water. But then she noticed something weird. As she stared at the river and shifted her gaze to stationary objects nearby, like a rock or a riverbed, it almost looked like those objects were moving too, but in the opposite, direction? So I thought that that was something that I had noticed for the first time ever. And no, it's a very old illusion.

In fact, Aristotle was the first person to describe this waterfall illusion or motion after effect. And as a scientist, Susanna now understands why this happens. Neurons in your visual cortex are sensitive to specific motion directions. So if you look at water flowing in one direction, those specific neurons that are sensitive to that direction stop firing as much. They almost adapt to the flow. And when you look elsewhere, the neurons that are responsive to the opposite direction of motion,

those are going to become a lot more prominent in your perception. So that's why you see motion in the opposite direction to the direction that your visual system is adapted to. Susanna is a professor of ophthalmology, neurology, physiology, and pharmacology at SUNY downstate Health Sciences University. And she loves illusions because illusions are perceptual experiences that do not match physical reality. How we see the world is driven entirely by our brains and illusions really reveal that. She and her collaborator, now husband, neuroscientist Stephen Macknick, made a game of spotting illusions whenever they went to professional vision conferences. They always wanted to know what new illusions had been discovered by other vision scientists.

Back in 2005, they had this idea. Would it be cool to have an event where we can have all the new illusions come together? And we thought, let's have an illusion contest. Susanna and Stephen developed the illusion of the year contest. Artists, magicians, any illusion creators can upload a one-minute video sharing some kind of novel illusion. And Susanna has been genuinely shocked that 20 years later, new illusions are still coming in. Illusions appear to be a fundamental feature of how our brains work, almost like the curtain falling down to reveal the wizard of Oz. And they're not the situations in which the brain gets it wrong, so to speak, but illusions are intrinsic to how we,

we construct our perception of the world. Today on the show, we kept lost, or maybe found, in the brain magic of visual illusions and reveal the most recent first place winner in the Illusion of the Year contest. I'm Emily Kwong and you're listening to Shortwave from NPR. All right, Susanna, so today we are going to get lost in the world of illusions. You were saying earlier how this is not a bug, but is in fact just a fundamental fact about how our brains work? Why? Exactly. From the very beginning of the visual pathway, we were talking about the retina

at the back of the eye, and there you already have neurons that are specialized on detecting contrast. So the brain never cares about absolute terms. There is no black, there is no white, there is no big or small. Everything depends on what you're comparing it with. And so this inevitably gives rise to illusions. And so in fact, I would go beyond to say that we never have 100% correspondence between reality and perception. What we call illusions are those cases in which the discrepancy is most spectacular, that we can no longer ignore it. And in some cases, really important cognitive dissonance because we can be in situations in which we know for a fact that what we're seeing is different from what's there, but we cannot force ourselves to see it any differently.

All reality is perceptual, you're saying. It's all filtered by what our brain is telling us. You never have a direct experience of reality. you've always been inside of your simulation. And your brain structure is so critical to the simulation that your view of the world, your sense of the world, is going to be just so different depending what kind of brain you have. And I'm not just talking about individual differences, but your experience of the world is going to be different from your cat's experience of the world or a beast experience of the world and so on.

These are different brains and different realities. All right. Well, let's talk about two visual illusions. The first one I want to discuss is an image you studied called rotating snakes. It's an example of what's called peripheral drift. It's a still photo that your mind thinks is moving. And I'm looking at it now, rotating snakes. And to describe it, it's like six kaleidoscopic-looking circles

that are made up of smaller overlapping different colored circles. And I know they're not moving, but as I focus on one, it looks like the others are moving. What is happening? Yes, so the rotating snake's illusion was created by one of the most famous illusion creators, Akiyoshi Kitaoka. And as you change your gaze around the image, these snakes appear to rotate, when in reality the image is perfectly stationary. these color patches, what's critical to them is not the color itself, but the luminance. So their brightness. And in the proper order, this sequence of patches tricks, again, the motion sensitive neurons in your brain into, quote unquote, believing that there is motion.

And what we discovered in my laboratory some years ago was the specific type of eye movement that you need to trigger this illusion. And that is the sort of transient eye movements such as eye jumps. All of our eyes jump. They jump every second of every waking hour. So by the time you go to bed, you will have made about 200,000 of these. eye jumps in a given day and you're only aware of, you know, maybe a dozen of them if I were to guess. So you cannot make more jumps on purpose, but what you can do is you can briefly suppress them. And so if you are looking at the rotating snake's illusion and you choose a single point,

like the center of one of the snakes and try to keep your eyes there as precisely as you can, just fixture as much as you can, you're going to get the snakes to slow down or even stop completely. But if you relax your gaze again, the motion will come right back. So you announced the winners of the Illusion of the Year contest recently this month. And I wanted to talk about the winner. And this is a static spin by Salida Kadir and Bernard Eger out of Germany. So this is a ballerina, an illustration of a ballerina

and she's rendered entirely in gray scale. She seems to be spinning, right? She's not. And it's like the edges of her are almost blinking. It's inspired by the paper motion without movement. The edges of her

are being adjusted through computer programming. We've taken this two de illusion into the third dimension by first estimating the depth of the image and then calculating the speed per pixel for a dynamic life-like effect. And there you have it, a mesmerizing blend of art and science. It's so cool. What did you think when you first saw this submission? Well, I thought that it was a beautiful illusion. I still do.

And I think that it brings together so well the science and the art because you're not just watching the implementation of a brain principle, of a perception principle, but you're actually watching art, and I think that that's what makes illusions so captivating in a way. Why study illusions at all? As scientists, what is the value in studying them and for these early career scientists to take time to do it? The definition of an illusion is a disconnect between objective reality and subjective perception. Because of this disconnect in illusions between perception and reality, we can, as scientists, go in and analyze the neurons and the brain circuits that support

activity, neural activity that matches perception. And those could be part of the neural basis of consciousness, whereas if you have neurons and circuits that have activity that matches reality rather than perception, those would not be part of the circuit that underlies consciousness. The contest will presumably continue next year, so people can submit from anywhere in the world? Yes, absolutely. Anybody can participate. You can be an illusion creator, and you can participate as a contestant, and you can also participate by voting for the best illusions. Susanna Martinez-Conde. It was so good to talk to you. Thank you so much for coming on Shortwave.

Thank you for having me. Shortivers, I like to think of every episode of our show as a combination of science and magic, just like an illusion. So if you never want to miss another one, subscribe, follow, like, whatever app you're listening to. And that way you'll never miss out. This episode was produced by Burley McCorme. It was edited by our showrunner, Rebecca Ramirez, and fact-checked by Tyler Jones. The audio engineer was Maggie Luthor. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening to Shorewave from NPR.