Why Illusions Are A Brain Feature, Not A Bug

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 https://illusionoftheyear.com/, open to illusion makers around the world and co-created by neuroscientist Susana Martinez-Conde https://www.downstate.edu/faculty/ophthalmology/martinez-conde.html. Today on the show, we get lost in the magic and science of visual illusions. Have a neuroscience question? Email us at [email protected] mailto:[email protected]. 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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://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 https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:16Emily Kwong?

  You're listening to Short Wave from NPR.

• 0:22Emily Kwong?

  When Susana Martinez-Grande was around 10, she was on a school trip near her hometown of Acuérno, Spain, when she found herself by a river, just staring at the water.

• 0:38Emily Kwong?

  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.

• 0:53Speaker 02

  So I thought that that was something that I had noticed for the first time ever. And no, it's a very old illusion.

• 1:01Emily Kwong?

  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.

• 1:12Speaker 02

  Neurons in your visual cortex are sensitive to specific motion directions.

• 1:19Emily Kwong?

  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...

• 1:31Speaker 02

  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.

• 1:50Emily Kwong?

  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 Macknik, 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.

• 2:28Speaker 02

  Wouldn't 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.

• 2:38Emily Kwong?

  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.

• 3:06Speaker 02

  And they're not the situations in which the brain gets it wrong, so to speak, but illusions are intrinsic to how we construct our perception of the world.

• 3:23Emily Kwong?

  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 Short Wave from NPR.

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• 5:09Emily Kwong?

  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?

• 5:23Speaker 02

  Exactly. From the very beginning of the visual path, 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.

• 6:05Speaker 02

  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, a 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.

• 6:37Emily Kwong?

  All reality is perceptual, you're saying. It's all filtered by what our brain is telling us.

• 6:42Speaker 02

  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 bee's experience of the world and so on. These are different brains and different realities.

• 7:19Emily Kwong?

  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?

• 7:51Speaker 02

  Yes, so the rotating snakes illusion was created by one of the most famous illusion creators, Akio Shi Kitaoaka. 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...

• 8:19Speaker 02

  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,

• 9:03Speaker 02

  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. But 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 illusion and you choose a single point, like the center of one of the snakes, and try to keep your eyes there as precise as you can, just fix your eyes 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.

• 9:44Emily Kwong?

  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 Egger, out of Germany. So this is a ballerina, an illustration of a ballerina. And she's rendered entirely in grayscale.

• 10:08Speaker 04

  She seems to be spinning, right? She's not.

• 10:12Emily Kwong?

  And it's like the edges of her are almost blinking.

• 10:16Speaker 04

  It's inspired by the paper Motion Without Movement.

• 10:19Emily Kwong?

  The edges of her are being adjusted through computer programming.

• 10:23Speaker 04

  We've taken this 2D illusion into the third dimension by first estimating the depth of the image and then calculating the speed per pixel for a dynamic, lifelike effect. And there you have it, a mesmerizing blend of art and science.

• 10:40Emily Kwong?

  It's so cool. What did you think when you first saw this submission?

• 10:44Speaker 02

  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.

• 11:12Emily Kwong?

  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?

• 11:23Speaker 02

  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.

• 12:08Emily Kwong?

  The contest will presumably continue next year, so people can submit from anywhere in the world?

• 12:13Speaker 02

  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.

• 12:25Emily Kwong?

  Susana Martinez Conde, it was so good to talk to you. Thank you so much for coming on Short Wave.

• 12:30Speaker 02

  Thank you for having me.

• 12:33Emily Kwong?

  Troivers, 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 Burleigh McCoy. It was edited by our showrunner, Rebecca Ramirez, and fact-checked by Tyler Jones. The audio engineer was Maggie Luthar. 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 Short Wave from NPR.

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