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Short Wave

Why Illusions Are A Brain Feature, Not A Bug

Short Wave

NPR

Daily News, Nature, Life Sciences, Astronomy, Science, News

4.76K Ratings

🗓️ 30 July 2025

⏱️ 12 minutes

🧾️ Download transcript

Summary

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.

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Transcript

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0:00.0

Support for NPR and the following message comes from the Robert Wood Johnson Foundation.

0:05.4

RWJF is a national philanthropy working toward a future where health is no longer a privilege but a right.

0:12.1

Learn more at RWJF.org.

0:15.7

You're listening to Shortwave from NPR.

0:21.5

When Susanna Martinez-Gonde was around 10, she was on a school trip near her hometown of Acorunia, Spain,

0:27.6

when she found herself by a river just staring at the water.

0:37.2

But then she noticed something weird.

0:39.6

As she stared at the river and shifted her gaze to stationary objects nearby, like a rock or a riverbed.

0:47.4

It almost looked like those objects were moving too, but in the opposite direction?

0:52.4

So I thought that that was something that I had noticed for the first time ever.

0:57.0

And, no, it's a very old illusion.

1:00.0

In fact, Aristotle was the first person to describe this waterfall illusion, or motion-after effect.

1:07.0

And as a scientist, Susanna now understands why this happens.

1:11.7

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

1:19.2

So if you look at water flowing in one direction, those specific neurons that are sensitive

1:24.1

to that direction stop firing as much. They almost adapt to the flow.

1:29.3

And when you look elsewhere,

1:30.9

the neurons that are responsive to the opposite direction of motion,

1:36.7

those are going to become a lot more prominent in your perception.

1:42.5

So that's why you see motion in the opposite direction to the

1:46.9

direction that your visual system is adapted to. Susanna is a professor of ophthalmology,

1:52.3

neurology, physiology, and pharmacology at SUNY downstate Health Sciences University. And she

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