4.7 • 6K Ratings
🗓️ 30 July 2025
⏱️ 12 minutes
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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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