First, take a look at the image below. You may have seen it elsewhere: it’s a very famous visual illusion called the Adelson checkerboard illusion.
In the image, regions A and B appear to be different: A looks dark gray, while B looks light gray. Right?
But that’s an illusion!
If you use screenshot software to crop regions A and B separately and place them side by side, you’ll see that there’s absolutely no difference in how light or dark their colors are.
How is that possible?!
How does this illusion happen?
Broadly speaking, the reason isn’t mysterious. Our perception of how light or dark a region is doesn’t depend on the amount of light reflected from that region into our eyes. It depends on the ratio of the light reflected by the region to the light shining onto it—in other words, its “reflectance”:
Reflectance = light reflected by the region / light falling on the region
The higher the reflectance, the brighter the region looks; the lower the reflectance, the darker it looks.
But our eyes can directly sense only the amount of light reflected from that region (in other words, we know only how large the numerator in the formula above is). So how does the brain “calculate” a region’s reflectance? How does it estimate the denominator?
The answer is that it has to use contextual information from the environment, or background knowledge about the situation, to infer it indirectly.
In fact, even the size of the numerator in the formula isn’t determined simply by the absolute amount of light received by the retina. Background information also affects the brain’s estimate of the numerator.
When you look at the checkerboard below, you see part of it sitting in the cylinder’s shadow. Shadow versus direct light—that’s background knowledge about the denominator.
At the same time, your mind comes with a preset expectation: a checkerboard has alternating light and dark squares. That’s background knowledge about the numerator.
So, with just one glance, you’ve actually combined at least those two pieces of background knowledge to make a judgment: square B is a light square in shadow, while square A is a dark square receiving direct light.
So the essence of this illusion is that you can’t treat the photograph as a two-dimensional pattern made up of patches of gray. Your brain has no way to ignore the three-dimensional scene depicted in the image.
When we discover that we experience an illusion like this, should we be startled and conclude that our vision is less perfect than we imagined?
Quite the opposite!
With just one glance, the brain automatically combines vast amounts of information: the physical stimulation of light on the retina, our intuitive grasp of light’s various physical properties, and the three-dimensional structure of objects. This “illusion” proves precisely that our visual perception is very good at understanding the three-dimensional environment we live in. And because we’re familiar with images, we’re also very good at understanding two-dimensional pictures depicting three-dimensional scenes.
When people experience illusions, it often isn’t because their senses are defective. It’s because the cerebral cortex responsible for those senses is so damn good at automatically calculating all kinds of background knowledge in the environment, then handing you the result of a complicated computation. You want “simple”? Your brain just won’t let you keep it simple.
Being too capable is a kind of helplessness, too~
Note
- This series of “Psychology New Knowledge: Mini-Lessons” articles was written at the time to accompany the related content of Zhichao Wei: Psychology New Knowledge Course, and published as short popular-science essays on the now-closed platform “F团.” The original link no longer exists.
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