Being Wrong - Kathryn Schulz [28]
You don’t believe me, for the very good reason that you do believe your eyes, and your eyes are telling you that these squares look completely different. Actually, it’s not your eyes that are telling you this; it’s a handful of interpretative processes of the kind I just described. These processes are in play because, when it comes to determining the color of objects around us, our visual system can’t afford to be too literal. If it were, it would do nothing but measure the wavelength of light reflecting off a given object. In that case, as the psychologist Steven Pinker has pointed out, we would think that a lump of coal sitting in bright sunlight was white, and that a lump of snow inside a dark house was black. Instead, we’re able to correct for the presence of light and shadow so that the coal still appears fundamentally black and the snow still appears fundamentally white.
One way we do this is through local contrast. In nature, if something is lighter than its immediate surroundings, it’s probably light in an absolute sense, rather than just because of the way the sun is or isn’t striking it. That’s one reason why, in this illusion, we read Square B (which is lighter than the dark checks around it) as light, period. The same phenomenon applies in reverse, so that we read Square A (which is darker than the squares around it) as dark, period. This interpretation is reinforced by several other interpretative processes, including the fact that we automatically adjust for cast shadows, mentally lightening whatever objects they fall on—in this case, Square B.
The net effect of these visual “corrections” is an illusion that is absolutely unshakeable. When I first saw it, I was so incredulous that I finally took a pair of scissors and cut the picture apart—whereupon, lo and behold, the A and B squares became indistinguishable from each other. In an effort to discourage you from mutilating this book, I offer a second image:
Not quite as convincing as slicing and dicing, perhaps, but a good start. (If you must cut it apart yourself to be persuaded, the original image—and a lot of other fun stuff—is available on the website of its creator, Edward Adelson, a professor of vision science at MIT.)
What makes this illusion both irksome and fascinating is that knowing how it works does not prevent it from working. No matter how many times you read the above explanation (or how many copies of the image you cut to pieces), the two shades of gray will still look strikingly different to you. Likewise, Robert Bartlett’s knowledge that he was 350 miles from Iceland could keep him from getting lost, but it was powerless to prevent him from seeing the Icelandic coast looming up before him. This is one of the defining features of illusions: they are robust, meaning that our eyes fall for them even when our higher cognitive functions are aware that we are being deceived. A second defining feature is that they are consistent: we misperceive them every time we come across them. Finally, they are universal: all of us misperceive them in precisely the same way.*
These characteristics make sense when you recall that illusions are the product of unconscious and universal perceptual processes. But here’s the important part: those same processes—the ones that cause us to screw up when we encounter illusions—serve us extremely well in everyday life. This helps explain why a scientist at one of the most respected academic institutions in the world is paid to sit around developing optical illusions. The real object of study is not the illusions themselves but the processes that give rise to them—processes that would be far harder to study (or even know about) if they didn’t occasionally produce surprising and erroneous results. Moreover, because illusions trick