Down the street and around the corner from my home, there's a face on the sidewalk. I first noticed it two or three months ago. Out for a walk, I thought I saw the moon, nearly full, hanging low and orange among the trees. I wanted to see it better, to get out from under the trees for a clear view. The trouble is, I live in a city that takes special pride in its tree-lined streets - a beautiful place to live, but far from ideal for stargazing. As I ventured through the quiet neighborhood to the nearest main avenue, I lost sight of the moon, and finally found myself wondering if it was only a streetlight I had seen among the dark treetops. As I headed for home, a streetlight that had blinked off came suddenly back on, and there, on the sidewalk in front of me, I saw the face in the streetlight's orange glow.
It was the face, maybe a little larger than life, of a man of African heritage. He seemed to be speaking, and there was passion and enthusiasm in his features. I thought perhaps I ought to recognize him. The image seemed to have been rendered in black spray paint, using a stencil. Night after night, it became part of my routine to go around the corner and see the face on the sidewalk.
Then, for the first time, I went to see the face in daylight. To my astonishment, although it still appeared to have been spray-painted onto the sidewalk with a stencil, the face wasn't black at all. It was a bright, vivid blue. How had I not noticed?
The answer to that is simple enough: the paint on the sidewalk reflects blue light, causing it to register as blue to the human eye. However, the light from the streetlamp was in the red-green portion of the visible spectrum. The reason I didn't see any blue when I saw the face in the glow of the streetlight was that there wasn't any blue to reflect.
If, instead of the face, I had caught a clear glimpse of the moon I was originally chasing after, I would still have experienced deception by my own senses. The moon famously appears larger when it's close to the horizon than when it's high in the sky, even though it is actually the same size.
Through the middle of July, there was a special issue of Scientific American Mind on newstands, all about optical illusions and the science of visual perception. Even if you aren't interested in brain science, this one is worth picking up just to look at the pictures. All the old favorites are here (afterimage tricks, parallel lines that seem to tilt toward each other, the impossible triangle) as well as some you've surely never seen before - including numerous winners of an annual "Best Illusion" contest - perhaps the only art competition in the world in which the winners' trophies are as much fun to look at as their entries. (This year's winner couldn't be featured in the magazine, but it's certainly worth a look. I guarantee you'll want to watch the video more than once.)
There are sixteen pages of illusions that take advantage of the neurological mechanisms that recognize and interpret faces and the direction of eye gaze. The secret of Mona Lisa's smile is revealed, and the reader is treated to a truly two-faced Paris Hilton, an Einstein that morphs into Harry Potter, and some creepy variants of Tony Blair and Margaret Thatcher. Eight pages about a select group of brilliant artists who specialize in making three-dimensional images of famous two-dimensional illusions (including a Lego reproduction of Escher's Ascending and Descending) are, alone, enough to justify the purchase price of the magazine. (If you're an Escher fan, visit the downloads page at the official Escher site to enjoy computer-generated virtual tours of Ascending and Descending and two of the artist's other works.)
One of the most intriguing, and unsettling, articles in the magazine is "Colors Out of Space," a glimpse into color-perception illusions that drives home just what a fickle and easily deceived thing the human brain is. So much of what we see is context-dependent. A pair of pictures, side by side, depicts the same Rubik's Cube, bathed apparently in yellow light on the left and in blue on the right. But it turns out that the parts of the cube that appear blue in the yellow picture and those that appear yellow in the blue picture are both printed in the same shade of gray. The brain essentially filters out the yellow or blue to compensate for the color of the lighting, and so the same color stimulus is interpreted differently based on the colors around it. Likewise, the squares that appear red in both pictures are printed in orange on the left, purple on the right. The brain filters out the yellow and blue and is left with what apppears to be the same shade of red in both pictures. What's truly unnerving about illusions such as these is that, even when you know how they work, you can't not see them. You see blue and yellow and red, not gray and orange and purple. If you isolate the colors by blocking out the rest of the image, you can prove to yourself that the authors' claims are accurate - but view the image as a whole again, and once more, your brain deceives you. Deception is possible even when the cube is depicted under white light. The center square of the brightly-lit top of the cube appears brown; the center square of the side of the cube facing the viewer, which lies in shadow, appears to be orange. Of course, they're actually printed in the same color. (You can see the blue- and yellow-lit versions of the cube about two-thirds of the way down the page here.)
The brain is a marvelous instrument. The neurological processes behind optical illusions are crucial in interpreting the world around us in meaningful ways. The reason that we can't recognize the actual colors used in printing the Rubik's illusion is the same reason that we are able to recognize the faces of our friends, by sunlight or by moonlight or under colored spotlights on the dance floor. More important than merely translating the wavelengths of light that hit the back of the retina into our personal language of color and shadow is the process of interpreting what it is that we see, in such a way that we can navigate our surroundings, seek out what is beneficial, and avoid what might cause us harm. Our brains must interpret the world in realtime, in greater detail than any computer rendering - and they do it excellently. Just as any complex computer program can be pushed to its limits, however, our brains find themselves struggling when they must interpret data that don't fit the established rules - and we are reminded that everything we know and believe about the world is filtered through the same marvelous but imperfect instruments of perception.
