Is what you see what’s really there, or is it all in your head?

“Well, I don’t know about you,” I hear you say (which is a good trick, considering this is a newspaper column), “but I see what’s really there. This newspaper is really here in my hands–I’m not imagining it.”

No, you’re not. But it’s all in your head just the same.

That’s because the sense of sight isn’t quite as straightforward as most of us think. We tend to think of the eyes as two little cameras, focusing upside-down, reversed images of what we see on the retina, light-sensitive cells at the back of the eye corresponding to the grain of film or the dots of a newspaper photograph.

But it’s not that simple. The fact is, the image on the retina is nowhere near as sharp or clear as the image a camera lens focuses on film. Nor do the light-sensitive cells (called rods and cones–rods detect only shades of gray, while cones sort out colour) break that image down to precise dots like a newspaper photo. We see clear, sharp images of the world only because our brains perform complex calculations to make sense of the input from our eyes.

The task is spread through various sections of the part of the brain concerned with vision, called the visual cortex. Some cortex cells are concerned with lines, others with depth, others with colour and still others with motion. Together they produce the final image.

The fact the final image provides us with so much information is due to the power of our brains. For example, we can easily distinguish between a real worm and a simple line crawling across the computer screen. A toad, however, will try to eat the crawling line, because to his brain, anything with that kind of motion is a worm. (The toad will ignore a line that moves vertically, because worms don’t crawl on their ends.)

But before you feel too cocky, remember how easily your own eyes can be fooled. Prior expectations are the toad’s visual downfall, and they can be ours, too.

Most optical illusions make use of such prior expectations. We know, for example, that as parallel lines such as railroad tracks stretch into the distance, they appear to come together. An artist makes use of that expectation when he draws a picture of railroad tracks, giving his work the illusion of depth by bringing the lines together at the vertical line representing the horizon.

Another preconception we have is that the further away an object is, the smaller it will appear. Artists use this, as well.

But if you draw three figures of identical size against a background of converging lines, our preconceptions come into conflict and cause us problems. “Look, converging lines!” says the brain. “That means that figure over there where the lines are closer together is farther away than the other two figures. Now, I know the further away something is, the smaller it appears. But that figure looks about the same as the other two. Wow, it must really be a lot bigger!”

People who have lived all their lives in dense forest, where they’ve never been able to see very far, have an opposite preconception concerning apparent size. They’ve never seen something that’s so far off it’s shrunk significantly by distance. If they’re transplanted to someplace open and flat–anybody know of such a place?–they have trouble judging the size of things, because when they see something on the horizon, their preconception tells them it’s actually much closer and much smaller than it really is.

Another good example of the role of the brain in vision is an experiment in which volunteers wore lenses that turned everything they saw upside-down. Their brains eventually adjusted, and the world looked right-side-up again. But guess what happened when they took the lenses off? (Anyone who has ever had a new, more powerful pair of glasses is familiar with this phenomenon. At first you find yourself lifting your feet too high, or tripping over nothing. We say our eyes eventually adjust, but actually it’s our brains doing the adjusting. In a day or two, the new lenses are no problem.)

Our brains do everything in their power to make sense of the images they receive. The Man in the Moon and various Great Stone Faces on mountainsides around the world are good examples. In our lives it’s so important for us to recognize faces that it takes only faint markings on an object in the approximate locations of human features for us to see a face in it.

Sometimes, though, the brain simply cannot identify what it sees, or even misidentify it. The result is confusion, an abstract play of light and colour that we can make no sense of at all.

Good examples are in the “Can you identify this?” photo features you sometimes see in magazines and newspapers, in which familiar objects have been photographed from unfamiliar angles and distances. Sometimes, especially if influenced by misleading titles on the photographs, our brains jump to a (usually wrong) conclusion about what they’re seeing, mistaking, for example, the tip of a melting icicle lit from behind by the setting sun as a piece of shining metal. Other times, our brain simply balks, and we have no clue at all as to what we’re seeing: we see the shape and the texture and the colour, but it means nothing–until it’s explained to us, either verbally or with a wider-angle photo of the same object. Then our brains say “of course,” and we wonder why we couldn’t see it to begin with.

As any policeman who’s gathered eye-witness accounts of a crime or accident can tell you, people don’t really see things as clearly as they’d like to believe. We assume, we approximate, and then we file these assumptions and approximations into our memories for future reference and base future assumptions and approximations on them.

And yet, it works. Vision is the sense on which most of us depend for the majority of our information about the world around us–a world, as you can see, that’s really just all in your head.

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