Head sizes: Here is another example of the baffling misperceptions of size constancy. The next time you are in a crowd of people—in a lecture hall or a theater—look around and ask yourself if people’s heads appear to be more or less all the same size. Then, roll up a piece of paper into a tube about an inch in diameter. Look through the tube (which has become an ad hoc measuring device) at a person fairly near you. Reroll the tube, if necessary, so that the person’s head just fits the diameter of the tube. Then, keeping the tube at the same diameter, turn and look through the tube at a person’s head far across the room. It may appear to be as small as one-quarter the size of the head near you. Yet, when you take the tube away and look at that person across the room, the head will instantly “jump up” in size to what we take for “normal.” Because the brain is actually changing the apparent size of things to match what the brain knows them to be, you cannot depend on what you think you are seeing. The brain knows that human heads are mostly about the same size, and it refuses to accept the actual reduction in perceptual size that distance causes. But here we confront a double paradox:
If you accept the brain’s version and draw all the heads, near or distant, about the same size, the brain will sense that something is wrong with the drawing. But if you draw the proportions of heads perceptually correct—diminishing in size with distance —the brain will see them as usual, that is, all pretty much the same size.
