Usually this process is adaptive in that it allows you to perceive the object as it really is: constant in size regardless of distance and retinal image size. But in the case of an afterimage, the processing backfires. The afterimage does not change size on the retina with changes in viewing distance, but your brain still interprets it as doing so. Thus, when the afterimage is superposed on a far wall, your brain expects the retinal image to have shrunk from the size it would have been on a near wall. Your brain therefore expands the apparent size to compensate. It is important to realize that all this occurs on a kind of autopilot. There is no conscious reasoning or decision making such as: “If the object is far, it must have a small image; therefore, object size must be....” That type of cogitation would be much too time-consuming to be effective. Why this entire process results in the image actually looking large rather than simply knowing it is large is a $64,000 philosophical question called the riddle of qualia. (We will stay away from this question in this column, even though we personally believe size constancy might one day help solve the riddle more readily than asking, “Why is red red?”)
Emmert’s law also works in complete darkness. This is because when you look at an imaginary object at different distances, the angle between the two eyes’ lines of sight (vergence angle) changes, and the brain measures this change in eye position. So the afterimage shrinks and expands in darkness, depending on how far away you gaze.
Next try the following experiment. Generate an afterimage with another flash. Then, in darkness, stand perfectly upright and move your head forward and back from the (invisible) wall in front of you. When you stick your neck out, you will find that the afterimage shrinks because the brain “assumes” it is a real object expanding and therefore applies a (false) correction. Perhaps signals from the neck muscles are sent to the visual centers to zoom the perceived size. Alternatively, when the motor-command centers in the brain send commands to neck muscles, they may send a kind of cc (as in e-mail) to the visual centers.
These facts about Emmert’s law are pretty straightforward, but the best is yet to come.
Affix a tiny, luminous spot on the center of your right palm and, in complete darkness, hold your hand out at arm’s length and look at the spot. Have a friend look over your shoulder, then take a flash aimed at your outstretched hand.
Now look straight head. You will see a vivid ghostly afterimage of your hand. Keep gazing forward so that the hand image is hovering in front of you—nothing surprising so far. But now move your real hand toward your nose, and you will get the impression that the hand image is shrinking. This miniaturization will happen even if there is an image in only one eye, so the source of distance information cannot be the vergence angle.
Gregory’s ingenious idea was that the proprioceptive information from muscle and joint sense in the arm must be going all the way to the brain’s size-perception centers; the messages do not have to originate in the eye muscles. The effect feels spooky, because you would expect your real hand image to grow as it approaches your nose, but (try it in a fully lit room) it actually shrinks because of proprioception, driven by Emmert’s law. The arm muscles are telling your brain that the glowing hand is approaching you, yet it appears to expand. So you are startled. Moreover, if you move the hand too close to yourself, the expansion of the ghost ceases. This result may occur because you do not usually bring or see your hand that close, so your size-constancy mechanisms are not “wired” for it. It might be equally interesting to affix a long dummy arm to artificially lengthen your arm to see what happens.