Maurice Hershenson, professor of psychology at Brandeis University. “The ‘moon illusion’ is one of the oldest known psychological phenomena; records of it go back to ancient China and Egypt. It may be the most ancient scientific puzzle that is still unexplained. “There are many misconceptions surrounding the moon illusion. People trained in the physical sciences often think that the illusion is real, that the moon actually looks large when it is near the horizon because of refraction of light by the atmosphere. In fact, there is a very small refractive effect, but it is not the cause of the illusion. “There are a couple ways you can prove to yourself that the light reaching the eye from the moon remains the same as the moon changes position in the sky. For instance, if you photograph the moon at various heights above the horizon, you will see that the images of the moon are all the same size. My students frequently send me photos of a ‘giant’ harvest moon in which the moon looks like a small spot in the sky. (The same thing happens in photos of seemingly spectacular sunsets–the illusion works for the sun as well.) Another way to break the hold of the illusion is to cup your hand into a fist and look through it at the ‘large’ horizon moon. It will immediately shrink in size. “Clearly, this is a psychological effect. There are many different theories (perhaps 10) for why it happens, taken from fields ranging from cognitive psychology to neurophysiology. “My own view is that the moon illusion is linked to the mechanism that produces everyday size-distance perception, a genetically determined brain process that allows us to translate the planar images that fall on the retina into a view of rigid objects moving in space. I believe the moon illusion results from what happens when the mechanism operates in an unusual situation. In normal perception, when rigid objects move in depth (distance), the angular size of the light image stimulating our eyes grows or shrinks. The brain automatically translates this changing stimulation back into the perception of rigid objects whose position in depth is changing. “When the moon is near the horizon, the ground and horizon make the moon appear relatively close. Because the moon is changing its apparent position in depth while the light stimulus remains constant, the brain’s size-distance mechanism changes its perceived size and makes the moon appear very large. “The history of the moon illusion and the details of alternative explanations can be found in my book: The Moon Illusion, by Maurice Hershenson (Lawrence Erlbaum Associates, Hillsdale, N.J., 1989).

Eye gaze is critically important to humans, as social primates. Maybe that’s why illusions involving eyes are so compelling.

FOLLOW MY FINGER: No matter from which direction you look at this image, the finger appears to be pointed directly at you.

The eyes are the windows to the soul. This fact is why we ask people to look us in the eye and tell us the truth. Or why we get worried when someone gives us the evil eye or has a wandering eye. Our everyday language is full of expressions that refer to where people around us are looking. Particularly if they happen to be looking in our direction.
As social primates, humans are very interested in determining the direction of gaze of other humans. It’s important for evaluating their intentions, and critical for forming bonds and negotiating relationships. Lovers stare for long stretches into each other’s eyes, and infants focus intently on the eyes of their parents. Very young babies look at simple representations of faces (such as smileys) for longer than they look at similar cartoonish faces in which the eyes and other features have been scrambled.

In this slide show, we’re going to investigate a series of illusions that take advantage of the way the brain processes eyes and gaze. It turns out that it’s fairly easy to trick us into thinking that someone is looking somewhere else, or that Albert Einstein is actually Marilyn Monroe.