Go to a mirror and look at either of your eyes. Then, while keeping your head still, look at the other one. As you do this, your gaze will change targets, as you are now looking at something different than before. But your eyes will not appear to move.
Now, go find a friend and repeat the experiment. Ask him or her to tell you if they see your eyes move as you glance from one eye to the other. Invariably, they will tell you that your eyes did indeed move — and obviously so. Switch roles and the illusion becomes obvious: your friend, staring into the mirror, is moving his or her eyes — but unlike the rest of the world, sees no movement.
What’s going on here? Our brains are protecting themselves from the fuzzy, blurry imagery which we’d otherwise “see” as our eyes glance quickly from point to point. That movement — called a “saccade” (pronounced “sah-COD”) — is simply too quick for our brains to deal with. So the brains, in effect, ignore what the eye sees, in a phenomenom called “saccadic masking.” Instead of processing and recording the blurred image otherwise caused by the eye movement, the brain replaces that milliseconds-long moment with a still image of the second item your eyes look at. This image replacement aspect can create an eerie effect if one quickly darts his or her eyes at an analog clock, causing the clock’s second hand to appear momentarily frozen in time (known as the “stopped clock effect“). It also is the reason why the image, above, seems to move — even though it is entirely static. (A larger version of the image, and more like it, can be found here.)
During these saccadic masking moments, we are, effectively, blind. According to some, these tiny moments of time lost down the memory hole add up to as much as 30-45 minutes a day — leaving us temporarily blind for roughly 2% of our lives.
From the Archives: The First Photograph of a Person: Early photographs did not contain people, because the exposure period was too long for subjects to remain still. Here’s the story about the first photograph containing a person — an accident in the making.
Related: “Optical Illusions: The Science of Visual Perception” by Al Seckel. A collection of 275 optical illusions with some explanation as to why they fool the mind.