PRETEND YOU ARE a member of an audience watching several people dribbling and passing a basketball among themselves. Your job is to count the number of times each player makes a pass to another person during a 60-second period. You find you need to concentrate, because the ball is flying so quickly. Then, someone dressed in a gorilla suit ambles across the floor (right). He walks through the players, turns to face the viewers, thumps his chest and leaves. Astonishingly, as Daniel J. Simons, now at the University of Illinois, and Christopher F. Chabris of Harvard University learned when they conducted this study, 50 percent of people fail to notice the gorilla.

We think of our eyes as video cameras that make a flawless recording of the world around us, but this demonstration shows how little information we actually take in at a glance.

The gorilla experiment is the culmination of a long line of related studies on attention and vision that began more than three decades ago by, among many researchers, Ulric Neisser of Cornell University, Ronald A. Rensink of the University of British Columbia, Anne Treisman of Princeton University, Harold Pashler of the University of California at San Diego and Donald M. MacKay of Keele University in England.

Researchers refer to the gorilla effect as “inattentional blindness” or “change blindness,” which in turn is part of a more general principle at work in our visual system. Our brain is constantly trying to construct meaningful narratives from what we see. Things that do not quite fit the script or that are not relevant to a particular task occupying our interest are wiped wholesale from consciousness. (Whether such deleted information is nonetheless processed unconsciously has yet to be investigated.) A simple example of how the brain’s running narrative can interfere with perception is the children’s game “spot the difference” (below). The two images are similar enough that the brain assumes they must be identical; it takes minutes of careful inspection to locate the disparities.

The value of having an underlying brain “story” becomes clear when you consider how jumbled sensory inputs can be. As you survey the room around you, the image on your retina is jumping rapidly as various parts of the scene excite different bits of retina. Yet the world appears stable. Researchers once believed that the experience of having an unbroken view was entirely created by the brain sending a copy of the eye movement command signals originating in the frontal lobes to the visual centers. The visual areas were thought to be “tipped off” ahead of time that the jumping image on the retina was caused by eyes moving and not by the world moving.

But an effect you can demonstrate for yourself at home shows that this cannot be the entire reason. (Jonathan Miller, an opera director in London, and one of us [Ramachandran] independently observed the effect in the early 1990s.) Turn a television set upside down. Gently! Better yet, flip the TV’s image optically with a prism. Alternatively, you can turn the TV sound off and then stand slightly to the side of the set, looking at the screen with your peripheral vision. Put the TV on any channel and watch what happens. You will see sudden, jarring changes and visual jolts. Next, gaze at the broadcast with the TV right side up, viewing it straight on and with the sound at normal volume. Now the cuts and pans of the camera flow smoothly and seamlessly into one another—in fact, you do not even notice them. Even when the scene switches, say, from one talking head to the other as they alternate in conversation, you do not see a head transforming or morphing from one to the other as your mind alternates between each of the two speakers. Instead you experience your vantage point shifting.

What is going on? The answer is that when the TV is right side up and you can hear the sound, the brain can construct a sensible narrative. The cuts, pans and other changes are simply ignored as irrelevant, however gross they might be physically. In contrast, when the scene is upside down or viewed with peripheral vision and the sound off, it is hard for the brain to make meaningful sense out of what the visual centers perceive, so you start to notice the big changes in the physical image. This effect is not true just for visual scenes on the boob tube but also for your entire life’s experiences; the unity and coherence of consciousness is mostly convenient, internally generated fiction.

The scene does not have to be complex for change blindness to occur, either. In 1992 Colin Blakemore, now chief executive of the Medical Research Council in London, and Ramachandran conducted an experiment on attendees of a seminar we gave at the Salk Institute for Biological Studies. We first showed a movie frame containing three abstract, colored shapes: a red square, a yellow triangle and a blue circle (above). We left this frame up for two seconds, then replaced it with the same three shapes, which were each shifted in position by a small degree. The audience observed that all three appeared to flicker or “glitch” slightly. The big surprise came when we then swapped one of the three shapes—the circle—with a different form—a square (left). Most people simply did not notice, except in those few instances when someone accidentally happened to be focusing all his or her attention on that particular object. Even with three simple objects, we experience sensory overload and change blindness.

Finally, imagine that you are staring fixedly at a little red X. Slightly off to the left we briefly show you a cross. All you have to tell us is which is longer—the cross’s vertical or horizontal line. That task is something people can do effortlessly. Now we surreptitiously introduce a word directly on the cross during the second that you are judging line lengths. Arien Mack of New School University and Irvin Rock, then at Rutgers University, discovered that people will not spot the word.

Maybe you are reading this article in a busy cafe. Have you noticed any gorillas walking by? Given the Simons demonstration, how can you be so sure that none did? We suppose it depends on how interesting and attention grabbing you have found this article to be.