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.

This article was originally published with the title How Blind Are We?.

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