IMAGINE you are walking down the street and a passer-by asks you for directions. As you talk to him, two workmen rudely barge between you carrying a door. Then something weird happens: in the brief moment that the passer-by is behind the door, he switches places with one of the workmen. You are left giving directions to a different person who is taller, wearing different clothes and has a different voice. Do you think you would notice?
Of course you would, right? Wrong. When researchers at Harvard University played this trick on 15 unsuspecting people, eight of them failed to spot the change.
What this demonstrates is a phenomenon called "change blindness". It happens because of a chronic shortage of a crucial mental resource: attention. You are blithely unaware of most of what is going on around you, to the point where you can fail to notice "obvious" changes in your surroundings.
Attention is not well understood, but whatever it is, we have a limited amount. Of all the information entering or being generated by your brain at any one time - sights, sounds, memories, ideas and so on - only a tiny fraction enters your consciousness. Object-tracking studies suggest that the maximum number of items we can attend to at any one time is around five or six (see demos at http://ruccs.rutgers.edu/finstlab/demos.htm).
Scientists studying attention spend a lot of time playing with change blindness because it provides direct access to the attentional system. In the door experiment, the subjects fail to see the change because their attention is elsewhere and the door conceals what would otherwise be attention-grabbing motion.
You can experience the same thing by watching "flicker images". These consist of two consecutive images that differ only in one key feature - two cowboys who swap heads, say. If the images are flashed up in quick succession with a brief blank screen between them (which acts like the door), most people take an astonishingly long time to spot the difference (see demos at http://www.psych.ubc.ca/~rensink/flicker/download, or try flicking your attention between the two images in the diagram below).
Similarly, we often fail to notice blatant continuity errors when films cut from one scene to another. We also usually fail to detect gradual changes to a static scene, such as the addition of a large building (see demos at http://viscog.beckman.uiuc.edu/djs_lab/demos.html and http://nivea.psycho.univ-paris5.fr/Slow%20changes%20bis/intro.html).
"Basically, the explanation is that attention is needed to see change," says psychologist Ronald Rensink of the University of British Columbia in Vancouver, Canada. "Attention is drawn automatically to the motion signals that accompany a change. But if these are swamped, then the observer can't rely on automatic control, but needs to hunt around with their attention."
A similar phenomenon is motion-induced blindness, in which concentrating on a moving pattern causes what should be very prominent static objects - such as bright yellow dots - to disappear (see demos at http://pantheon.yale.edu/%7Ebs265/demos/MIB-percScotoma.html). Motion-induced blindness was only discovered in 2001 and it is still unclear why it happens, but most researchers think it has something to do with attentional resources.
There is a related and even more counter-intuitive demonstration of our limited capacity for attention. If you are deliberately concentrating on something, it can render you oblivious to other events that you would normally have no trouble noticing. This "inattention blindness" is probably the reason why motorists sometimes collide with objects such as pedestrians and buses that they simply "didn't see".
The most famous demonstration of inattention blindness was staged in 1999 by Daniel Simons and Christopher Chabris of the University of Illinois at Urbana-Champaign. It involves a game of basketball. Chances are you've seen it or read about it before. If not, have a look at http://viscog.beckman.uiuc.edu/grafs/demos/15.html. The task is to count the number of passes made by the team in white. You won't believe your brain.