http://tinyurl.com/2jzwe
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In one experiment, people who were walking across a college campus were asked by a stranger for directions. During the resulting chat, two men carrying a wooden door passed between the stranger and the subjects. After the door went by, the subjects were asked if they had noticed anything change.

Half of those tested failed to notice that, as the door passed by, the stranger had been substituted with a man who was of different height, of different build and who sounded different. He was also wearing different clothes.
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Working with Christopher Chabris at Harvard University, Simons came up with another demonstration that has now become a classic, based on a videotape of a handful of people playing basketball. They played the tape to subjects and asked them to count the passes made by one of the teams.

Around half failed to spot a woman dressed in a gorilla suit who walked slowly across the scene for nine seconds, even though this hairy interloper had passed between the players and stopped to face the camera and thump her chest.

However, if people were simply asked to view the tape, they noticed the gorilla easily. The effect is so striking that some of them refused to accept they were looking at the same tape and thought that it was a different version of the video, one edited to include the ape.