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Imagine that you are on the street and someone pulls you aside to ask you directions. You begin to engage this person in conversation. For mysterious reasons, in the middle of this conversation, workers, carrying a door, walk between you and the other person. When the door is gone, the original person has been replaced by a different person. Would you notice if Stranger One was replaced by Stranger Two? The intuitive answer is" Of course, I would." How could you fail to realize that you were no longer talking to the same person? The empirical answer, from a clever experiment by Dan Simons and Dan Levin (1997) is that people fail to notice this change about 50% of the time. Surprising as this finding might be, it is not an isolated phenomenon. In recent years, a growing number of reports have seemed to show that we see much less than we think we see. Simons and Levin have produced a number of variations on this basic theme. In one series of experiments, subject watch a video tape of a conversation. The camera cuts from one speaker to the other. Speaker One is sitting at a table. She pours herself a cup of Diet Pepsi. The camera cuts to Speaker Two. When the camera return to Speaker One, the Pepsi bottle has been replaced with a cardboard box. Subjects might remember that Speaker One poured herself a drink but they frequently fail to notice that the bottle has vanished.

Grimes obtained a similar effect when he changed the contents of images during eye movements. He found that observers were strikingly insensitive to substantial alterations in the content of scenes. These were unsubtle changes such as trees disappearing from …