Why can you miss a huge change that happens right in front of you?

Change blindness is what happens when something in a visual scene changes and the observer does not notice. The classic version is a picture that flickers off and on with a change hidden in it: observers often fail to notice even major differences introduced while the image flickers. In some cases it can take over a minute of constant flickering before someone finds the location of the change.

Some of the earliest observations came from outside the lab. When film editing was introduced, editors saw that changes to the background were not noticed by the people watching the film. William James had already mentioned the lack of ability to detect change in his 1890 Principles of Psychology. In 1988, Harold Pashler showed that observers were poor at spotting changes in arrays of letters when the display flickered off and on, even if the gap was only 67 milliseconds long.

That result struck Pashler as odd. People generally report having a clear sense of apprehending the identities and locations of large numbers of objects in a scene, yet their detection of changes is surprisingly poor. Part of the reason is that, unless a change produces a localizable transient signal at a specific position on the retina, people typically fail to detect it.

It is not just a matter of pictures. In one experiment, researchers called participants on the phone and replaced the speaker in the middle of the conversation. Participants rarely noticed. Those who were explicitly monitoring for a change did better. People's poor ability to detect changes has been argued to reflect fundamental limitations of human attention.