From Wikipedia:
In visual perception, change blindness is the phenomenon that occurs when a person viewing a visual scene apparently fails to detect large changes in the scene. For change blindness to occur, the change in the scene typically has to coincide with some visual disruption such as a saccade (eye movement) or a brief obscuration of the observed scene or image. When looking at still images, a viewer can experience change blindness if part of the image changes.
For an intriguing example of change blindness, check out the video below depicting an experiment by Daniel J. Simons and Chris Chabris.
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For a sample of related Situationist posts see “‘The Grand Illusion’ — Believing We See the Situation,” “Neuroscience and Illusion,” “Brain Magic,” “Magic is in the Mind,” “The Situation of Illusion” “The Heat is On,” and “The Situation of Climate Change,” or click here for a collection of posts on illusion.