When viewers are told beforehand that there is a hidden seventh face, they immediately begin searching for one. Once a person believes there is something unusual to find, the brain becomes much more likely to interpret ambiguous shapes as meaningful.
Psychologists have long known that expectations influence perception. Two people may look at the same image and notice completely different things depending on what they were told beforehand.
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Why Viral Optical Illusions Spread So Quickly
Images like this often become internet sensations because they encourage participation.