Does this person look normal to you?
How about now?
The eyes, and the brain, can fall prey to a range of optical illusions, for a variety of reasons.
One such illusion is known as the Thatcher Effect. Named for former British Prime Minister Margaret Thatcher, since it was a photo of her that was famously used to illustrate the illusion, shows that the brain cannot properly process a photo of a face which is upside-down.
It demonstrates this by modifying upside down faces, flipping the mouth and eyes right side up. Because of this, and how our minds have learned to recognize faces, the brain believes nothing is wrong...until the image is turned right side up. Only at that point does the brain finally realize that yes, something was indeed very wrong with the image.
The Thatcher Effect demonstrates the importance of the brain’s processing of visual information, and the fact that recognizing faces is something we learn to do as our vision develops.