Even upside down, the photo could appear normal at a first glance. A simple photo, nothing more. And yet, if you look at the image again, you could suddenly see something horrifying: the fact that what you thought it was put together normally is, in fact, a nightmarish collage intended to play a trick on your mind. If you weren’t fooled straightaway, you have been the victim of the Thatcher effect, a visible phenomenon that even exists in primates.The brain, the machine that tells us what we seeOur brain was created to recognize images. For a highly developed social species, and as is part of our survival instincts, it is crucial for us to know how to identify the rudimentary forms of an image through its many details. Whether it is to identify something close up or to detect a predator, this know-how is a part of our genes. It can also lead us to find its paroxysm in pareidolia, which causes us to see certain things in everyday objects.The Thatcher effectHowever, the way that the brain deals with information can sometimes be alarming. As evidenced by the Thatcher effect. This effect was brought to light for the first time in 1980 by Peter Thompson, a psychology professor, who noticed it in a portrait of the British Prime Minister, Margaret Thatcher (which is where the name comes from). It shows that when faced with an image that has been flipped upside down, it is a lot more difficult for our minds to identify local changes, such as the abnormalities that you can see in the photo.In order to identify this effect, Thompson created collages of images in which certain elements are inversed. By flipping the final image back, we notice a reversed image, in which the mouth and eyes are shown the wrong way around, which is a change that our brain seems to find very difficult to detect. Even once the trick was revealed, it is still difficult to look at the photograph again other than as a perfectly normal portrait.A visible phenomenon even found in non-human primatesA study carried out in 2009 has shown that the Thatcher effect can also be observed in non-human primates. There is no difference between our responses that can be seen when looking at an image that has been tampered with and one that hasn’t, suggesting that this phenomenon came to us from our apelike ancestors. This is certainly an effect that will make your head spin!Check out the video above to see this mind-bending illusion in action!