The Thatcher Illusion is a very well known illusion in the field of upside down illusions. The illusion is often called the Thatcher illusion, because its discoverer, Peter Thompson, used a photo of Margaret Thatcher in his experiment.
What to see?
That’s easy… all you have to see are two pictures of two faces of a man or woman and to spot the resemblances or differences. One of the pictures is represented in a normal position and the other is represented upside down.
What to do?
If you look at them as we shown them to you in the first case you will probably see nothing wrong, maybe just a few shades in the upside-down picture. Now… if you look at them in the same way, the right way… What do you see? A weird face, with plenty differences. In the inverted photo the mouth and eyes are upside down. This doesn’t bother us all that much. In fact, we often don’t even notice it, and the expression looks pretty close to the normal face. But when we see both faces in a normal way the facial features strike are revealed.
How it works?
There are several hypotheses about this kind of illusion. The first I’m going to present you is the one I like the most. The expression illusion it’s created because we have a problem in seeing the grotesque. The inversion disrupts the perception of grotesqueness a Thatcher’s face. In a simpler way: we try not to see the unusual because the human brain is made to see the good things, things we can accept and live with. For example we have a problem looking at the grotesque image because it represents something abnormal in nature.
The second hypothesis is linked with the reference frames. When we view an object like a face, we use two reference frames, one of which is based on the object, and the other on our egocentric or contextual sense of orientation. When the faces I are upside down, the top of the mouth and eyes differs for the two reference frames, but when the faces are in the right place, the two reference frames are in agreement, and we get an ugly image.
The last hypothesis is based on dual process theories of facial perception, which is well supported empirically. This theory is based on the idea that we process the features of the image and its configuration by looking at the pictures. When the images are inverted we don’t process the information because we can’t see the configuration in a normal way.
Some studies have shown that the brain sees all the differences even when the pictures are inverted, but the consciously we can’t process the grotesque so we prefer to see them the same.