That illusionary smile

Do illusions tell us anything about how our brain processes faces?

In everyday life we encounter faces that are mostly in the right order: eyes above nose, nose above mouth. Researchers call this a configuration, and our brains seem to understand that a face is there even if it’s fairly low in details. A good example is the smilies (aka emoticons) used in text messages. There is very little information in :-) or :-( but we still see a happy or sad face. That’s because the configuration (eyes, nose, mouth) still looks like a face even if it’s on its side. But what happens when you turn a face upside down? Our brains still recognise it as a face, but we can do strange things to it and our brains won’t notice. The Thatcher Illusion, named after the famous UK prime minister, is a classic example. Two upside-down faces are put next to one another. They both look like normal faces, but when you turn them both the right way up, you see that one picture has had the eyes and mouth turned upside down. It now looks grotesque and obvious, but you didn’t notice this when the pictures were inverted. Why? Because your brain has problems with inverted faces. You don’t see many of them in daily life, so all it can manage to do is recognise all the parts of a face in roughly the right upside down order. The fine detail – the fact that the eyes and mouth are the wrong way – is lost.

You can also take a look at the Thatcher illusion for yourself.

Facing up to the potato

Another illusion with faces comes about when you take a look inside a plastic Halloween mask. They are made by stretching plastic over a mould of a person’s face. Looking at the mask from the front it’s clearly a face protruding out at you. Now take a look behind. It starts out looking hollow, but if you look at it for a bit, perhaps closing one eye, it starts to look like a face peaking out at you too. Researchers have argued that this is because we don’t normally experience faces that curve inwards, only faces that point outwards, so given the same information our brains will see a hollow face as a normal face. Perhaps this is a special face thing? Well perhaps not. You can take any shape – for example, a potato – and make a plaster cast of it. Looking into this cast it’s hollow, but your brain sees the potato shape bulging out, just like the real thing.