A 35-year-old optical illusion is still astounding people. And it all revolves around a rotated picture of Margaret Thatcher.

But, as with many optical illusions, the strange effect shows a lot about the way that the brain processes images.

The Margaret Thatcher Illusion — which also gets called the Thatcher Effect, along with other names — occurs when a picture is turned upside down. But instead of changing everything in the picture, the effect happens when the features, like the mouth and eyes, are kept the right way up.

The brain is often unable to see that anything strange has happened when the picture was turned the wrong way up.

When the Thatcher Illusion was first developed, in 1980, it presented a huge challenge to the way that scientists understood how the human brain processes faces. It showed that people don’t just think about the face as a whole, but rather understand each of its component parts and assemble them into a whole.

The picture was first shown in a paper by Peter Thompson, of York University. He published it in Margaret Thatcher: a new illusion, in Perception.

In the 35 years since, it has been used on a range of different people. A site, thatchereffect.com, allows users to try the effect on famous people — as well as themselves.

The effect seems to work on almost anyone. The only thing that sometimes throws it out is if a person’s smile is especially distinctive, or notably shaped, which seems to pull the brain out of the trick and force them to notice that there is a problem.

Optical illusions Show all 5 1 /5 Optical illusions Optical illusions These aren't moving either The effect comes from much the same place Optical illusions The squares at A and B are the same colour It's for much the same reason as #TheDress — it's about the way that your brain understands colour Optical illusions And neither is this Yep, this is also a still image Optical illusions These circles aren't moving The trick comes from the way that our brains scan images over and over Optical illusions There's only two colours in this picture The effect comes from the way that the brain receives different parts of the image at different times

Research has found that many monkeys also respond to the illusion in the same way, seeing the face as normal even when it’s turned wrong and lighting up the same parts of the brain. What’s more, that discovery shows that primates have the same ways of understanding faces as humans do.