How can you experience this yourself?
You will need:
- A stand-up table mirror.
- Another person.
How to do it:
- Person 1 should sit or stand at a table, opposite Person 2, with the mirror lengthwise between them.
- Person 1 places their hands on the table – one either side of the mirror – and looks into the mirror at the reflected hand. They need to spread out their fingers and keep watching the mirrored hand.
- Person 2 needs to be able to see both of Person 1’s hands, as they need to touch both hands at the same time and in the same place.
- Person 2 should count Person 1’s fingers out loud whilst touching each of them in turn.
- Then, starting with the thumb, Person 2 should stroke the first four fingers of both hands at the same time.
- Upon reaching the little finger, Person 2 needs to do something different on each hand. On the visible hand (seen hand), they should stroke the top of the little finger as they say five, then stroke the table in a line from the base of the little finger as they say six. On the hidden hand (behind the mirror), they should stroke the inside of the little finger as they say five and the outside of the little finger as they say six.
What should happen?
Most people experience the feeling that they have an extra finger (that they cannot see) on the hand behind the mirror. A few people briefly see the extra finger in the mirror, but it quickly disappears.
How does this relate to pain?
The brain puts information together to come to conclusions about the body. This illusion shows that the brain can easily come to the wrong conclusion about the body. Even though you have (most likely) had five fingers for your whole life, you can quickly feel like you have six. If the brain can come to the wrong conclusion about something as fundamental as the number of fingers that you have, it makes sense that it can also come to the wrong conclusion about possible danger to the body. In persistent pain, the brain often concludes that the body is under threat and alerts us to this with pain, even when there is no danger. The brain becomes overprotective, meaning that we feel pain before we even get close to damaging our body. The brain is trying to protect us from things that it sees as threats, but it repeatedly comes to the wrong conclusion, making the body continuously feel different to how it really is.
How does this illusion work?
The brain is doing what it does all the time – combining what you can see with what you can feel, so that it can make sense of the body. The brain uses the assumption that the hand reflected in the mirror is your other hand – a very reasonable assumption for the brain, as it looks similar, is in a similar place, and feels touch at the same time. Then, when someone touches your actual hand in a way that is different to how it looks in the mirror, the brain comes to the wrong conclusion, because its assumption was wrong.
More information
The brain isn’t perfect at knowing where on the body has been touched, so when the demonstrator touches the inside of your hidden little finger, while touching the top of your reflected finger, it usually trusts what you can see. Then, when it feels a touch that is further across than the fifth finger, and sees a touch that is where a sixth finger could be, it combines that information and concludes that you suddenly have six fingers! For the brain, this is the most sensible explanation. It is amazing how adaptable the brain is.
Fun fact: this illusion was invented by Dr Roger Newport and he called it the Anne Boleyn Illusion because she was rumoured to have six fingers on one hand – but she didn’t really.
Additional fun
- Can you wiggle your extra finger?
- Can you make the extra finger really long? Try this by stroking further on the reflected table whilst stroking the outside of the hidden little finger more slowly.
- Can you make the extra finger look like it is going in a different direction?
- Can you make the extra finger wiggle-shaped?