How can you experience this yourself?
You will need:
- A hexagonal pencil (a pencil with edges)
- A smooth-edged pencil (or a marble, or other small, smooth item)
How to do it:
- Roll the smooth pencil between your thumb and first finger and notice how smooth it feels.
- Then, squeeze the ridges of the hexagonal pencil hard (not so hard that it hurts, but as hard as you can otherwise) between your thumb and first finger for around a minute, until there are small dents in your skin.
- Swap back to the smooth pencil and roll it again between your thumb and first finger, where the dents are – notice how it feels now.
What should happen?
The smooth pencil should feel different to the first time. It may feel like it has edges, is bumpy, or like it is not smooth anymore.
How does this relate to pain?
Our brains are constantly making assumptions and coming to conclusions about our body to help us. The brain assumes that the skin is always pretty flat, so concludes that the dents in the finger must actually be on the pencil, which makes the pencil feel as though it is bumpy. If the brain concludes that part of our body is under threat, it may bring this to our attention in the form of pain, even if our body is actually safe. This means that it has become overprotective, which happens a lot with persistent pain. If we can change how the brain interprets the body, we can change how the body feels to us.
How does this illusion work?
In this illusion, the brain uses the assumption that the skin is essentially flat. This is quite a reasonable assumption for the brain to make, as it is almost always true, so it is easier for the brain to treat the surface of the skin like a flat map. When edges are felt, the brain decides that they must belong to the object in the hand – the pencil – not the skin. The brain has come to the wrong conclusion, interpreting the information before bringing it to our attention, meaning that the pencil feels different to how it really is.