What do you think of this fact?
Your brain knows where your body ends. Or so you’d think. A simple trick with a fake hand and two paintbrushes shows that this border is far more flexible than it feels.
Two brushes, one rubber hand
In 1998, Matthew Botvinick and Jonathan Cohen published a paper in Nature called “Rubber hands ‘feel’ touch that eyes see”. The setup, as ScienceDaily described it: each participant rested their left arm on a table, hidden behind a screen. In front of them, in plain view, lay a rubber model of a left hand and arm.
While the participant looked at the rubber hand, an experimenter used paintbrushes to stroke the fake hand and the hidden real hand at the same time, matching the strokes as closely as possible.
The result was bizarre. Participants reported that the touch they felt seemed to come from the brush they could see on the rubber hand. Eight of the 10 people who experienced this illusion also said the rubber hand felt like their own.
Your body sense is an estimate
Why would anyone accept a piece of rubber as part of themselves? The authors write in Nature that three things interact here: vision, touch and your sense of where your body parts are. When seeing and feeling match perfectly, your brain goes with the most convincing story, even if that story includes a fake hand.
According to the Lancaster Glossary of Child Development, the illusion even changes where people think their real hand is: their judgment of its position shifts toward the rubber hand. The authors of the original study see this as a clue to how we decide what belongs to our own body in the first place.
Bonus fact: Children experience the rubber hand illusion too, and according to the Lancaster glossary, even more strongly than adults.
You don’t just have a body. Your brain keeps deciding which parts of the world count as you.