Donders Wonders

With a tail between your legs

This post is also available in Dutch.

Does your brain know where your body ends and where the world begins? Research shows that this boundary is more vague than you might think. But how far does this flexibility go?

Picture by Lisa E. Rombout (CC BY 4.0)

The rubber hand illusion shows that sometimes we can feel as though external objects are a part of our body. Do it yourself: Hide your own hand under the table, put a fake hand in its place, and ask someone to touch both hands in the same way and at the same time. In no time, and against all your rational considerations, your brain will decide that the rubber hand is now part of your body.

A trick with serious consequences
This is a nice little joke, but it is such a powerful illusion: Tricking the brain! But it goes much further than that. If someone suddenly hits the rubber hand with a hammer, everything in your body will react as if you’ve really been hit yourself. Adrenaline is released, your heartrate speeds up, and your immune system sends a bunch of white blood cells to the ‘damaged area’. All in all, that rubber hand is taken quite seriously by your brain.

The most likely explanation for the power of this illusion is the combined information from two senses. You see and feel your hand being touched, and therefore, it is quite unlikely that both your eyes and your touch-sensors are lying to you at the same time. By combining signals from the senses, your brain makes a prediction about what belongs to your body and where all its bits are at this particular moment.

All hands on deck!
Your brain is prepared to overlook a lot in order to make sense of these different signals. You can test this if you happen to own a whole collection of fake hands (we’re not judging). A hand with a different skin-color? No problem. More or less hairiness? Fine. A bright purple hand with tiger-stripes? It will be considered a new body part as long as it’s in the right place, has the right size, and looks vaguely hand-shaped.

The artist Stelarc experimented years ago with an extra robot-hand. However, more recently, including additional parts to the body have been studied more closely in virtual reality. Adding a third hand turns out to be perfectly possible. It takes a bit longer for the illusion to establish itself, but after a short while you become just as attached to it as you are with your own two hands.

Your head can make tails of it
People can even be convinced that a tail is a part of their body. Researchers in London gave people a virtual tail in a game. This peculiar new body part reacted to hip movements, which gave participants a strong feeling that it was part of their actual body. At the end of the game the tail burst into virtual flames and was completely reduced to ashes – scientists can be pretty mean sometimes. Again, the body reacted strongly with stress-hormones and increased activity of the immune system. All that for a completely new body part!

So how far can we go with this, and how can it be applied in a useful way? Will future humans have extra arms, tails, and maybe even extra senses…? What would you like to add to your body?

This blog was written by Lisa E. Rombout. She is a PhD candidate in cognitive science & artificial intelligence at Tilburg University and organizes events around coding. Apart from that she likes reading, good whisky and the absurdity of being. Read more on

Edited by Angelique.

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