Translated by Rumia Bose

What is the boundary of your body? Where does your body end? That seems fairly clear: at your fingers, toes, crown, in fact everywhere on the outside of your body. This may be clear to you, but not to your brain! The rubber hand illusion is a powerful example. Watch this:

Your brain decides

In the rubber hand illusion, your brain is taken for a ride. The information from your various special senses is at first separate and is then integrated. In this case, touch enters in part of your cortex (the sensory cortex) and what you see in another part of the cortex ( the visual cortex), and this is combined in yet another part of the cortex ( the temporal and parietal lobes). What you feel on your own hand and what you see on the rubber hand are exactly the same, so it is combined in your brain. You might think: the woman did say, “ it feels like you are doing that ( brushing) to my hand, but that’s not my hand.” Therefore, she probably is aware that the place where she is being touched with the brush is somewhere different to what she is seeing. What she sees does not look much like her hand. And yet the combination of perceptions is so strong that she has a nasty shock when the man hits the rubber hand with a hammer. And then, “…my brain decided that you hitting that was hitting me..”. She expresses what is happening very accurately. She is being cast back and forth between the various interpretations of her perceptions. But when it comes to the crunch, the decision is made – outside of her consciousness – that the rubber hand is hers.
There is a constant stream of information to the brain from all the special senses. The brains are continuously occupied in combining this information. In addition to this, the number of possible combinations is almost infinite. We can however only react to one combination: our actions are based on one of these combinations. A choice must be made between those combinations, one of them must take precedence. Normally you do not notice anything of all this. When this decision is made then this one combination penetrates your consciousness.

Tools become a part of your body

You might find it strange that you can experience a rubber hand as real as your own. It is pretty far-fetched to experience something outside your own body, something which you know is made of rubber, as if it were your own limb. But this is not as unlikely as it seems. When you reach out to grasp something, such as a sweet, you use your hand as a grasping tool. In order to apply the tool correctly, the hand has to be moved accurately by the brain, and that must be adjusted according to what you see: where the sweet lies, how big it is. For this to happen, the information about what you feel with your hand is combined in the parietal lobe with information about what you can see around the hand. What is special about this is that if you move your hand somewhere else, then the visual information which comes into that part of the parietal lobe remains centred around your hand. So your hand, the movement and what you see close by your hand is – and stays – linked. This is known as the PeriPersonal Space (PPS). For your brains, this makes your body limit hazy. What you see close to your hand and how your hand feels become one.
In general, the PPS extends to about a foot beyond your hand. But it gets more interesting. If you use a different tool then this can change. For instance, if you see a sweet that is two feet away from you. Normally you would bend forward to pick it up and it would be within reach of your hand again. But if you were bound to your chair, then this is not possible. Imagine that you have a small rake within your reach. You can grasp this and pull the sweet towards you. If you have done this a few times, it works easily and smoothly. What has changed in the parietal lobe? The rake is handled in the parietal lobe as if it were a part of your body. The PeriPersonal Space gets bigger, in this case up to two feet beyond your hand.
The tool becomes – as it were – a part of your body. And in this way you can use it just as quickly and effectively as your own hand. This can go even further with a lot of talent and practice. Try asking Max Verstappen if he has to think about how to move his hands and feet while driving his Formula One car. That is not the case. It feels for him as if the car is a part of himself. And he knows, without looking or thinking, where the wheels and the side of his car are positioned. The whole car is part of his body in his PPS. This way he can race at top speed right next to a wall. A different example is provided by people who are wheelchair-bound. The wheelchair also takes its place in their perception as a part of the body.

If you can’t choose consciously, then your unconscious does the trick

Which brings us back to the rubber hand illusion. You feel how your hand is brushed and you see the brush moving along the rubber hand. In your brain, all sorts of different combinations of perceptions are struggling for precedence. The illusion that the rubber hand is yours wins the struggle because the hand is so important for the use of tools, and because there is a special part of the brain that makes this link.
The woman in the experiment somehow senses this struggle. She says, “When you hit that thing I thought I was going to lose my hand…and that you hitting that was hitting me”. Whereas “somewhere” she knows that the rubber hand is not a part of her body. “Somewhere” is not a random word here. Because it does not hurt when the rubber hand is struck. A different combination of perceptions – that was lying there in wait all along – wins here; she knows at once that the rubber hand is not hers, although it takes a while to recover from the shock.

References

Clark, A. (2016). Surfing uncertainty : prediction, action, and the embodied mind. Oxford University Press. eISBN 978–0–19–021703–7, Ch8.9; Extended Predictive Minds

Martel, M., L. Cardinali, et al. (2016). “Tool-use: An open window into body representation and its plasticity.” Cognitive neuropsychology 33(1-2): 82-101.

Serino, A., E. Canzoneri, et al. (2015). “Extending peripersonal space representation without tool-use: evidence from a combined behavioral-computational approach.” Frontiers in behavioral neuroscience 9: 4.

Galli, G., J. P. Noel, et al. (2015). “The wheelchair as a full-body tool extending the peripersonal space.” Frontiers in psychology 6: 639.

Cardinali, L., C. Brozzoli, et al. (2009). “Peripersonal space and body schema: two labels for the same concept?” Brain topography 21(3-4): 252-260.

Clark, A. (2008). Supersizing the mind; embodiment, action, and cognitive extension. Oxford University Press. ISBN 978-0-19-533321-3; p. 28, p.41

Frith, C. D. (2007). Making up the mind; how the brain creates our mental world. Blackwell Publishing. ISBN 978-1-4051-6022-3; p.65

Obayashi, S., T. Suhara, et al. (2001). “Functional brain mapping of monkey tool use.” Neuroimage 14(4): 853-861.