The illusion of the rubber hand: a curious psychological effect.

An optical illusion that has therapeutic purposes.

The study of optical illusions has been of great help to psychology because of what they can reveal about perceptual processes. For example, understanding how our brain functions with respect to proprioceptionhas been of great help to amputee patients. Thanks to techniques such as the mirror box, it is possible to reduce their phantom Pain and improve their quality of life.

For several decades, science has been interested in these phenomena. And technological progress has made it possible to acquire new knowledge and better understand what happens in our brain. A group of psychologists in Pennsylvania (USA) discovered a curious illusion, known as the "rubber hand illusion".

Researchers realized that if we put a rubber hand in front of us and, at the same time, cover one of our arms so that it looks like the rubber hand is part of our body, when someone caresses our rubber hand, we will feel that they are caressing our real hand.

Below you can visualize how the rubber hand illusion happens:

The rubber hand illusion, more than just a trick for illusionists.

The rubber hand illusion not only became a trick for illusionists, but it was also an important finding because it it provided insight into how sight, touch and proprioception (i.e., the body's sense of position) combine to create a convincing sensation of the hand. to create a convincing sense of body ownership, one of the foundations of self-awareness.

Body ownership is a term used to describe the sense of our physical self and differentiate it from not being part of us. It is what allows us to know that a hammer we are holding in our hand is not part of our body or, in the case of animals, to know that they should not eat their paws because they belong to their own body.

The discovery of the rubber hand illusion has inspired many researchers.

For Henrik Ehrsson, a neuropsychologist at the Karolinska Institute in Stockholm (Sweden), "The rubber hand illusion has inspired many researchers, and many studies have tried to find the answers to this phenomenon. Science has wanted to know how the body is perceived by our mind, and how the integration of this information happens."

Scientists have found that the greater the intensity with which the rubber hand illusion is experienced, for example by hitting it hard, the greater the activity in the premotor cortex and parietal cortex of the brain. These areas are responsible for integrating sensory and movement information.. But of course, stroking the hand is not the same as hitting it. And although the individuals who have performed experiments with the rubber hand are aware that this hand is not part of their body, the brain regions that are activated by fear and threat, and that correspond to flight, are also more active.

What happens with the real hand that is hidden?

Another interesting finding was carried out by a group of scientists at Oxford University, who wanted to know what happens to the hand that is hidden during the experiment. If the brain reacts to the rubber hand, does it also react to the hand that is hidden? Well, it seems that just as the brain falsely recognizes the rubber hand as its own, the temperature of the real, hidden hand drops, the temperature of the real hand, which is hidden, decreases.. In contrast, the rest of the body remains the same.

Moreover, when the experimenter stimulates the hidden hand, the subject's brain takes longer to respond than when the other authentic hand is touched. These results seem to show that, when the brain thinks that the rubber hand is a real hand, it forgets about the other hand.

This has been really interesting for medicine because it shows that thermal regulation of the body also depends on the brain.

Mirror box therapy: another example of optical illusion

Experiments based on illusions have helped amputee patients who continue to feel pain even though the limb is no longer part of their body, known as "phantom pain."

The neurologist at the Centro Brain and Cognition Center at the University of California, San Diego, Vilayanur S. Ramachandran, also became interested in this type of optical illusions to design Mirror Box Therapy, which works to reduce phantom pain.

The mirror box has similarities to the rubber hand illusion.. In the mirror box, the good hand is placed next to a mirror and moved so that the person thinks he or she is moving the imputed hand. In this case, the mirror hand acts as the rubber hand and, thanks to this, the pain disappears by visual feedback and by eliminating potentially painful positions. With this technique it is possible to give feedback to the brain and relieve the pain that the person feels.

If you want to know more about the mirror box, you can read this article: "The phantom limb and the mirror box therapy".