The study of optical illusions has been of great help to psychology in that it can reveal about perceptual processes. To give an example, understanding how our brain works with respect to proprioception , has been very useful for patients who have suffered an amputation. 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 allowed us to acquire new knowledge and to better understand what is going on in our brain. A group of psychologists from Pennsylvania (United States) discovered a curious illusion, known as the “rubber hand illusion”.

The 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 illusion of the rubber hand happens:

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

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

The property of the body is a term used to describe the sense of our physical self and to differentiate it from the fact that it is not 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 its legs because they belong to their own body.

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

For Henrik Ehrsson, neuropsychologist at the Karolinska Institute in Stockholm, Sweden, “The illusion of the rubber hand 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 more intensely the rubber hand is experienced, for example by hitting it hard, the more activity there is 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 individuals who have conducted experiments with the rubber hand are aware that the hand is not part of their body, the brain regions that are activated by fear and threat, and which correspond to flight, are also activated more.

What about the real hand that is hidden?

Another interesting finding was made by a group of scientists from the University of Oxford, 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 when the brain falsely recognizes the rubber hand as its own, the temperature of the real hand, which is hidden, drops . Instead, the rest of the body remains the same.

In addition, 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

Illusion-based experiments 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 of C within the Brain and Cognition at the University of California, San Diego, Vilayanur S. Ramachandran, also took an interest in this type of optical illusions to design the 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 moves in such a way that the person thinks he is moving the accused hand. In this case, the hand in the mirror acts as the rubber hand, and because of this, the pain disappears through visual feedback and by eliminating potentially painful positions. This technique provides feedback to the brain and relieves the pain 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”.