2023-07-12 15:55:00
A test involving a humanoid robot endowed with proprioception, iCub, on November 18, 2014 in Madrid ©BelgaImage
Have you ever wondered how you stand up, or how you manage to perform all the gestures that you don’t even consciously think regarding anymore? The view helps but it is not everything, otherwise you would not be able to walk with your eyes closed. If you manage to do all this, it is thanks to proprioception. In summary, it is the ability to perceive the position of the different parts of our body in space. A true sixth sense which is sometimes described as such. In other words, it is essential to us. But then why is it so little known? And what if we lose it?
Proprioceptive sensors throughout the body
As explained to Arte Edith Ribot Ciscar, researcher at INSERM and the University of Aix-Marseille, “we feel our body because it is equipped with neurosensory sensors in the muscles, joints, tendons… which will be activated by movement“. “When there is excitation of these receptors, there is a nervous message conveyed by the nerves to the brain“. It is this message that is at the origin of this meaning.
You can test your proprioception by doing a little exercise. Hold out one arm and close your eyes. Make exactly the same gesture but with your other arm and open your eyes. Normally, your two limbs end up in the same position, symmetrically. It may seem simple but it is only possible thanks to these proprioceptive capacities. “I often ask my students to close their eyes and touch their noses“, explains to Popular Mechanics magazine Hamid Charkhar, professor of biomedical engineering at Case Western Reserve University in Cleveland. A person who is devoid of proprioception cannot do it.
A drama when proprioception disappears
This is what happens in a few rare individuals in whom these famous neurosensory sensors do not work or only slightly, and this in the absence of any other pathology. “These people are not able to make simple movements, such as holding their hand in one position. We see that their arm is moving in one direction or the other and they do not realize it“, declares to Arte Fabrice Sarlegna, researcher at the CNRS and at the University of Aix-Marseille, who works with patients concerned.
For these people, their sight is absolutely necessary in order to make precise movements, which is not always enough. For example, it is almost impossible to walk. Carrying a glass to drink becomes an ordeal. You not only have to aim your mouth well, but you also have to control the force that you put into this gesture. It is therefore not improbable that if this glass is a plastic cup, it will end up completely crushed, the gesture having been too intense.
A still mysterious meaning
In summary, proprioception is vital. However, it remains little known to the general public, who are more accustomed to the five senses identified since Aristotle (hearing, taste, sight, smell and touch). “The sense of proprioception is less known, in particular because the pathologies that affect proprioception are less known.“, believes Fabrice Sarlegna. “We have all heard of blind or visually impaired people and we can put ourselves in their place simply by closing our eyes. For proprioception, these are internal senses that are difficult to manipulate and understand.“.
For a long time, proprioception was little studied by scientists. This tends to change today. In 2021, the Nobel Prize in Medicine was awarded to two researchers, David Julius and Ardem Patapoutian, who worked on temperature and touch receptors that act in particular on proprioception. But there is still much to discover. In particular, there is still no treatment for people who have lost this sense. Neuroprostheses might represent a hope for these patients, but this system must be perfected before entering the practice.
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