The “rubber-hand illusion” demonstrates how easily this occurs. In essence, the illusion works by convincing a person that a fake hand belongs to his or her own body. To do so, one hand is hidden from view and a rubber hand is placed in front of the participant. By simultaneously stroking the rubber hand and the hidden one with a brush, the person will eventually feel as if the fake hand were their own. This is because the mismatch between visual and proprioceptive input causes the brain to shift its map of hand position in an uncanny phenomenon known as “perceptual drift.”
Scientists are still uncertain why this happens. Yet research on such phenomena may help explain how the brain creates and maintains a representation of the body, which could have wider applications in medicine. Some studies suggest that fear of falling in older people may be due to declining proprioception, for instance.
In Your Genes
Far less is known about proprioception than about the standard five senses — until recently. In a study published in Nature Neuroscience, a team of researchers finally identified the molecule responsible for proprioception.
Scientists had been meticulously searching for it for 50 years, having already figured out taste and smell, says Patapoutian, who led the study. The sensor protein, called “Piezo2,” converts stretching and muscle tone into extremely fast nerve impulses similar to an electric current. When the researchers switched off the signaling protein in mice, they exhibited severe movement abnormalities.
Recently another group of scientists detected mutations in the Piezo2 gene in two girls who have rare disorders affecting both proprioception and normal musculoskeletal development. Both subjects, who are unrelated, have difficulties walking and cannot run or jump. The results of the study, by Drs. Carsten Bonnemann and Alexander Chesler, were published in The New England Journal of Medicine.
The researchers also performed a series of tests and compared the patients to healthy volunteers. When they tried to walk blindfolded, the girls stumbled, staggered, and had to be prevented from falling. Nor were they able to reach for an object in front of their face. Controls were able to perform the tests without much difficulty. The authors say the results suggest the girls totally lack proprioception.
Whether proprioception might play a role in musculoskeletal development is still unclear. The authors believe that lacking proprioception could lead to an inability to hold certain postures or positions, which in turn might directly or indirectly influence the development of your own skeleton.
Commenting on the study, Patapoutian called the results “intriguing.” Could proprioception, or touch, play a role in skeletal development? “I think it’s very plausible,” he says. No one knows how many patients have the mutation, but the implications could be far-reaching. “This is how new syndromes and diseases are discovered,” says Patapoutian.
Scientists are still unraveling the complex processes underlying proprioception. Further studies will certainly yield more answers about this mysterious sixth sense. In “The Man Who Mistook His Wife for a Hat,” the neurologist Oliver Sacks wrote of a client who called proprioception “the eyes of the body.” Indeed, proprioception may turn out to be just that — and much more.
This article was first published in Brain World Magazine’s Summer 2017 issue.
