When you close your eyes and bring the fingertips of your right hand to touch those of your left, you somehow sense that they will meet. Yet how did you know where your hands were positioned? The reason you know this is a phenomenon called proprioception.
Sometimes called the “sixth sense,” proprioception is crucial for controlling our movements and knowing where our body is in space. While senses such as sight convey information to the brain from the outside world, proprioception allows us to perceive the inner state of our bodies. Unlike the other senses, there is no single sensory organ, nor are we consciously aware of proprioceptive signals that the brain receives.
Proprioception is vital to our daily life, however. “It’s a sense that we depend on very deeply,” says Dr. Ardem Patapoutian, a professor in the Dorris Neuroscience Center at the Scripps Research Institute in La Jolla, California. “Without it we couldn’t walk or stand up.”
It is also crucial to our sense of body ownership. Proprioception forms the basis for distinguishing “self ” from “nonself.” Although we tend to take it for granted, a feeling of body ownership is the foundation of our self-awareness.
The idea that we have five senses — vision, hearing, smell, touch, and taste — can be traced back to Aristotle. After defining them, he went on to reject the notion of there being more than five. Yet for centuries scientists have debated over the possible existence of other senses.
It was the neurophysiologist Charles Sherrington who coined the term “proprioception” in 1906, a word that literally means “one’s own” (from the Latin proprius). His definition was based on a division between what he termed exteroception and interoception. Exteroceptors receive and transmit sensory information from our surroundings to the brain, while interoceptors send us signals about our internal organs.
Sherrington identified two types of sensory neurons for perceiving our inner world that he called proprioceptors: muscle spindles and Golgi tendon organs. Located within the muscles, the muscle spindles detect changes in muscle length and joint angles; while the Golgi tendon organs, found where tendons connect muscle to bone, convey information about muscle tone. When you move your arm, the signals are transmitted up the spinal column to the cerebellum, which collects the information and recalculates your limb position. All of this occurs near instantly, in a matter of milliseconds.
Proprioception also helps us perceive our body when we are motionless. This is how we sense where our foot is, even when it’s still. Other proprioceptive inputs that affect our physical awareness include a sense of force, effort, and heaviness. Effort, for instance, refers to how much effort we feel a movement requires, which can be influenced by factors such as muscle fatigue.
How we perceive our movements and our body in space usually relies on more than proprioceptive sensations. It is also affected by our internal representation of our body, and of our touch and vision.
Many scientists have suggested that the brain has a stored “body model.” In 2010, Dr. Matthew Longo, a professor of cognitive neuroscience at Birkbeck, University of London, co-authored a paper with Dr. Patrick Haggard on the subject. They noted that the brain receives inadequate information about the size and shape of individual body parts for picturing the actual space occupied by the body. According to their hypothesis, the brain uses previous interactions with our environment to create an accurate mental map of the body.
“A receptor in the elbow joint can tell you something about the angle of the elbow,” explains Longo — but that itself is not enough. Without a mental representation, he says the brain’s calculations would amount to “a bunch of angles.” Having a stored mental map explains why when you close your eyes you can still walk in the dark, for instance, and why a pianist can play without looking at their fingers.
Proprioception is also intertwined with other senses. The brain takes information from your eyes, muscles, and skin and combines it to perceive your body. The process of multisensory integration is not well understood, however, and our proprioception can sometimes be misguided.
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 2015, 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 last year 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.