Close your eyes and touch your nose. If everything is
working properly, this should be easy because your brain can sense your
body, as well as its position and movement through space. This is called
proprioception. But how does this “sixth sense” work — and
what happens when it clashes with other senses?
Top image: Sculpture by Antony Gormley.
We’re all familiar with the five standard senses, which
include vision, hearing, smell, taste and touch. The idea that there are only
five of them has been rooted in our minds since the time of Aristotle, who
explicitly rejected
the idea of a sixth sense. But for centuries scientists have seriously
entertained the idea of a sixth sense that allows us to perceive our bodies.
There remains a lot of debate about whether this sense, which later
became known as proprioception, can be considered an additional sense alongside
the five standard ones. After all, the five senses all allow us to experience
the outside world, whereas proprioception allows us to understand our physical
place within that world.
Sixth sense or not, proprioception is recognized as being
vital to our daily experiences and something that contributes to our overall body
ownership. As Nature’s Allison Abbot says:
“Without it, our brains are lost.”
https://gizmodo.com/scientists-say-theyve-found-the-neural-origin-of-human-1280151948
What Is Proprioception?
Let’s start with what proprioception is not.
Proprioception
is not the vestibular
system — the master controller of our
balance and spatial orientation. However, the vestibular system does contribute
to the guidance of our bodily movements.
And
though the two terms are often used interchangeably, proprioception is not kinesthesia. Like
proprioception, kinesthesia involves the senses of limb position and movement,
but scientists typically view the focus of these two as being quite different.
That is, kinesthesia is behavioral in nature, and
it places an emphasis on the body’s motions, as well as incorporates routine or
habitual behaviors to improve movements. Both hand-eye coordination and muscle
memory involve kinesthesia — the more you perform certain actions, such as during sports, the
better at them you will become. Comparatively, proprioception has more to do
with body position, and focuses on the cognitive awareness of the body in
space.
Importantly,
the proprioceptive system is often considered to include both the vestibular and
kinesthetic systems.
According
to a 2012 review on the topic, proprioception
includes “the senses of position and movement of our limbs and trunk, the
sense of effort, the sense of force, and the sense of heaviness.” Proprioception
uses receptors located in the skin, muscles and joints to build the internal
sense of our bodies.
Surprisingly,
the study of proprioception began hundreds of years ago. In 1826, neurologist
Charles Bell questioned what
functions the muscles had, other than to contract under the command of motor
nerves. He concluded that they must provide our brains with the position of the
body and limbs when the brain has no other way of knowing these things (such as
if you can’t see your limbs). In 1887, neurologist Henry Charlton Bastian proposed
“kinesthesis” to replace the common terms “muscular
sense” and “sense of force.” To Bastian, kinesthesia, then, was
the perception of the position and movements of the limbs, as well as their
degrees of resistance and weight.
Almost
20 years later, in 1906, neurophysiologist Charles Scott Sherrington coined the
terms proprioception, interoception and exteroception. Proprioception, he said,
was an awareness of the body, which stems from sensory receptors
— proprioceptors — in the muscles, tendons and joints. Interoceptors and
exteroceptors provide the brain with information about the internal organs and
external world, respectively.
Originally experts believed we figured out the positions of our limbs from our muscles. But in the 20th century they ascribed this to joints instead, based primarily on studies on the stretch receptors
in the knee joints of cats. Today the view has flipped again, with most
physiologists believing that the principle proprioceptors are housed in the
muscles.
How Does It All Work?
For
the most part, proprioception works because of muscle spindles (below), a kind of
sensory receptor within muscles. Research in the 1960s and 1970s found that
muscle spindles detect the changes in length and velocity of a muscle, and then
transmit that information to the central nervous system via sensory neurons.
The changes detected by the muscle
spindles also signal the angle of related joints, though joints have
receptors that may contribute some information about limb positions. Additionally,
the skin has stretch receptors, or mechanoreceptors, that can detect small
movements of the limbs.
These
proprioceptors, of course, can also work in the absence of movement — this is
how we instinctively know where our hand is, even when it’s still. But there
are other proprioceptive sensations that are always associated with movement,
which are known as the senses of effort,
force and heaviness.
Within
the tendons that attach muscles to bones are proprioceptors called Golgi tendon organs, which
provide the brain with information about muscle tension. This is your sense of
how much force you’re exerting. Relatedly, the sense of effort
refers to how much effort is required to produce a given motion, and this sense can be thrown off
by fatigue from, say, exercise. Tendon organs and muscle spindles also convey
to the brain the sense of heaviness, which relates specifically to those
occasions when you pick up and move objects.
The
brain — particularly the cerebellum — takes information from all of these proprioceptive inputs to try to
determine the location and movement of body parts. But the way we sense our
body and its motion generally involves more than just proprioception.
For
instance, a 2010 study noted that there are no sensory signals that inform the
brain about the size and shape of body parts. The researchers concluded that
our sense of position
must refer to a “stored body model,” which we may unconsciously
develop through our interactions with the environment.
Scientists
have also known for many years that our vision strongly affects our sense of
body in space. In 1999, researchers found that visual cues and proprioceptive
cues about the position of the arm are encoded on to the same neurons in the premotor cortex of monkeys. Research also suggests that our vision is involved
with charting out the path and kinematics of our reaching movements, while proprioception
turns this plan into action. One study even suggests that vision sometimes trumps proprioception
when it comes to determining the positions and movements of body parts.
https://gizmodo.com/the-people-who-can-see-in-pitch-darkness-1455790525
Building A Sense of Ownership
Importantly,
proprioception and our ability to detect the movement and location of our body
parts contribute
strongly to our sense of body ownership, or the knowledge that our
body belongs to us and not someone else. Recently, scientists discovered that
the brain doesn’t require visual input for body ownership — only proprioception is
necessary (though vision helps). And a few studies have showed that
proprioception and multisensory integration are important for the development of body
awareness during infancy. Body perception may even be present
in newborns.
But
what does this mean for people whose proprioception isn’t working because of
spinal cord injury or other issues? Is their sense of their bodies different? According
to some research, the lack of these inputs results in a “disturbed
sense of an embodied self.”
Though
proprioception is vital to sensing our bodies and developing a sense of ownership of your body,
it’s not infallible. This is especially clear when you consider just how easy it is to get your sense of your own body to
contradict with what your see with your eyes.
The most well known example of this is the
“rubber
hand illusion.” This illusion basically boils down to convincing someone
that a rubber hand is part of their body by gently stroking the rubber hand and
the person’s (hidden) hand at the same time. This causes the person’s brain to
shift its sense of hand position from the real hand to the rubber hand, a
phenomena that’s called proprioceptive drift. Research has shown that this proprioceptive
drift can also occur using virtual reality.
Body ownership can be screwed with even
further. In one experiment, researchers were able to trick participants into
thinking that they had three
arms. The
researchers explained how this worked in their study:
https://gizmodo.com/how-to-really-feel-like-youve-got-three-arms-5769559
Taken together, these results indicate that
ownership of the supernumerary hand depends on achieving a match between the
visual information about the spatial orientation of the rubber hand and
proprioceptive information about the orientation of the real hand, and on a
match between the correlated visual and tactile information from the two hands.
Misguided
proprioception may also be behind phantom limb phenomena — an amputee’s
sensation that their missing limb is still there. A few years ago, scientists
proposed that phantom limb pain is the result of “proprioceptive
memory,” a kind of conflict between vision and memory. The brain
remembers the positions of the limb and its relation to the body and other
limbs (based on proprioception), but the eyes don’t see the limb. The
researchers argue that proprioceptive memory may be why so-called mirror treatment
— which involves viewing the reflection of the intact limb superimposed
over the missing limb — works to relieve phantom limb pain. The treatment, in a way, resolves the
conflict between the visual and memory systems.
It’s still unclear, however, just how the brain
builds and maintains its internal representations of body form. It’s also
curious as to why, if we can form a sense of body ownership based on
proprioception alone, it’s seemingly so easy to mess with this perception
using other senses (vision and touch). Future research will, no doubt, solve these mysteries, as well as many others, including how proprioception changes
with age — it is thought, for example, that declining
proprioception is partly responsible for the elderly’s increased tendency to
fall.
Inset images via Tsutomu Takasu/Flickr and A. Pedro Marinho/Wikimedia Commons.
