You may not see dead people, but that doesn't mean your senses are limited to five. Whether it’s extrasensory perception (ESP), seeing dead people, or your mom’s uncanny ability to detect shenanigans anywhere within a 30-mile radius, we’re all familiar with the concept of a fabled “sixth sense”. Much like the ability to use more than 10 percent of your brain, it’s become shorthand for superhuman powers – an extra sense, over and above the normal five, with which to perceive the world in a way most people couldn’t even imagine. And again, much like the ability to use more than 10 percent of your brain at any time, it’s nonsense. Ask a scientist whether there’s such a thing as a “sixth sense”, and the answer will likely be simple and intriguingly non-supernatural: a sixth sense exists, we pretty much all have it, and it’s called proprioception. Or, perhaps, you might have heard it called kinesthesia. Look up either of the terms online, and you’ll likely see the words used fairly interchangeably, but in fact, they’re quite different phenomena – although highly interlinked with each other. So what’s the difference? Are these really a “sixth sense”? And are there any other senses out there we’ve been ignoring all this time? Position vs Movement Perhaps the easiest way to distinguish between proprioception and kinesthesia is with a demonstration, so: close your eyes, and put your hand on top of your head. Could you do it? We’re going to guess that task was not only possible, but laughably simple for almost everyone who tried it – and yet, if you think about it, quite a lot went into making it happen. You had to be aware, somehow, of where your head was in space; you had to know where your hand was, too, not just in general, but in relation to your head; you had to know exactly how and in what direction to move your arm in order to get it to its cranial destination. And what’s more, you had to do all that without being able to see it. The reason you found it so easy was thanks to those two important senses: proprioception and kinesthesia. The two abilities were working together closely to make that simple movement possible, but each was playing a very different role – in simple terms, it was proprioception that told you where your hand and head were in space, and it was kinesthesia that told you how to move one on top of the other. “Proprioception can be thought of as a cognitive awareness of your body in space,” explains Ronald Sahyouni in an explainer video for the Khan Academy. “Your sense of balance [and] your sense of position are… taken care of by your proprioceptive sense.” Kinesthesia, on the other hand, is “a little bit more behavioral,” Sahyouni continues. It’s the sense that kicks in when you decide to put one foot in front of the other, or swing a bat to hit a baseball: the constant awareness of the position and movement of the various parts of your body, fed to your brain thanks to special neurons, slightly confusingly known as proprioceptors, found in your joints and muscles. In fact, this cognitive-versus-behavioral distinction is one of the main ways we can differentiate between the two phenomena. “Proprioception… [is] a little more subconscious,” says Sahyouni. “You’re not always thinking about exactly where your body is in space, exactly how you’re oriented, if you’re walking or if you’re running – your main concern isn’t, ‘I hope I’m not going to fall down.’” Meanwhile, kinesthesia is something you’re not only aware of, but you can actually use to your advantage when it comes to learning or improving your muscle memory and coordination. “Every time you actually swing [a] bat or swing [a] golf club, your body is able to detect exactly how it’s moving,” Sahyouni explains. “And so over time, if you learn that, ok, if I move in this certain direction I’m able to hit the golf ball, or if I move in this direction I’m able to hit the baseball, then over time your body is able to detect exactly what that movement is and start to undergo that movement more and more often, so you’re able to actually teach yourself exactly how you should move in order to successfully complete whatever task is at hand.” How does our sixth sense work? So what’s going on inside our bodies to make all this possible? “Let’s say [we’re looking at] an arm muscle,” explains Sahyouni. “There’s a tiny little receptor inside of the muscle, and this receptor… is sensitive to stretching.” As your muscle contracts, this receptor – called a spindle, in this particular case – senses that it’s become stretched out. That prompts it to fire out a signal to the brain, letting it know that something is going on in the arm muscle, so it should probably update its location information on that particular limb. ”This is the basic principle behind proprioception,” says Sahyouni. “We’re able to tell exactly how contracted or how relaxed every single muscle is in our entire body, and this allows us to know exactly where our body is in space.” Spindles aren’t the only type of proprioceptor our bodies rely on, however – they’re just the ones that hang out in our skeletal muscles. We also have Golgi tendon organs, which look after the interfaces between our muscles and tendons, as well as joint receptors, which live, unsurprisingly, in our joints. But even this expanded arsenal isn’t enough for the full proprioceptive effect – because when it comes to things like walking, running, or even just standing up, there’s one sensory organ that simply cannot be overlooked. And it may not be the one you’re expecting. “Imagine you are blindfolded and I tilted you forwards slowly. You’d immediately have a sensation of how your body’s position was changing in relation to gravity,” points out Christian Jarrett for BBC Future. “This is thanks to the fluid-filled vestibular system in your inner ear, which helps us keep balance.” That’s right: your ears aren’t just for hearing. It’s the fluid sloshing around inside our ear canals that tells us what’s going on with our acceleration and orientation – vital background information for the intel coming in from our proprioceptors. And in case you’re doubting just how important your inner ears are for proprioception, just think back to the last time you had a bad ear infection or sat on a carousel for too long. Dizziness, drunkenness, motion sickness – these are all caused by some kind of mismatch between the information coming in from your inner ears and your other senses, and they’re also a great example of what it’s like to lose your sense of proprioception. And here, too, we see a distinction between proprioception and kinesthesia – because it’s perfectly possible to lose one without the other. “An inner ear infection, for example, might degrade the sense of balance,” explains a University of Minnesota textbook on Sensation and Perception. “This would degrade the proprioceptive sense, but not the kinesthetic sense,” it continues. “The affected individual would be able to walk, but only by using the sense of sight to maintain balance; the person would be unable to walk with eyes closed.” Other senses So it’s settled, then: the human body has not five senses, but seven. Right? Well, it depends on who you ask. “Neurologists would count, and agree on, at least nine senses,” points out the UK’s Sensory Trust. “Because there is some overlap between different senses, different methods of neurological classification can yield as many as 21 senses.” Not enough for you? According to some experts, your superhuman seeing-dead-people movie ought to be called the 34th sense, not the sixth – and at least one puts the number of human senses at a whopping 53, the Sensory Trust notes. Why the disagreement? It all comes down to how you define a sense – and at what point one sense blends into another. After all, it’s well known that what we see can influence how food tastes, but it turns out our vision is in turn intertwined with our heartbeat, which is part of our interoceptive sense. We’re generally happy separating these phenomena out, but then where do you stop? “Some might… argue that the senses should be defined by the types of receptors we have; a different sensor means a different sense,” Jarrett writes. “If that were the case, then even well-known senses quickly split into different varieties… it becomes even more absurd if we turn to smell: humans have over 1,000 distinct olfactory receptors tuned to different odorous molecules. Should each one be counted as a different sense?” So, the question of how many senses the average human has is no doubt one that’s going to cause headaches in the scientific world for a long time. But one thing’s for sure: the answer is definitely not five – and thanks to your senses of proprioception and kinesthesia, you’re now fully equipped to flee from any neurologist wanting to argue otherwise. Source
