When I was growing up, my teachers taught me about the five senses. Most people have these:
But the spiritually-inclined (and the directors of really weird Bruce Willis movies) like to talk about “a sixth sense.” This is an extra information conduit by which people (often people who want you to call a dodgy hotline) can get information about the past or future, or about the floaty spirits of the dead.
The thing is, we all have a sixth sense. Every one of us. And let’s just say it’s not quite as impressive as being able to see dead people:
You have an accelerometer in your head (which sounds like a rejected line from Total Recall). It’s called the vestibular system, and it looks like an alien:
Just think, you’ve got two of these creepy snail-demons living in your head. And you should be happy about that, because they stop you from tipping over all the time. You’ll notice that each of those canals (the semicircular canals) is approximately perpendicular to the other two. Evolution went and did it again: it built a fucking accelerometer in our heads. That’s awesome.
Its operating principle is relatively simple. The semicircular canals are full of a fluid called endolymph. When you’re sitting still or moving at a constant velocity without rotation, the endolymph is stationary. If you should start spinning, though, your body turns while the endolymph, thanks to inertia, tries to stay still. But the semicircular canals are rotating with your body, so as far as they’re concerned, the stationary endolymph is moving. This movement creates pressure on a creepy onion-shaped structure called the cupula, which senses the flow and transmits a position and acceleration signal to your brain.
This system is handy to have, and you’ll be especially aware of that if you’ve ever had an inner-ear infection or been really drunk: when you don’t know which way is up, you develop a nasty tendency to fall over and bash parts of yourself on furniture and rocks and things. You also tend to tell long-winded stories to your friends and laugh a lot, but I don’t think that’s an inner-ear thing.
Damage to the vestibular system is bad news. It can happen when the bones around it fracture and move during a head injury. It can happen from infections. It can also be caused by a toxic side-effect of the antibiotic gentamicin. Either way, if your vestibular system quits working, you lose your ability to properly sense rotation, acceleration, and orientation. You can simulate this yourself by behaving like an 8-year-old: go outside and spin around for a long while. Keep spinning until you don’t feel like you’re spinning anymore. Then stop suddenly. That’s what it’s like when your vestibular system doesn’t work: you stumble around, your eyes have trouble pointing at things, and oftentimes, you get nauseous and puke. I probably should have warned you about that ahead of time.
So there’s no doubt that the vestibular system gives us unique information of a completely different kind to any other sense. So we all have a sixth sense, and scientists have an awesome-sounding name for it: equilibrioception. I want to name my first-born daughter Equilibrioception. I should probably let the mother pick the baby names.
So that whole “human beings have five senses” thing is bullshit. Here’s the list so far: Vision, Hearing, Touch, Taste, Smell, Equilibrioception. We all have a sixth sense, which is good, because without it, we’d all stumble around like drunken toddlers.
But actually, we all have a seventh sense, too. This one’s called proprioception. I want you to try an experiment. Don’t keep reading. Try this experiment. I’m not being a smart-ass here, and I’m not going to make you punch yourself in the genitals or anything. Here’s the experiment (finish reading it first): 1) Hold your arm with your elbow at your side and your hand out in front of you. 2) Close your eyes. 3) Move your arm so that it’s extended out to the side, parallel to the floor. Go. Do it now. Seriously. I’m trying to make a point here.
Did you do it? Good. I’m glad we’re getting along again. Anyway, you’ll notice that, without any visual input, you were probably able to position your arm exactly as I described. And that’s because your body has a built-in system for sensing the angles of your joints and the force you’re applying with your muscles. Next time you’re lying in bed, close your eyes and try to picture in your mind how your limbs are arranged. Unless you’ve got particular kinds of brain damage, you should be able to do it without effort. That’s proprioception. It’s like Douglas Adams used to say “Always know where your towel is.” Except this is even more important: “Always know where your limbs are.”
So we all have seven senses: Vision, Hearing, Taste, Touch, Smell, Balance, and Proprioception. It would be very difficult to argue that these senses aren’t independent of one another. Plus, you can lose one of these senses while the others remain intact. Vision, Hearing, and Smell can be destroyed by infections, trauma, and brain tumors. Touch and Proprioception can be lost in spinal-cord or brain injuries. Balance can be lost to ear infections, trauma, cancer, or even drug side-effects. Before we go on and try to see if there are any other senses my elementary-school teachers missed, let’s establish some criteria for what counts as a sense. Here’s what I’m going to use:
A. A sense gives you unique information that you can’t get any other way.
B. A sense is unconscious and doesn’t necessarily require the higher brain in order to function. (This is so we don’t have to include a million little things like language-sense and face-sense, which are important, but are derived from other senses by the brain.)
Simple enough. And, using these criteria, human beings have at least eleven senses. This is because “touch” is not a single sense. It’s five senses lumped together by careless elementary-school teachers (who, for some silly reason, didn’t want to try to teach somatosensory anatomy to six-year-olds.)
Time for another thought experiment. Imagine that you close your eyes and I touch a metal rod to your skin (Which is kinda creepy, but I’ll keep things professional.) You know immediately that there’s an object touching you. That’s the first sense contained under the umbrella of “touch,” and it’s provided by specialized skin receptors called Meissner’s corpuscles or tactile corpuscles. Now imagine the metal rod starts vibrating (I promise this isn’t going anywhere creepy. It just sounds like it is.) That’s a different sense altogether, provided by Meissner’s corpuscles, Ruffini corpuscles, lamellar corpuscles, and Merkel disks. Now imagine I turn off the vibration and start pressing the rod harder against your skin. Your lamellar corpuscles detect the change in pressure, and if I maintain the pressure, the Merkel disks detect that the pressure is continuous. So far, touch is actually three separate senses: touch, vibration, and pressure. But now imagine I start pressing too hard with the rod, so that it starts hurting. Before you open your eyes and punch me, take note: pain is a sense of its own, called noiception. Now imagine I take the rod away (in order to keep from getting punched) and press it lightly against your skin again, but this time it’s hot. Not hot enough to burn, but hot enough to notice. That’s another sense: thermoception.
So far, we have:
6. Vibroception (good name for a band)
9. Pressure sense
Eleven distinct senses.
You might take issue with my sub-dividing the sense of touch like this. I can understand that. I mean, if we divide touch into five different senses, then why not divide vision into color-sense and brightness-sense, and why not divide taste into the classic “basic flavors”: sweet, sour, salty, bitter, and savory (or umami)? If we did that, then human beings have sixteen different senses. To a degree, how many senses you count will depend on what you count as an independent sense. But the point I’m trying to make is that my elementary-school teachers were full of shit. Whatever the number of human senses, it isn’t five. I think most people would agree that balance and proprioception count as individual senses in their own right, so in the end, human beings have at least seven senses. And I will teach that to my children, so that they can argue with their teachers. I’m going to make an…interesting father.