Myth: You have five senses

For this week's mythconception, Kat Arney has been sniffing out the truth about our senses.

Kat - Even a child knows that we have five senses: touch, smell, sight, taste and - of course, for fans of the Naked Scientists - hearing. Together, they enable us to perceive the world around us and - literally - make sense of reality.

The Merriam-Webster dictionary defines ‘sense’ (in this context) as “a specialized function or mechanism (as sight, hearing, smell, taste, or touch) by which an animal receives and responds to external or internal stimuli”. But it might surprise you to know that the textbook idea of five senses is actually a myth.

The latest scientific research suggests that we have far more than five senses, and maybe as many as 33 in total, or even hundreds more. Alternatively, a reductionist approach could knock those five down to just three. So, where did the idea of five senses come from, and what are all these other senses that we have? Let’s listen and learn…

The idea of the Big Five, as I’ll call them, probably dates back to Aristotle and his book De Anima, or On the Soul, where he explains the classic five senses of smell, taste, touch, vision and hearing. And since then, this sensory quintet has been the starting point for pretty much every sensory discussion from science to philosophy.

As scientists learned more about the nervous system and the inputs that go into it, the total number of senses went up to nine - namely the famous five plus pain, mechanoreception (that’s balance and direction), temperature, and so-called interoreceptors, which measure things like blood pressure or how stretched your bladder is (meaning that it’s time to take a trip to the loo!)

The latest research suggests we have at least 21 senses, and some researchers put that number up at around 33 - notably a large science and humanities research project called Rethinking the Senses.

The list includes: Smell, taste, low light vision (rod cells in the eye), colour vision (that’s cone cells), sound, pressure, heat, cold, itch, proprioception (where bits of your body are in space), tension (such as in muscles and tendons), stretch (bladder, lungs and things like that), pain, balance (or equilibrioception), thirst, hunger, and chemicals in the blood such as hormones.

Then there’s magnetoception - the ability to detect magnetic fields. This is very weak in humans, but does seem to be there. And then there’s the ability to accurately sense time passing, which is more hotly debated but does seem to go awry in people with neurological disorders.

Then there is the question of when to stop counting senses. Do we count each type of taste receptor? That’s sweet, salt, sour and umami. And what about the weird effects of sichuan peppercorns, which actually cause vibration-like stimulation of a facial nerve. And should we count all the thousands of individual odorant receptors in the nose as individual senses? After all, they all pick up different smelly chemicals.

Added to this is also the discovery that some people have senses that others don’t. For example, some people can learn to ‘see in the dark’ using echolocation, making clicks with their tongue and listening to how the sound bounces off the walls around them.

And if you want to get really reductionist about it all, you could argue that humans have just three senses, based on the kind of input coming into the nervous system - mechanical senses that measure physical changes (which would include touch, hearing and proprioception), light detection, and chemical sensing, which takes in taste, smell and all the sensory inputs that come from chemical signals within the body.

Before we start suffering from a sensory overload, let’s take a step back. In fact, the bigger question is not exactly how many senses we have, but how our brains integrate all the information coming in from our bodies to build up a picture of the world around us. The exact number doesn’t really matter - it’s what we do with the information that counts.

Finally, there’s the spooky question of whether we really have a so-called ‘sixth sense’ that enables us to sense magical or paranormal weirdness? Sadly not. A study in 2014 by Australian researchers debunked the idea that some people can somehow sense changes that they can’t identify. And many researchers have shown that claims of extra-sensory perception or sixth sense just don’t hold up under scientific scrutiny.

Rather than any unusual senses, these phenomena are either due to natural, rational explanations, tricks, or are the work of that wonderful human organ - an overactive imagination.