5, 9, 21, 53… how many senses?

Lars Stenberg

Unsurprisingly there is often talk at the Sensory Trust about our senses. There are some that may be peculiar to our office such as the sense that someone should put the kettle on or, in my case, that I should stop talking and let folks get on with their work, but on the whole the debates usually range around sense spotting. Like big game hunters we try to hunt down those tricksy senses that slip out of our sights just as we try to define them. We’re not the first to have this problem and so there follows a Brief History of the Senses to demonstrate that exactly how many senses we have is unlikely to be agreed any time soon.

Aristotle (384-322 BC) is credited with first numbering the senses in his work De Anima. Even if someone had numbered them prior to that, it's certain that the Big Five have been known for thousands of years, are known to all of us, and are what most of us mean when we talk of The Senses.

• Sight or vision
• Hearing or audition
• Smell or olfaction
• Taste or gustation
• Touch or tactition

However, neurologists would count, and agree on, at least nine senses. A broadly acceptable definition of a sense for neurologists would be a group of sensory cells that responds to a specific physical phenomenon, and that corresponds to a particular region of the brain where the signals are received and interpreted. Some things that we lay-folks might refer to as “senses”, such as the sense of direction for instance, are defined by neurologists as post-sensory cognitive activities and don’t count in this definition.

Because there is some overlap between different senses, different methods of neurological classification can yield as many as 21 senses. And this number does not include some physiological experiences such as, for instance, the sensation of hunger or thirst. Generally agreed senses for neuroscientists currently include:

• Thermoception - the sense of heat (there is some debate that the sense of cold may be a separate sense)
• Nociception - the perception of pain
• Equilibrioception - the perception of balance
• Proprioception - the perception of body awareness (close your eyes and touch your nose. Got it first time? That's proprioception in action)

Not happy with up to 21? Eco-psychologist Michael J Cohen puts the number of senses at our disposal at 53. His definition of a sense goes beyond the physiological phenomenon/nerve sensor definition. He breaks the senses down into four categories:

• The radiation senses: sense of colour, sense of moods associated with colour, sense of temperature.
• The feeling senses: sensitivity to gravity, air and wind pressure, and motion.
• The chemical senses: hormonal sense, such as pheromones, hunger for food, water or air.
• The mental senses: pain, external and internal, mental or spiritual distress, sense of self, including friendship, companionship and power, psychic capacity.

We can’t reproduce the whole list here, but you can find it in his book Reconnecting With Nature.

Cohen’s point is that we are all sensory creatures and that our senses are a large part of who we are. Our senses are given to us not to be indulged, to be playthings or for decoration, but are mechanisms originally designed to help us survive and thrive in the natural world. Because life in the “developed” world is now so confined (Americans for instance spend an average of 95% of their lives indoors) our senses have little to do and consequently become either atrophied or over-sensitive, which in turn leads to many of the common ailments of today’s existence, such as stress, anxiety and depression.

Next time you’re out and about go somewhere new or take a new route. Concentrate on what you experience, not just the smells and sounds but all the sensations: cold, hungry, expectant, nervous… Go on, forget about indulging your senses and think more about exercising them.

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