Ask Harvard Medical School researchers how many senses humans have and you’re bound to receive a range of answers. This lack of consensus isn’t limited to Harvard: Neurologists and others who study perception have long disagreed on the number of senses we possess to help us navigate our way through life.
No one doubts the Big Five. Vision, hearing, smell, taste, and touch have been classified since the days of Aristotle, who reportedly catalogued them himself. From the yeasty aroma of freshly baked bread, to the silky feel of a cat’s sleek coat, to the Technicolor brilliance of a sunset, most of us continually experience our world through the major senses.
These days, however, some scientists believe that this quintet can be split into subgroups. Sight, for instance, may be divided into perceptions of brightness, color, and depth. Other researchers argue that true senses are bodily systems consisting of a group of sensory cell types that not only respond to a specific physical phenomenon but also correspond to a particular region in the brain. Using that definition, many neurologists recognize additional human senses.
Equilibrioception. Whether you’re slaloming down a slope or strutting down a street, this sense—otherwise known as balance—helps keeps you upright. Although vision plays a role in equilibrioception, the vestibular system of the inner ear is mainly responsible.
Nociception. If you’ve touched a boiling kettle or stubbed a toe, you’re likely all too familiar with nociception, the sense of pain. Recent research shows that what was once viewed as a subjective experience related to touch is, in fact, a distinct phenomenon that corresponds to a specific area in the brain.
Proprioception. Close your eyes and touch your fingertip to your nose. Quick: Where’s your hand? Unless you suffer from a deficit of this kinesthetic sense, you know where your hand is, even though you can’t see it. This sense, the awareness of where your body parts are, sounds silly—until you consider that without it, you’d have to constantly watch your feet to make sure they were planted on the ground.
Thermoception. You notice a chill in the air, so you don a jacket on your way to work. Later, as you enter your warm office, you shed that garment. That’s thermoception, the sense of heat and cold, which relies on temperature sensors in your skin to keep you from overheating or freezing.
Temporal perception. There’s no doubt that the perception of time can be subjective: Three hours spent at a party with friends may speed by, while a three-hour meeting can seem to drag. Yet our sense of time is rooted in biology. Research shows that the basal ganglia and other parts of the brain are responsible.
Interoception. When we take our internal perception into account, we have even more senses. These are linked to sensory receptors found in internal organs, such as those in the lungs that control respiratory rate.”
Perhaps most fascinating, though, are the senses we don’t have. Ever wish you could sniff out hidden objects, see in the dark, or detect magnetic and electric fields? In humans, these senses are the domain of superheroes. Yet they’re a natural advantage to some animals. Dogs have an extraordinarily keen sense of smell. Bloodhounds, for example, have noses up to 100 million times more sensitive than ours. Cats can see in just one-sixth the light level we require, while vampire bats and some snakes can see in infrared, and bees and dragonflies can see in ultraviolet. Birds and bees navigate—and migrate—based on their perception of magnetic fields. And sharks can sense changes in electric fields, as can platypuses. Even some plants get in on the act: Venus flytraps can sense vibration, light, water, and odors.
These and other abilities remain a fantasy for humans. Belts and other devices are in the works that may allow us to sense magnetic fields, for example, but their success is still limited. For now, we’ll have to continue to rely on our five standard-issue senses. Or is it ten? Or sixteen?