"We interact with our environments so effectively and so effortlessly, it is difficult to appreciate the extensive computations that underlie even the simplest sensory experience."—SENSORY EXOTICA—A WORLD BEYOND HUMAN EXPERIENCE.
PICTURE yourself cycling along a quiet country road. As you pedal, sensors in your legs enable you to apply just the right pressure to maintain your speed. Your organs of balance keep you upright; your nostrils smell the aromas; your eyes absorb the panorama; your ears are attuned to the chirping of birds. Thirsty, you grasp your drink bottle, aided by touch receptors in your fingers. Your taste buds and hot-and-cold sensors reveal the flavor of the liquid and its temperature. Sensors in your skin and those attached to your body hair tell how strong the breeze is and, in cooperation with your eyes, how fast you are going. Your skin also informs you of the ambient temperature and humidity, while your awareness of time tells you approximately how long you have been on the road. Eventually, internal senses will compel you to rest and to eat. Yes, life truly is a superb symphony of the senses!
Just Five Senses?
During such a bicycle ride, how many senses come into play—just the traditional five: sight, hearing, smell, taste, and touch? According to the Encyclopædia Britannica, these five senses were enumerated by the ancient philosopher Aristotle, whose "influence has been so enduring that many people still speak of the five senses as if there were no others."
However, according to Britannica, studies in skin sensitivity alone "yield evidence that the human senses num
ber more than five." How can that be? Certain functions once lumped together under touch are now regarded as senses in their own right. For instance, pain receptors respond to and distinguish between mechanical, thermal, and chemical forces or agents. Other sensors signal an itch. Evidence suggests that we have at least two kinds of pressure sensors—one for light surface pressure, another for deep stimulation. Our body also has a broad range of internal senses. What is their role?
The Wonder of Human Touch
The human hand has a particularly refined sense of touch. According to Smithsonian magazine, researchers found that our hand can detect a dot just three microns high. (A human hair has a diameter of 50 to 100 microns.) However, by "using a texture rather than a dot, the researchers found the hand can detect roughness just 75 nanometers high"—a nanometer being one thousandth of a micron! Such remarkable sensitivity is attributed to about 2,000 touch receptors in each fingertip.
The Internal Senses
Internal senses detect changes taking place inside our body. They signal things like hunger, thirst, fatigue, internal pain, and the need to breathe or to go to the toilet. In cooperation with our biological clock, internal sensors make us feel tired at day's end and jet-lagged if we have flown across time zones. In fact, because we can consciously "sense" the flow of time, it has been suggested that time awareness be added to the catalog of senses.
We also have a vestibular sense, or sense of balance, which is located in our inner ear. It responds to gravity, acceleration, and rotation. And finally we have a kinesthetic sense, which enables us to detect muscle tension and, even with eyes closed, the movement and position of our limbs.