Everything we know about the world comes to us through our senses. Traditionally, we were thought to have just five of themsight, hearing, touch, smell, and taste.
Scientists now recognize that we have several additional kinds of sensations, such as pain, pressure, temperature, joint position, muscle sense, and movement, but these are generally included under "touch." (The brain areas involved are called the "somatosensory" areas.)
Although we pay little attention to them, each of these senses is precious and almost irreplaceableas we discover, to our sorrow, if we lose one. People usually fear blindness above all other disabilities. Yet deafness can be an even more severe handicap, especially in early life, when children learn language.
This is why Helen Keller's achievements were so extraordinary. As a result of an acute illness at the age of 19 months, she lost both vision and hearing and sank into a totally dark, silent universe. She was rescued from this terrible isolation by her teacher, Anne Sullivan, who managed to explain, by tapping signs into the little girl's palm, that things have names, that letters make up words, and that these can be used to express wants or ideas.
Helen Keller later grew into a writer (her autobiography, The Story of My Life, was published while she was still an undergraduate at Radcliffe College) and a well-known advocate for the handicapped. Her remarkable development owed a great deal to her determination, her teacher, and her family. But it also showed that when a sense (or two, in Helen Keller's case) is missing, another sense (in her case, touch) may be trained to make up for the loss, at least in part.
What we perceive through our senses is quite different from the physical characteristics of the stimuli around us. We cannot see light in the ultraviolet range, though bees can, and we cannot detect light in the infrared range, though rattlesnakes can. Our nervous system reacts only to a selected range of wavelengths, vibrations, or other properties. It is limited by our genes, as well as our previous experience and our current state of attention.
What draws our attention, in many cases, is change. Our senses are finely attuned to change. Stationary or unchanging objects become part of the scenery and are mostly unseen. Customary sounds become background noise, mostly unheard. The feel of a sweater against our skin is soon ignored. Our touch receptors, "so alert at first, so hungry for novelty, after a while say the electrical equivalent of 'Oh, that again,' and begin to doze, so we can get on with life," writes Diane Ackerman in A Natural History of the Senses.
If something in the environment changes, we need to take notice because it might mean dangeror opportunity. Suppose an insect lands on your leg. Instantly the touch receptors on the affected leg fire a message that travels through your spinal column and up to your brain. There it crosses into the opposite hemisphere (the right hemisphere of the brain controls the left side of the body, and vice versa) to alert brain cells at a particular spot on a sensory map of the body.
The brain's map of the body extends along a vertical strip of cerebral cortex near the center of the skull. The cortexa large, deeply wrinkled sheet of neurons, or nerve cells, on the surface of the brain's two hemispheresgoverns all our sensations, movements, and thoughts.
The sensory map in humans was originally charted by the Canadian neurosurgeon Wilder Penfield in the 1930s. Before operating on patients who suffered from epilepsy, Penfield stimulated different parts of their brains with electrodes to locate the cells that set off their attacks. He could do this while the patients were awake, since the brain does not feel what is happening to it.
In this way, Penfield soon learned exactly where each part of the body that was touched or moved was represented in the brain. He then showed it in his famous "homunculus" cartoons of the somatosensory and motor areas.
Surprisingly, these maps do not accurately reflect the size of body parts but rather, their sensitivity. Arms and legs take up very little space, despite their length. The face and hands, which have greater sensitivity and complexity, are given more spaceespecially the tips of the fingers. Nevertheless, the signal that a mosquito has landed on the back of your left leg comes through loud and clear. In a fraction of a second, through a decision process that is not yet understood, this signal leads you to swat the insect at just the right place.
< Previous | Top of page | Next >
These famous maps by Wilder Penfield show that each part of the body is represented on two strips of the brain's cerebral cortex.