If you were asked, "Does your hand belong to you?" you would naturally say, "Of course."
But ask neuroscientists the same question and they will turn the question back on you: How do you know it's your own hand? In fact, how do you know that you have a body? What makes you think you own it? How do you know where your body begins and ends? How do you keep track of its position in space?
Try this little exercise: Imagine there is a straight line running down the middle of your body, dividing it into a left half and a right half. Using your right hand, pat different parts of your body on the right side -- cheek, shoulder, hip, thigh, knee, foot. With your finger, trace a line over your right eyebrow and over the right portions of your upper and lower lips.
You are able to tell these body parts from one another because each is faithfully mapped in a two-dimensional swath of neural tissue in your left brain that specializes in touch. The same thing goes for the left side of your body: All its parts are mapped in a similar region of your right brain. Your brain maintains a complete map of your body's surface, with patches devoted to each finger, hand, cheek, lip, eyebrow, shoulder, hip, knee, and all the rest.
A map can be defined as any scheme that spells out one-to-one correspondences between two different things. In a road map, any given point on the map corresponds to some location in the larger world, and each adjacent point on the map represents an adjacent real-world location. The same holds broadly true for the body maps in your brain. Aspects of the outside world and the body's anatomy are systematically mapped onto brain tissue. Thus the topology, or spatial relationships, of your body's surface is preserved in your touch map to a high degree: The foot map is next to the shin map, which is next to the thigh map, which is next to the hip map. Whenever someone claps you on the shoulder, nerve cells in the shoulder region in this map are activated. When you kick a soccer ball, the corresponding part of your foot map is activated. When you scratch your elbow, both your elbow region and fingertip regions are activated. This map is your primary physical window on the world around you, the entry point for all the raw touch information streaming moment by moment into your brain.
This touch information is collected by special receptors throughout your body, funneled into your spinal cord, and sent up to your brain along two major pathways. The more ancient of these pathways carries pain, temperature, itch, tickle, sexual sensation, crude touch -- sufficient, say, to know that you bumped your knee and not your shin, but not acute enough to tell a penny from a dime -- and sensual touch, which includes the gentle maternal caresses that were vital for your body map development as a baby.
The evolutionarily newer pathway carries fine touch information -- the kind you need in order to thread a needle or leaf through a book -- and position-and-location information from receptors embedded in your joints, bones, and muscles.
Once these many channels of sensory information reach your brain, they are combined to create complex, composite sensations such as wetness, hairiness, fleshiness, and rubberiness. The same goes for the many varieties of pain. Through a combination of pain -- and touch -- related signals, you have access to the rich diversity of unpleasant experience that includes the smarting pain of a sunburn, the shooting pain of carpal tunnel syndrome, the piercing pain of a stab wound, the dull throbbing pain of an abused knee, the itchy pain of healing, and so on.
You also have a primary motor map in your brain for making movements. Instead of receiving inputs from your skin, this map sends output signals to your muscles. Just like the touch map, this movement map is also found in both sides of the brain. It is vital to your ability to guide your body parts to make fine-tuned movements and assume complex positions in space -- like doing the hokey-pokey, playing hockey, or assuming a poker face in a high stakes card game. When you wiggle all your toes, the toe and foot regions of your motor map are active. When you stick out your tongue, the map's tongue and jaw regions are active. Thanks to this map, all the low-level, mostly unconscious tasks of coordinated movement unfold smoothly without a glitch.
Elsewhere in your brain you also have a very different but no less critical body map of all your body's innards. This is your primary visceral map, a patchwork of small neural swatches that represent your heart, lungs, liver, colon, rectum, stomach, and all your various other giblets. This map is uniquely super-developed in the human species, and it gives you a level of access to the ebb and flow of your internal sensations unequaled anywhere else in the animal kingdom. You feel lust, disgust, sadness, joy, shame, and humiliation as a result of this body mapping. These visceral inputs to the psyche are the wellspring of the rich and vivid emotional awareness that few other creatures even come close to enjoying. The activity in this map is the voice of your conscience, the thrill of music, the foundation of the emotionally nuanced and morally sensitive self.
The Embodied Self
The idea that your brain maps chart not only your body but the space around your body, that these maps expand and contract to include everyday objects, and even that these maps can be shaped by the culture you grow up in, is very new to science. Research now shows that your brain is teeming with body maps -- maps of your body's surface, its musculature, its intentions, its potential for action, even a map that automatically tracks and emulates the actions and intentions of other people around you.
These body-centered maps are profoundly plastic -- capable of significant reorganization in response to damage, experience, or practice. Formed early in life, they mature with experience and then continue to change, albeit less rapidly, for the rest of your life. Yet despite how central these body maps are to your being, you are only glancingly aware of your own embodiment most of the time, let alone the fact that its parameters are constantly changing and adapting, minute by minute and year after year. You may not truly appreciate the immense amount of work that goes on behind the scenes of your conscious mind that makes the experience of embodiment seem so natural. The constant activity of your body maps is so seamless, so automatic, so fluid and ingrained, that you don't even recognize it is happening, much less that it poses an absorbing scientific puzzle that is spawning fascinating insights into human nature, health, learning, our evolutionary past, and our cybernetically enhanced future.
Your body is not just a vehicle for your brain to cruise around in. The relationship is perfectly reciprocal: Your body and your brain exist for each other. A body that can be moved or stilled, touched or evaded, scalded or warmed, frozen or cooled, strained or rested, starved, devoured, or nourished, is the raison d'être of the senses. And the sensations from your skin and body -- touch, temperature, pain, and a few others you will learn about -- are your mind's true foundation. All your other senses are merely added-on conveniences in comparison. After all, human beings can get by just fine in life without vision or hearing. Even people like Helen Keller who lack both these senses can thrive both mentally and physically. The brains of people born deaf don't develop auditory maps, and the brains of congenitally blind people never form visual maps, but even deaf-blind people have body maps. In contrast, vision or hearing without a body to relate sights and sounds to would be nothing but psychically empty patterns of information. Meaning is rooted in agency (the ability to act and choose), and agency depends on embodiment. In fact, this all is a hard-won lesson that the artificial intelligence community has finally begun to grasp after decades of frustration: Nothing truly intelligent is going to develop in a bodiless mainframe. In real life there is no such thing as a disembodied consciousness.
The sum total of your numerous, flexible, morphable body maps gives rise to the solid-feeling subjective sense of "me-ness" and to your ability to comprehend and navigate the world around you. You can think of the maps as a mandala whose overall pattern creates your embodied, feeling self. All your other mental faculties -- vision, hearing, language, memory -- hang supported in the matrix of this body mandala like organs on a skeleton. Developmentally speaking, it would be impossible to become a thinking, self-aware person without them.
Copyright © 2007 Sandra Blakeslee and Matthew Blakeslee
The above is an excerpt from the book The Body Has a Mind of Its Own
by Sandra Blakeslee and Matthew Blakeslee
Published by Random House; September 2007;$24.95US/$32.00CAN; 978-1-4000-6469-4
Copyright © 2007 Sandra Blakeslee and Matthew Blakeslee
Author
Sandra Blakeslee is a regular contributor to The New York Times who specializes in the brain sciences. She has co-written many books, including Phantoms in the Brain with V.S. Ramachandran, On Intelligence with Jeff Hawkins, and Second Chances: Men, Women, and Children a Decade After Divorce with Judith S. Wallersein. She is the third generation in a family of science writers.
Matthew Blakeslee is a freelance science writer in Los Angeles. He represents the fourth generation of Blakeslee science writers. This is his first book.
Writing 
