When the Brain Says, 'Don't Get Too Close'

A century ago, neurologists noticed that when ladies wearing big feathered hats ducked through entryways, they would align their bodies just so. It was as if they could feel the tops of doors with the tips of the feathers.

From this and other observations, the scientists concluded that each person holds within the brain a mental representation of the body and its parts -- even the clothing it wears -- as it moves through space.

Those early scientists could not explain how the brain creates such sensations, or body schemas. But using modern methods for probing brains, researchers are uncovering the cells and circuits that are responsible.

For example, research has found that brain cells become active as objects approach the space around the body. These cells will fire when, say, you see an insect fly toward your face. This so-called peripersonal space extends to arm's length; people with longer arms have a bigger peripersonal space. And when they use a tool, a rake, a joystick or an automobile, their body schema and peripersonal space expand to include it.

Moreover, perceptions change as the body schema changes in response to outside stimuli. A hill looks steeper when you wear a backpack than when you do not.

The findings, from laboratories worldwide, offer tantalizing biological explanations for many phenomena, including anorexia and syndromes in which stroke patients neglect one side of the body. They may explain why people are sucked into video games, and even why drivers get so upset when their car is dented.

''To act efficiently, we need to locate objects in the space around our bodies,'' said Dr. Angelo Maravita, a psychology professor at the University of Milan. ''We need to hold a constantly updated report on the body's shape and posture.''

The new research draws on the principle that the brain forms internal maps of the external world; groups of cells hold mental models of everything a person sees, hears, feels and knows. The brain also forms a mental map of the body itself. Clumps of brain tissue represent each hand, foot, trunk or lip. If someone touches your hand, cells in the brain's ''hand area'' become active.

Neurons respond to both vision and touch in at least six brain areas. For example, a cell will fire when the right hand is touched, or when the person sees an object moving toward it. The closer the object, the faster the cell fires.

Such cells encode the space around the body, within arm's reach. It is as if you walked around in your own private soap bubble. But the brain also has cells to map space farther away.

Dr. Atsushi Iriki, a cognitive neuroscientist at the Riken Institute in Japan, was one of the first to explore body schema using modern techniques. He inserted single electrodes into monkey brains and identified single cells that responded to both a touch on the hand and visual space next to the hand. Then he gave the monkeys a rake and for three weeks trained them to pull in food pellets with the tool. After training, he found that the cells that represented the hand and arm, as well as space around the arm, changed their firing pattern to include the rake and the space around it.

The moving tool was incorporated into the monkey's body schema, Dr. Iriki said. When the monkey held the tool passively, its body schema shrank to normal size.

In another experiment, Dr. Iriki allowed the monkeys to see a virtual hand on a video monitor while the monkey's real hand, hidden from view, operated a joystick. When he made the image of the hand larger, the monkey's brain treated the virtual hand as if it were an enlarged version of its own; the brain's hand area blew up like a cartoon character's hand. When he put the image of a spider or snake on the screen and made it approach the image of the hand in the monitor, the monkey suddenly retracted its own hand.

These neurons may constitute the neural basis of a person's feeling a sense of reality when playing video games, Dr. Iriki said. People say they can feel the joystick touching objects in the monitor as they extend their bodies into far space.

Stroke patients who neglect one side of their body also reveal changes in body maps. Dr. Anna Berti, a psychologist and physician at the University of Turin in Italy, asked a patient to indicate the midpoint of a line held in front of her body. She put the mark way to the right of the midpoint. But when the same line was shown to her in far space, beyond her reach, she found the midpoint with a laser pointer. The neglect caused by the stroke extended only to the space around her body.

But when Dr. Berti asked the patient to find the midpoint with a stick in the distant line, she made the earlier mistake. Using a tool extended her neglect further into space.

Dr. Berti also tested a stroke patient who denied that the wedding rings on her left hand were hers. But when she was shown the rings in front of her or on her right hand, she described how she came to own them. Dr. Berti concluded that objects closely associated with a hand are part of the body map.

People do not notice they have a body schema until they lose it or feel it is permanently altered, said Dr. Michael S.A. Graziano, a psychology professor at Princeton. Certain kinds of brain damage result in a sensation of floating outside one's body. The feeling can be induced in healthy people by bombarding a region of their brains with a powerful magnetic force.

In a condition called body dysmorphic disorder, people perceive a normal part of their body, like the nose, ears or buttocks, as grotesquely large. And there is recent evidence that anorexia is partly a disorder of the body schema, Dr. Graziano said.

Social psychologists have long studied how personal space expands or shrinks depending on personality, culture and circumstances, although they do not know the underlying mechanisms. For example, when a person is threatened or anxious, body space expands in an effort to keep others away. A conversation with someone from a different culture can produce the feeling that his face is uncomfortably close, though it may be the same distance as that of someone from the same culture. Dr. Dennis Proffitt, a psychologist at the University of Virginia, studies how the body schema affects our perception of the environment. For example, just about everyone overestimates the inclines of hills so that a 5-degree hill looks like a 20-degree hill. But people who are encumbered, tired, out of shape or elderly and in declining health may perceive the incline as 25 or 30 degrees.

Finally, researchers say that large machines can become part of the body map. The lines between parking spaces appear closer together from a Humvee than from a Miata, Dr. Graziano said. An automobile is automatically absorbed into peripersonal space. And that helps explain why a minor dent in a fender can provoke a major blowup. The driver's body space has been harmed.

Meanwhile, expert riders describe how their body space becomes integrated with the horse's. Imagine what that does to the ego.