Limits to Plasticity

A User's Guide to the Brain: Perception, Attention, and the Four Theaters of the Brain
by John J. Ratey, M.D.

(Page 9 of 12)

Despite the brain's amazing ability to adapt, there are limits to its flexibility. Age does make it harder to reroute and establish new circuits. Music teachers, chess champions, and star athletes all advise parents to start their disciples young. We've all seen how much easier it is for young children to pick up second languages. In an American family that is transferred to Tokyo for a year, the preschool child will learn to converse in Japanese while her mother is still struggling with basic communication.

Children who are exposed to two languages from birth learn to speak both fluently. Linguistic researcher Patricia Kuhl, at the University of Washington in Seattle, likes to say that all babies are born "citizens of the world," meaning that they can learn any language perfectly. She has tested newborn infants with sounds unique to African languages, English, and Japanese. No matter where a baby is being reared, he or she can distinguish the fine auditory cues typical of any non-native language and is presumably ready to learn any language heard.

From about six months on, however, if babies have not heard a particular speech sound, they can no longer distinguish it. Infants whose parents speak English have formed different linguistic connections than infants whose parents speak Japanese, based on the phonemes they hear: the long "oooo" and abrupt "ba" of English, the clipped "toh" and slurred "rr/ll" of Japanese. By its first birthday an infant can no longer process phonemes it hasn't heard; it is functionally deaf to foreign sounds, having learned to ignore sound distinctions not necessary for its native language. In fact its babbling, though not yet words, is confined to sounds that the infant has already heard in its own language. To learn Japanese after childhood, we conjugate long lists of verbs and endlessly repeat dialogue from language tapes, but we can never speak like a native, because our language circuits are unable to form new basic connections.

Brain development in the fetus and baby occurs through a series of critical periods, "windows of opportunity," when the connections for a function are extremely receptive to input. Once the window closes, neural connections are pruned down to the most efficient, according to how much they are used. Then the battle is over: the closed eye and the deciphering of foreign phonemes will never regain space in the brain. It is clear that it is possible for adults to learn to speak a new language with little or no accent, but it is also clear that they do not do this the way a baby does, and instead use altogether different systems to learn. The adult systems are not nearly as good as the baby ones.

These precious windows of opportunity are also times of great vulnerability to irreversible damage. "Closet kids" found by police provide the strongest evidence. These children have been locked in closets or basements for years by psychotic or brutal parents. They grow up without hearing human conversation and are never able to master the sounds and grammatical rules necessary for smooth speech. Through long instruction after they are found, other pathways compensate to some extent, but tragically, owing to the extreme deprivation, the critical period for natural speech development has been missed.

One girl, Genie, was discovered in 1970 in Los Angeles at age thirteen. She had spent her entire life, from babyhood on, in one room, often chained for hours to a potty chair and beaten if she made a noise. Imprisoned and isolated by her psychotic father, she had effectively grown up without human contact. All she was able to hear was blurred conversation through the walls of her room. After four years of subsequent experiments and training she had learned a vocabulary and sign language, but her syntax remained disrupted. She could produce pidgin-like sentences such as "Applesauce buy store," but was per-manently incapable of mastering grammar. She had already passed the limited window of opportunity for language acquisition. (Unfortunately, Genie did not come out of this story well. Funding ran out and Genie regressed after passing through a string of foster homes, where she was beaten and abused.)

In contrast is Isabelle, who was six years old when she and her mute, brain-damaged mother escaped the silent imprisonment of her grandfather's house. With training, a year and a half later she had a 1,500-word vocabulary and could form complex sentences like "What did Miss Mason say when you told her I cleaned my classroom?" She had not yet passed through the window of opportunity for attaining syntax.

University of Chicago psychologist Janellen Huttenlocher has found that the frequency with which normal parents speak to and around their child during the child's second year significantly affects the size of the child's vocabulary for the rest of his or her life. The more words a child hears during this sensitive period, whether it's "cat" or "existentialism," the stronger the basic language connections.

Constraints on plasticity for many sensory and motor functions also depend on critical time periods. Most humans move all their body parts during the first two years of life. By age two the motor circuits become hard-wired. If for some reason a child never moved his arms these circuits would be lost and he would never be able to move his arms in a natural way. Regions for basic vision are complete by six months. Acquisition of other functions, however, such as academic learning, takes place over a lifetime, unconstrained by windows of development.

Understanding when the brain's circuits are most receptive to learning a particular skill can help create the optimum environment for a child's development. Psychiatrist Dan Stern at the University of Geneva believes that the critical period for developing emotions occurs from ten to eighteen months. Stern was one of the first and most long-term baby watchers, and has looked for evidence of emotional and social critical periods. His work indicates that if parents regularly respond to their baby with delight, the child's circuits for positive emotions are reinforced. If parents repeatedly respond with horror, the child will shut down those circuits and instead reinforce the fear circuits; the research shows that early fright conditions the baby's brain for more fright. Prolonged depression in the mother conditions the baby for depression, too. The key words here are "repeatedly" and "prolonged." An occasional scowl won't set the child up for a miserable life.

Stress management is also learned during a critical period early in life, according to research on newborn rats, which have neurons very similar to humans. The studies reveal that the more the rats are gently handled, the more they produce serotonin, a brain chemical that controls aggressive behavior. As adults, the rats who had received gentle handling were better able to cope with stress, had stronger immune systems, and actually lived longer than rats who had not been treated gently.

Many cognitive functions share pathways in our brain's complex tangle of neural connections. The development of one skill can therefore profoundly influence another that is seemingly unrelated. As the Mozart effect shows, music and spatial reasoning appear to be linked. Listening to words and reading share some of the same circuits, too.

Excerpted from A User's Guide to the Brain by John J. Ratey, M.D. Copyright © 2002 by John J. Ratey, M.D.. Excerpted by permission of Vintage, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

About the Author

John J. Ratey, M.D., is an associate professor of psychiatry at Harvard Medical School. He has lectured extensively and published many articles on the topic of treating adults with ADD. Dr. Ratey is the author of A User's Guide to the Brain and the co-author of Driven to Distraction. He lives in Cambridge, Massachusetts, where he has a private practice.