The good news emerging from recent work on the teenage brain is that it reinforces the notion that the brain is flexible and plastic, often able to catch up when deficiencies in early childhood are redressed later on. This is an important concept because it affects how we look at the ways children learn and, in particular, whether certain stages of a child's life are more important than others. Most experts who study children -- the way their brains work and the way they learn -- now agree that while the first years of a child's life are an important stage of brain development and learning, there are only a few critical times when children must be exposed to certain stimuli without which they will miss crucial developmental steps.

Some Critical Periods for a Young Child

Research suggests that there are certain critical developmental periods for particular functions, such as vision and hearing. William Greenough, James Black, and Christopher Wallace at the University of Illinois did most of their work on rats, but their concept of how the brain learns from experience is widely believed to apply to humans as well.

The researchers distinguish between two different mechanisms guiding brain development. One is called "experience-expectant." For instance, the brain "expects" certain visual images, patterns, and exposure to light in order for vision to develop normally. The developing neural system expects to hear sound in order for the auditory system to develop properly.

For certain functions, neurologists have found a critical period of time at which a child must have sensory or motor input. If a particular stimuli is not present at that time in early development, the window closes and the opportunity is lost forever. In a famous experiment, two scientists, David Hubel and Torsten Wiesel, illustrated that a kitten, temporarily blinded in one eye at a crucial developmental period, would never recover its sight in that eye even when the blindfold was taken off. It would not develop what is called binocular vision. In Greenough's framework, these "expected" experiences help shape and wire the basic neural systems, in particular those involving sight, hearing, and to some extent, movement.

The other mechanism guiding brain development is called "experience-dependent" and refers to experiences that are unique not to the species, but to the individual. Thus a child born in an igloo to Inuit parents will begin to learn about dog teams and skinning animals while a small child in Illinois might be experimenting with pots and pans, or puzzles, or picking out notes on a piano. The brain develops synapses, or connections, specific to what is being learned by the individual. Things that are specifically learned appear to be dependent on the particular environment of the child, whereas certain types of other experience -- spoken language, visual and auditory stimuli, motor input, something that the brain "expects" to find in any environment into which a child is born -- is not.

Most Learning Takes Place Throughout Our Lives

Most human learning is not dependent on sensitive or critical periods or on lessons being taught at a particular age. Unlike the development of the visual system, in which it is crucial for babies to see patterns and light from both eyes when they are infants, many things can be taught to children outside of the early years. There appears to be no particular benefit, for instance, to teaching music or number sense to toddlers, despite claims to the contrary. This is true even for reading. Children are taught to read at greatly varying ages in different countries, and yet the literacy of children who began reading relatively late sometimes is greater than those who learned to read much younger.

Acquiring spoken language is slightly more complicated, and there has been a large amount of work done in this area since it is so crucial for a child's performance at school. Experts used to think that there were critical periods for learning a first language, and mothers were taught to enunciate very clearly to their babies. However, we now learn that the kind of baby talk once so frowned upon is exactly what babies need in order to learn the language.

For language development as a whole, it appears that the critical period is much longer than it is for sight. For a child's native language, both grammar and accent appear to be best learned young, and the window does seem to close, slowly, around puberty. This is true of second-language learning, too. As Harry Chugani of Wayne State University in Detroit put it: "Who's the idiot who decided that youngsters should learn foreign languages in high school?" But the same is not true of vocabulary, or the semantics of language. Here again the brain shows flexibility, taking in new words and constructions well beyond childhood.

Some of the early research into how children come to know certain things -- i.e., into children's cognitive development -- emerged out of work on environmental complexity or "enriched environments" originally with rats, but also with monkeys. Researchers put the animals in three different kinds of cages. For instance, individual rats were isolated in a small lab cage; others inhabited larger cages with other rats; still others were put in complex environments, not only living with other rats, but given lots of toys and obstacles that were changed regularly so the rats weren't bored.

The rats who had the more complex cages seemed to produce more synapses. They could find their way through more complicated mazes than other rats and more quickly. In short, they appeared to be smarter. Their brains seemed to be bigger, too. In one study, the rats raised in complex cages appeared to have 25 percent more synapses in certain parts of their brains compared to the more deprived rats.

This research excited the community of neuroscientists and made its way into the popular press. Combined with a popularization of the idea of critical periods in an infant's life, the enriched-environments research appeared to argue for the benefits of stimulation for infants and toddlers. Legislators called for an Early Head Start program in 1995, arguing that the disappointing results of the program for children ages 3 to 5 were due to the fact that it started too late, after much crucial brain development had taken place. Movie director Rob Reiner, a charismatic advocate for children, picked up this theme, launching a national campaign called "I Am Your Child." Speaking to the National Governor's Association in February 1997, Reiner stated: "We now know through science that the first three years of life is the most critical time period. It is the time period when the brain develops at a greater rate than any time during the course of a person's life. ... But by age 10 your brain is cooked and there's nothing much you can do."

Parents bought mobiles, flash cards. The governors of Georgia and Missouri gave mothers of newborns classical music tapes and CDs because research seemed to indicate music would make babies smarter. And while this fervent interest in the brain development of little children swept through the nation, neuroscientists and others began to set the record straight about what neuroscience really had to say about enriched environments and critical periods.

Cautioning against using monkey and rat research, Dr. William Greenough argued that the "rich" environment for a rat is actually the normal environment for a child. Infants are never raised completely alone and they often have much to explore right at home -- just crawling around the floor might bring a baby to table legs, books to be pulled out of bookshelves, clothes hampers to be upended. His research, stated Greenough, did not argue for extra stimulation. There was no research at all indicating that those mobiles, flash cards, and music lessons created more synapses in a baby's brain.

What we learn about brain science and how children learn about the world around them should give solace to many parents who worry that they have missed giving their children crucial experiences in the early years. Much of what we learn happens throughout our lives. What we missed at one stage, given different circumstances, can be made up later on. The work of neuroscientists and of educators points to the resilience of the brain, to a lifelong ability to learn new things. Public policy that supports learning and interventions at all stages of a child's life can have a profound effect on our society.