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Blank slates, lumps of clay, empty vessels. These descriptions of newborn babies have prevailed through the centuries. Even so, the internal workings of the child - what is really going on in the minds of the junior members of our species - have always been a matter of fascination and speculation for both researchers and proud new parents.

Over the past several decades, professionals in the field of child development, led by advances in neuroscience, have ventured deep into the virtually uncharted territory of how a child's brain development affects thinking. The latest studies show that babies do indeed arrive with inborn, or preprogrammed, abilities. They come into the world with their senses primed to take in information, and with the thinking process already well under way.

In the beginning

The first glimmers of cognitive skills, those abilities associated with learning and thinking, appear in the womb.

Between conception and delivery, billions of brain cells form, including those that will become the cerebral cortex, the part of the brain that contains specialized areas for analyzing time, space and language (as well as for controlling sensory and motor functions). The fetus's responses are primitive, but they are the first step in cognitive development.

"A fetus will kick in response to a loud sound, and then, as the sound is repeated, will stop kicking," says Daphne Maurer, a psychology professor at McMaster University in Ontario and co-author of "The World of the Newborn." This action represents learning in its most rudimentary form.

The womb - long believed to be quiet and stable - is thought of as the home no one wants to leave. But this cozy notion of the womb isn't correct, says Maurer.

Actually, life in the uterus is noisy, unsettling and foul-tasting. Researchers who have plumbed its depths with microphones confirm noises that reach as high as 95 decibels, equivalent to standing next to a lawn mower. Mom's voice resonates deep within, her heart chugs like a heavy-duty pump, her stomach churns and gurgles like a waterfall.

"The womb is not the peaceful, insulated place that people envision. There's a lot going on, and a fetus' senses are constantly working," says Anthony DeCasper, a psychology professor at the University of North Carolina at Greensboro.

In fact, fetal intelligence is purely sensory. The fetus experiences his world through taste, touch, vision and hearing. With all the activity going on inside the womb, he is remarkably adapted and primed to respond to stimulation of all kinds.

And many parents happily (often unknowingly) provide it. Pregnant women touch and stroke their bellies repeatedly and commune with the developing fetus.

"The first one we kind of doted on in utero," says Sarah Kahn, a mother of four in New Canaan, Conn. "We talked to him in there and called him `the Cheeseburger." ' Some people go even further - reading stories, speaking in foreign languages, playing complex segments of postmodern music.

Is there any value to all of this? Probably not, says DeCasper. "In the normal course of development, the fetus gets plenty of stimulation. There's no need to expose it to more than it ordinarily gets."

By the seventh month, the fetus begins responding to sound. In a classic Japanese study, researchers generated tones directly against expectant mothers' bellies and found that during the third trimester the fetal pulse rate quickened - evidence of the ability to hear.

Other experiments have suggested that the fetus can actually remember what he's heard in utero.

DeCasper asked a group of pregnant women to read a portion of a book (for example, Dr. Seuss' "The Cat in the Hat") twice daily to their babies in utero over the last six weeks of pregnancy. Less than three days after birth, the babies listened to the same passage that had been read to them, plus a passage that had not been read to them. The majority of the newborns indicated - by sucking more vigorously on a pacifier - a preference for the tale they had heard while they were in the womb.

Like Maurer and others who study fetal behavior, DeCasper is convinced that learning does occur during the prenatal period. "It's perceptual learning, or learning through the senses," he says.

The first year

Once out of the womb, the newborn's challenges increase dramatically. The first year brings a series of rapid-fire changes - all fueled by a baby's myriad new experiences.

Jerome Kagan, a professor of developmental psychology at Harvard University, likens the process of cognitive development to the production of flowers: "A baby needs a variety of stimuli and the opportunity to use her growing competencies. Just as a seed needs water and sunshine to become a flower, a young infant needs to crawl, play, vocalize, smile and interact with others for her abilities to fully develop."

For much of early childhood, babies' intellectual exploration continues to be very much rooted in the senses and motor skills. A new toy immediately goes into the mouth. A furry blanket gets the fingers moving and stroking. Objects and movements attract her attention, and she looks at things that interest her.

Grasping, rooting, sucking and stepping, the infant is born with a bundle of reflexes that provide her first responses to external stimuli. For example, rooting - the way a baby turns her head when touched on the cheek - stimulates the infant to nurse. And grasping, involuntary in the newborn, becomes intentional as the baby develops.

Carolyn Krall, a mother of three in Minneapolis, was struck immediately by the power of her youngest daughter's grasp. "If Simone grabbed hold of something," Krall recalls, "she'd hang on so tightly that it was hard to get her to let go."

Babies seem almost comical in the first days after birth, their movements awkward and undirected. Within weeks, however, they've learned to take more control of their limb movement. In a study by Carolyn Rovee-Collier, a psychology professor at Rutgers University in New Brunswick, N.J., and the editor of "Infant Behavior and Development," researchers attached a ribbon from a baby's foot to the mobile above her crib. "These babies soon learned to kick their foot in order to make the mobile move," Rovee-Collier says.

At what experts call the "21/2-month shift," the brain undergoes major physiological changes that affect babies' behavior. Infants begin spending more of their time awake, their awareness of outside stimuli increases and they begin to react in slightly less random ways. The infant starts to remember objects she encountered just a second or two before. Her vision, which was extremely limited at birth, improves to the point that she can now focus on a face and maintain eye contact.

"Once Simone was 21/2 months, she could definitely recognize me," says Krall. "When I first get up in the morning, I don't wear my glasses, and although Simone would eat and go through her whole nursing routine, she didn't smile. Because I wasn't wearing my glasses, she would look at me as if she wasn't quite sure who I was."

One of a baby's earliest challenges is detecting the similarities and differences among objects and actions in her environment. The image of Mom's face quickly becomes familiar to the newborn, while a change in the mother's appearance can be temporarily distressing. The infant needs to integrate the difference into her original image of Mom. The process of habituation - by which a baby grows accustomed to a particular object, sensation or event - is a primitive form of learning, one skill in her limited but significant repertoire.

Another major development at this stage, says Dr. Robert Emde, a psychiatry professor at the University of Colorado Health Sciences Center in Denver, is the emergence of the social smile. "An infant is now apt to greet people with a warm smile and may even coo in their presence. It's an awakening of the child's sociability, the beginning of rewarding social interaction. At this point, a baby becomes more of a person in her parents' eyes."

As babies continue to take in sights and experiences - faces, voices, movements, patterns, tastes - they make them their own, forming what are called "mental schemas," or ways of remembering past events and using them to solve problems.

"My daughter has a toy trumpet," says Suzanne deYoung, a Manhattan mother of a 10-month-old. "When I blow into it, the air comes out and hits her face - she loves it. Today when I picked up the trumpet, she winced and smiled, as if she knew the air was about to come out."

But it's the novel or unusual events that stretch the brain and transform the familiar schemas. The child who looks at the same old mobile, with the same objects, day after day, will soon become bored. Change the objects that hang from it, though, and she'll stay interested.

"With each day, an infant acquires more knowledge of her world, becoming sensitive to deviations from what she knows," says Kagan. "If you show a baby a picture of a familiar face, she might be interested, but present her with a picture of an upside-down face and she'll be very attentive."

What's needed for intellectual growth, then, is a balance between familiarity and novelty. In order to understand the stimuli around her, a young baby has to do things over and over again.

"We need some repetition to learn," says Maurer. "The nervous system doesn't usually change from doing something once. There's often rhythm in a baby's actions - that's true of sucking, kicking or grasping a toy."

Such repetitive actions help organize the brain and set the stage for more complex thinking. DeYoung learned this when her daughter was about 2 months old. "She discovered the toy bar on her bouncy chair when she accidentally bumped it," she says. "After that she would intentionally knock it and grin from ear to ear."

With a baby's increased mobility and control of her body come greater interaction with the world as well as an improved understanding of the nature of how things work and how she can affect events.

Jean Piaget - the Swiss psychologist who conducted landmark studies in cognitive development from the 1920s through the 1970s - labeled the period from birth to two years the sensorimotor period, the first of four periods in the development of intelligence. His observations of his own three children - which he recorded in rich detail - convinced him of the inextricable link between activity and cognitive skills.

In the second half of the first year, infants make great strides. The brain continues to mature, which allows for enhanced learning ability. Six-month-olds now comprehend, with great pleasure, that their actions can affect their environment.

Still, the ability to intentionally produce a desired effect is not firmly established. It is not until about 8 months that the infant will consciously decide to kick the bars and make the bells ring.

"Sophie knew that crying or rattling her crib would get Mom or Dad to pick her up," says New York City father Sean O'Connor. "She'd also initiate a game of peekaboo by putting a sheet over her head, knowing we'd come to take it off."

At about the same time, babies begin to solidify a sense of object permanence, or the knowledge that something exists even when it is out of sight. As early as 4 months, an infant realizes that her beloved koala bear hasn't vanished when it falls behind the crib, even though she is unable to act on her understanding and reach under the crib to get it. As a result, she quickly forgets about it. However, the 9-month-old, more accomplished at reaching, is not distracted by the fact that the bear is out of sight for the moment. She can remember the location of her furry companion.

That's what Lucy Barnard observed her daughter, Paris, accomplish at 9 months. "She had a hand-shaped toy that she liked to chew on," says the Houston mother of three. "One day she left it on the floor, and the next morning, after I got her dressed, she crawled right to the toy and started to chew on it."

Paris had acquired what experts label "recall memory." "When babies use recall memory, they're calling up something that isn't present," says Jane Healy, author of "Your Child's Growing Mind: A Practical Guide to Brain Development and Learning From Birth to Adolescence." "That could be a visualization of an object or a past event, or a nonvisual memory of a piece of information, such as a word."

Once a child can form mental pictures of her world, she is on her way to a higher level of thinking. She will soon be able to recognize common characteristics among objects - the beginnings of categorization, a critical skill for language and math. And, perhaps most important, she will begin to play symbolically, another crucial building block in the development of language.

Most children utter their first words by the time they are a year old - but they are already skilled conversationalists. Through cooing and babbling, in wordless dialogues with Mom, Dad and others, the baby has learned the art of communication.

"One of the things we do as people is converse with each other," says Naomi S. Baron, a linguistics professor at American University in Washington, D.C., and the author of "Growing Up With Language." "The baby learns, `Aha, when you're with people, you talk,' " says Baron. The infant has adopted the "conversational imperative" and is now ready to take on the infinitely rich world of words.

The second year

Although the achievements of the second year - toddlerhood - are striking, their evolution tends to be slower than those of the newborn's first few months. The work during this period is more complex, and the time it takes for children to reach each level varies enormously. Some toddlers walk early and talk late; others are verbally precocious but happily creep and crawl until the middle of the second year. Whatever the timetable, the mind and body are flowering and learning is nonstop.

The 13- to 15-month-old is an entirely different creature than he was as a newborn. His brain is now better organized, and the different cognitive systems within it, which have been maturing at different rates, are reaching a kind of equilibrium. But the defining physical feature of a toddler is his ever-increasing mobility.

"The most exciting part of the transition from the first into the second year is the mastery of walking," says Daniel Stern, a psychology professor at the University of Geneva in Switzerland and the author of "Diary of a Baby." "Learning to explore opens up the world for children. They can now look at things from different perspectives and start to understand that the same thing can be experienced in different ways."

An upright and mobile toddler has a much different view of the world of objects, people and events. He is more of a participant with far more control over his environment.

"When they can finally walk, they're so thrilled," says Carolyn Krall. "They stand up and realize `OK, I can make things happen on my own!' "

With greater physical and mental control comes a whole new set of intellectual skills as well as the refinement of already-developing abilities. Most experts agree that the strides made in language are one of the major achievements of the second year. Says Stern, "Language structures experience. If you can't put an experience into words, it can never be examined fully."

The early schemas, or mental models, that infants construct are useful for learning many things, but they are limited in ways that language is not. "Language is the unique tool that makes humans go forward cognitively," says Stern. Quincy Cotton, a New York City mother of two, remembers how at age 2 her son, Luke, could speak in complete sentences and begin to express his emotions. "When I would drop him off, he would say things like, `I don't want you to go.' "

According to Baron, children take one of two approaches to language. Some begin with a single word and go on to sentences; others begin with sentences and then attribute meaning to the words.

"In this culture, children tend to start with one word. A lot of people around them are saying, `Can you say ball?' " explains Baron. "But in cultures where parents are less concerned about whether their children are going to speak at a certain time," she adds, children are more likely to start with "sentences," sometimes unintelligible, that have the structure of speech. Some of those children don't say individual words until they've spoken sentences.

However children approach language, conscientious parents are often distressed by what they perceive to be delays in speech. But, says Baron, "the most critical thing to remember is that there is an incredible amount of variation."

The two older children of Rachel Warner were extremely late talkers. "Both of them spoke only two- to three-word sentences until they were 3," says the mother of three in Arlington, Virginia. "Then their speech took flight."

In the twilight of what Piaget called the sensorimotor period, from about 18 to 24 months, the toddler's mental actions increase and get stronger. He no longer depends on his senses alone to understand what is happening around him; he can represent the world inside his mind. And he has words to express his thoughts. "At the end of the second year, children start to grasp general concepts and understand the way things work in the world," says Stern. "For the first time, they can understand abstract ideas: not only that this tree is green, for instance, but that all trees are green."

While the 9-month-old recognizes similarities and differences among objects, a child twice his age can actually create his own distinct categories. Researchers have discovered that this intellectual ability evolves slowly, over the second and into the third year.

In a study by Susan Sugarman, a psychology professor at Princeton University, children ranging in age from 1 to 3 years were given a group of objects - four boats and four dolls, for example. When asked to rearrange them, some of the 1-year-olds would randomly pick up a boat, look it over and touch it to the other boats, one at a time. The 18-month-olds would handle two or three objects of the same kind in sequence. The 2-year-olds, however, divided the objects into two separate categories.

"Categorization is basic to the way we operate - we think by comparing and contrasting," says Sugarman. "Being able to put things in categories might make it easier for a child to share. For example, a toddler can give a cookie to a playmate if he understands that there are other cookies where that one came from. It might also help him understand a sibling - that's another child in the family."

Toddlers can not only organize objects, they are also discovering that some things (like hats and balls) have different purposes and that others (like cups and saucers or spoons and forks) are related.

In pretend play, toddlers invent their own versions of objects and events in their world. "In this type of play, which typically appears during the last half of the second year, a child discovers how to take all those things that are going on in his life and cut them down to size. He's creating a small-scale reality," says Jerome Singer, a psychology professor at Yale University and coauthor of "The House of Make-Believe: Children's Play and the Developing Imagination."

At first, the creations are quite simple: An 18-month-old child might pretend to drink water from an empty cup or use a real fork to pretend to eat a piece of invisible cake. By the end of the second year, however, a child is capable of substituting one object for another. He might use a stuffed dinosaur to represent another child, or a wooden block for a table. As the child plays in this way, his imagination soars - and when he speaks aloud, he reveals glimpses of what he's thinking.

In order to pretend to be something he is not, a child has to know who he is. Along with a cache of new intellectual skills, the toddler is developing a sense of self. By a year and a half, he recognizes himself in the mirror. At 20 months, he can describe his own behavior. "Mike did it," he'll say, or "Finished."

"At this age, children also start to use pronouns like me, mine and I," says Stern. "That indicates the formation of a self that can be referred to, called the `self-reflective self.' " And by the end of the second year, he has become sensitized to adult standards, the way adults think things should be done.

In studies with children of this age group, Kagan asked adults to demonstrate various activities that they wanted the kids to imitate. He found real distress, starting at 18 months, among those toddlers who could not do what the adults had done. The child now knows that there is an expected way to do things, and he is determined to get it right. Whether building a tower or putting in the final piece of a puzzle, he is goal-driven. When he accomplishes it, his delight is reflected in what Kagan calls the "smile of mastery."

To parents, the changes of the first two years are nothing short of miraculous: A toddler's incredible curiosity - and formidable new learning skills - will lay the foundation for a lifetime of exploration. Yet the pace of a child's progress can also be a source of anxiety for mothers and fathers. Most professionals simply advise parents not to worry and to let their baby develop according to his own timetable.

"Although milestones can be important markers of intellectual growth or flagging development, they have little meaning by themselves," says Virginia Casper, a developmental psychologist at the Bank Street College of Education in New York City. "Every child goes about learning in his own way. What really matters is knowing your child and how he learns - and then using that information to help him along."