Thus begins a complex series of lessons on language. Within months after birth, a child learns to say, "ba!" or "Mama." In another year or so, she is already demanding, "Cookie me!" and "Hi, Daddy!" Language acquisition seems to happen at lightning speed. The challenge, says Johns Hopkins professor of psychology Peter Jusczyk, "is explaining how kids pick up language so rapidly."
When you stop to think about it, learning language is an amazing accomplishment. Mommy and Daddy do not sit down with Junior and say, "Okay son, now, `doggie' is a noun, and `run' is a verb. The proper order is noun-verb. `Doggie runs.'"
But somehow Junior learns what the words sound like, what they mean, how to order them in a sentence, and how to make them agree grammatically--quite often before his second birthday.
Moreover, as the months pass, Junior also learns that a word can mean one thing in one context--"Bees like honey"--and something entirely different in another--"Honey, give me a smile." Eventually, he also absorbs the overlayers of unspoken meaning conveyed through tone or context; he can detect sarcasm in a voice dripping with it, for example, or unctuousness in a voice oozing with sweetness.
How do babies do it?
Until about 30 years ago, language researchers focused their studies on infants who had already begun to babble, according to Jusczyk, who has written a book on how children acquire language titled The Discovery of Spoken Language (The MIT Press, 1997). Babies start to vocalize at around four months of age, and to babble in strings of words at around six or seven months.
"Theories around at that time said that infants perceived speech sounds by producing them," says Jusczyk. In other words, by listening to themselves babble, babies learned to tell one sound from another. Mom, Dad, or the babysitter would reinforce these sounds by repeating their utterances like, "Baba! That's bottle."
Researchers, however, had not developed methods of deciphering what went through a baby's mind before baby uttered his first "Ma" or "Papa." So Jusczyk and other experimentalists devised techniques that allow them to study the pre-babbler. They have demonstrated that speech is the culmination of a tremendous amount of learning. Long before a baby utters his first "baba," the researchers discovered, his mind is furiously sorting out the sounds and shapes of words and sentences.
Colleagues credit Jusczyk for being one of the key
experimentalists to bridge the gap between the study of infant
speech perception and language development. "Peter is the
father of a lot of this work," says Robin Cooper, an associate
professor of psychology at Virginia Polytechnic Institute in
Blacksburg, who studies infant language acquisition.
Jusczyk, who arrived at Hopkins in fall 1996, after six years at the State University of New York at Buffalo, says his interest in the field grows partly out of a lifelong love of language and literature. He has a penchant for Eastern European writers, Faulkner, and poetry. "I'm one of the few people I know who buy poetry books," he remarks.
In their decades-long search for the universal truths about language acquisition, Jusczyk and collaborators around the world have found that at every stage of development, babies know a lot more than they'd been given credit for. The very seeds of language learning, in fact, start to develop in the womb.
Researchers cannot easily investigate language perception in the womb, however. So they study newborn babies' reactions to sounds that mimic the muffled language that penetrates the womb. In this technique, newborn babies listen to filtered recordings of a woman (the baby's mother or another mother) speaking, while sucking on a pacifier that is attached to a pressure transducer. Filtering erases the crisp edges of words, while leaving intact other features such as rhythm, melody, pitch, and intonation-- similar to what a fetus hears in the womb. "It's kind of like listening to a stereo next door," says William Fifer, an associate professor of developmental psychobiology at Columbia University. "You hear a lot of bass, but not the crisp, clear high frequencies."
Using this technique, Fifer and his colleagues found that newborns suck harder on the pacifier when listening to filtered recordings of their own mother's voice in comparison to another mother's. The newborns thus recognize and prefer their own mother's voice, concludes Fifer.
In further studies, Jusczyk and postdoc Thierry Nazzi found that newborns prefer filtered recordings of their own native language over that of a foreign language. Babies like what they know, says Jusczyk. Newborns, he says, apparently learn the rhythm of their native language and of their mother's voice while in the womb.
How does a baby then build onto this rhythmic foundation?
When Jusczyk was an undergraduate at Brown University in the 1960s, he and psychologist Peter Eimas made a remarkable discovery. Using the pacifier technique, they found that one-month-old infants could already distinguish between "pa" and "ba." Subsequent studies by other investigators revealed that babies could do this even at birth.
"Babies come equipped with basic speech perception capacities, as if it were hardwired," Jusczyk concluded. "The ability is part of our biological endowment. Babbling is useful for learning how to produce the sounds. But babies don't need to babble before they can tell the difference between sounds."
Jusczyk's and Eimas's pa/ba findings, which were published in Science in 1971, provided some of the first experimental backing of theories on the hardwiredness of language proposed by noted MIT linguist Noam Chomsky. Their research also opened the doors to a renewed interest in infant speech perception, and encouraged more scientists to start exploring how language develops before a baby starts to speak.
Further, psychologist Janet Werker and colleagues at the University of British Columbia demonstrated that babies can distinguish between a large array of phonetic differences, including ones that are not part of the language spoken around them. For example, an English Canadian six-month-old knows that the hard English "da" is different from the softer Hindi "da," which is pronounced with the tongue tip touching the back of the palate rather than the upper teeth. Other researchers showed that babies born to speakers of the African language Kikuyu, which does not contain the ba/pa distinction, can hear the ba/pa difference, and that Japanese babies can distinguish "ra" from "la," even though Japanese speech does not include "ra."
But scientists have found that even chinchillas can distinguish
between "pa" and "ba," says Jusczyk. So, obviously, this ability
is not all that is required to launch into language. Jusczyk
believes that the ability to make phonetic distinctions is part
of a broader auditory skill, which is shared by humans,
chinchillas, and other mammals. The broader talent may include
the innate ability to distinguish different musical tones, for
example. (Jusczyk is currently testing that hypothesis in human
infants, see Arithmetic of the
Whatever it is that chinchillas and humans share, it is not long before a baby advances beyond all other creatures.
During baby's first year, as words flow across the folds and contours of gray matter, their sounds and rhythms sculpt the brain, and baby becomes sensitive to more idiosyncracies of her particular language.
To a nine-month-old baby, the contrast between the English "da" and the Hindi "da" is less noticeable than it was three months earlier. By the time the baby is a year, she does not appear to notice the difference, paying no more attention to one than the other. "It's use it or lose it," remarks Jusczyk.
The ability to detect these phonetic differences does not entirely disappear, however. As an adult, an English speaker can still hear the Hindi "da," just as a Japanese can learn to pronounce "ra"--but the adult will have to work harder at it.
A BABY'S TASK OF LEARNING LANGUAGE would be much easier if words were spoken in sharp, discrete packets. "But we run words into each other," says Jusczyk. Consider: The ants are my friend. They're blowing in the wind.
This headline, which accompanied a Buffalo News article about Jusczyk's research, illustrates the point. It takes a sophisticated listener to hear that the folk song is about an answer and not airborne ants.
As anyone listening to an unfamiliar foreign language or to certain lyrics knows, it is difficult to tell where one word begins and ends and another begins. But even children whose parents speak as rapidly as Abbott and Costello learn to pluck words out of this torrent of speech.
"How babies do this is a critical problem, and was avoided in speech [acquisition] research for many years," says Jusczyk. But Jusczyk has used an experimental technique called the head-turn procedure to conduct a series of studies on how infants locate words in fluent speech.
Which is why, one day in between snack and nap time,
nine-month-old Alexandra Elliot is sitting on her mother's lap in
a dimly lit booth on the Homewood campus listening to Dutch.
Dressed in her cutest outfit, with a bluebird barrette holding
back all three strands of her strawberry blond hair, Alexandra
stares straight ahead, at a green light on a wall in front of
her. The light starts flashing, as the experiment begins.
Seated at a computer behind this wall, graduate student Derek Houston watches Alexandra through a peephole. Both he and Alexandra's mother, Donna Pitts, wear earplugs and headphones that play loud music to mask the Dutch recordings. The researchers want Alexandra's reactions to be her own, not to be influenced by the data collector or by barely perceptible movements from her mom.
Pendel, announces a hidden speaker behind a pegboard wall on Alexandra's left. At the same time, a red light on the wall starts to flash.
Alexandra's eyes open wide and her mouth, framed by chubby cheeks, makes an "O," revealing two bottom teeth. "Dada!" she exclaims, and reaches toward the light.
Kusten, reports a hidden speaker to Alexandra's right, while a red light on the right wall flashes. Alexandra then stares at this light.
Die pendel ligt op het bureau van mijn oom... continues the recording. Several other Dutch passages follow. Some contain the words pendel or kusten, and some do not.
At the computer, Houston records precisely how long Alexandra stares at either of the lights.
Later, Houston explains the experiment's premise. Babies believe that flashing light equals sound, he says. If a baby is interested in the words she hears spoken, she will look to see where they are coming from. The more interested she is in the words, the longer she will stare at a light that flashes while the words play. Often, the difference is slight: a baby will stare at a light for six seconds while one passage is played compared to seven seconds while another is played. But even one second difference, says Jusczyk, can be statistically significant.
Obviously, Alexandra does not know that pendel means "hanging lamp," nor that kusten means "coasts," says Houston, but she can hear the prosody of these words. "Prosody," says Jusczyk, "means rhythm, melody, accentuation on syllables, and intonation--the suprasegmental information" contained in a word.
"The rhythmic properties of English are such that English words usually (about 75 percent of the time) begin with a stressed (or accented) syllable," says Houston. Think of "bottle," "carrot," "baby," "pencil." Babies raised in an English-speaking environment, the theory goes, apparently recognize this acoustical strong-weak pattern, and use it to pick out words in the sea of babble--that is, to segment speech.
Indeed, in earlier experiments, Houston, Jusczyk, and their colleagues found that American seven-and-a-half-month-old infants listen longer to unfamiliar words that follow this strong-weak pattern than they do to unfamiliar words such as "beret" or "guitar," which do not.
To add weight to their hypothesis, the researchers then tested Dutch infants while the babies listened to similar two-syllable English words and passages containing those words. Now the researchers are testing American infants, like Alexandra, while those babies listen to analogous recordings of spoken Dutch. "Dutch and English have very similar rhythmic properties," says Houston. "If infants are relying mainly on rhythmic properties to locate words in fluent speech, it may not matter that it's a different language, so long as the words follow the typical stress pattern of English," as pendel and kusten do.
If a word does not fit the strong-weak pattern, Englishspeaking parents make it conform to the pattern, adds Jusczyk: "horse" becomes "horsie." "Dog" becomes "doggie." It is as though parents instinctively know that the strong-weak pattern will help their baby learn the outlines of a word. Not every word in English obeys the strong-weak rule, but finding words that abide by the rule may be an entry point into the language.
But not all languages favor strong-weak accented two-syllable words. Consider chalet and touché and many other French words. They are syllable-timed. Both syllables receive approximately equal emphasis. "So how do French babies learn to segment words?" asks Jusczyk.
The last syllable of many French words is slightly longer than the first syllable (think of château). "Maybe French babies look for [a second] syllable that is accented just a little bit longer," posits Jusczyk. Jusczyk's wife, Ann-Marie, who is laboratory coordinator for the infant language studies, recently trained researchers in France who are now investigating this question. After France, the Jusczyks hope to conduct similar studies in countries where other languages are spoken.
Of course, language is not just about words. It also involves grammar and syntax. Jusczyk believes that information such as prosody can help babies learn the rules.
"When you grow up, your voice will change, my mother told me" has distinct syntactic units, says Jusczyk. "Children learn what the right packages are." They learn that "When you grow up" is a correct package, and that "When you grow up your voice" is an incorrect package.
In a series of experiments, Jusczyk's team played recordings of
speech in which the scientists had inserted one-second pauses in
one of two places: at the boundaries between clauses, or within
the clauses themselves. They then played the recordings for
seven- to 10-month-old infants. The researchers found that babies
listened longer to passages containing one-second pauses between
clauses than passages containing pauses that interrupted clauses,
"This line of work suggests that infants are learning about cues that are going to help them identify syntax," he concludes. "When you get to the end of a clause, the pitch drops, the syllable lengthens, and you tend to pause." These features appear to signal a shift from one phrase to the next, or from one clause to the next.
Likewise, says Jusczyk, information in speech helps language learners comprehend grammar. Nouns, for instance, are often preceded by "the" or "a." These words serve as cues, and help the baby learn which words are nouns.
But how does a baby know she must hunt for a noun in the first place?
"That is the $64,000 question," says Jusczyk. "These are issues that are still very much debated."
Some authorities argue that speech contains all the information a baby needs to understand language. Others, including Chomsky and MIT cognitive neuroscientist Steven Pinker, believe that this knowledge is, to some degree, innate. According to this view, babies are hardwired with an understanding of nouns and verbs from birth.
Jusczyk falls somewhere in between. Babies use information from speech to learn language, he says. But they also have an innate tendency to look for categories that we call noun and verb. Babies do not know that "dog" is a noun and "runs" is a verb, but they appear to be innately inclined to look for parts of speech that correspond to objects and actions, he says.
Parents probably unconsciously guide babies in learning the structures of words and phrases, says Jusczyk. Parentese, or child-directed speech, is the singsongy voice that parents use when speaking to an infant. Researchers find that moms and dads around the world appear to use it, and babies, in turn, appear to prefer parentese to adult-directed speech.
Parentese highlights important features of the language, Jusczyk hypothesizes. When adults speak to adults, they often mark syntactic boundaries by changes in prosody (such as changes in pitch, syllable duration, and pauses). When adults speak to babies, they exaggerate these prosodic cues. Mother says, "Look at the pretty babieeee." It is as though the adult is erecting linguistic signposts that indicate a) baby is a noun, and b) the sentence is ending.
More research is needed, however, says Hopkins cognitive scientist Michael Brent. He recently recorded 200 hours of mothers speaking to their babies. The eight mothers who volunteered for the study wore lightweight recorders tucked inside fanny packs that recorded their speech as they went about their daily routines and cared for their children. Recordings were made every two weeks from the time the babies were nine months old until they were 15 months. In addition, Brent periodically tested the babies' language abilities.
Brent is now writing computer programs to analyze his collection of child-directed speech. "I'd like to see how often mothers use isolated words, whether they are using mostly nouns or verbs, how long is each utterance, and what's the pitch."
Once he has the anthology, says Brent, he and other researchers will be able to study how mothers' child-directed speech changes over time, as their children learn language. He will also look to see how often mothers speak single words to their children--for example, just, "doggie" or "car"--without combining those words with others. Brent hypothesizes that babies first learn these single words, and build upon them, in learning how to segment speech.
AT SOME POINT, a baby does not just absorb language but begins producing words. Baby's vocabulary really takes off at around 18 months, says Jusczyk. During this vocabulary explosion, some researchers believe, babies learn to speak as many as nine new nouns per day. Scientists have struggled to explain how babies could learn words so quickly.
Jusczyk recently added a piece to the puzzle. Long before they speak words or know their meaning, babies appear to memorize the sounds of words they hear frequently.
Jusczyk's team recorded a woman reading children's stories, and played the stories for eight-month-old babies once a day for 10 days in the babies' homes. The stories included words unfamiliar to an eight-month-old, such as peccaries, python, and hornbill.
In the lab two weeks later, the babies listened to recordings of lists of words that occurred frequently in the stories. They also listened to words that were similar to the story words but had not been in the stories.
The babies preferred the lists containing words they had heard in the stories, the scientists reported in the September 26 Science. Clearly, the eight-month-olds didn't know that a hornbill is a bird, or that a python is a snake. But they did remember the sounds of these words--a helpful starting point for future word comprehension. "The mind is wired to retain sound patterns, and then you piece them together," concludes Jusczyk. It is as though the child first builds an acoustic frame, and then fills it in with a semantic picture.
FROM THE RHYTHMS OF LANGUAGE that override the whoosh of the womb to the complex rules of grammar and syntax, the stages and timing of language acquisition are being defined.
But the steps of language acquisition are just details of the process, notes Brent. "Generally, the state of the theory is that none of us can explain how children learn language. So far, it defies explanation.
"It would be like someone saying, `My computer works because someone stuck a central processing unit into it.'" But how does the CPU work? Likewise, we can assume that the concept of noun and verb is genetically programmed into our brains, but how does that program work?
Future research may uncover some answers. For example, Jusczyk is interested in studying how listeners integrate many types of information along with language. For example, a person listening to someone speaking not only pays attention to the speaker's words. He also thinks about the appearance, mannerism, and professional standing of the speaker, and may even ponder memories related to the speaker's topic. Jusczyk would like to know how the brain analyzes language in the context of this other information.
Likewise, says Jusczyk, "We're a long way from understanding the relationship between what's happening in the brain and in behavior." At around eight to nine months, for example, the long-range connections among different brain regions proliferate, and the brain's metabolism becomes more like an adult's. These changes are interesting, says Jusczyk, "but are they causal?" Does this proliferation of brain connections account for the ability of eight-month-old babies to memorize the sounds and rhythms of words? Or does baby's new ability to remember words spur a proliferation of neuronal connections? "Sometimes I don't think we even know if we're on the right page or not," says Jusczyk.
"I don't think anybody knows for sure how we learn language," says Jusczyk. "But we have certain hypotheses that make more sense than they did before."
Melissa Hendricks is the magazine's senior science writer.
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