How Do We Learn Language?
Most new moms and dads know a lot about language acquisition because they witness it first-hand. They can tell you that babies start to babble around six to ten months of age, and that not long afterward they say a few words like “no” or “uh-oh.” At around two years, already more like children and less like babies, they begin speaking grammatically correct sentences and their vocabulary undergoes a growth spurt. And by three years, most children can speak in a manner that is essentially adult-like.

But how do children learn so much about language in so little time? Adult-like knowledge of language requires many complex skills, and it’s not at all obvious how we acquire them. We possess phonological skills: the ability to perceive and analyze sounds in real time, and to produce sounds by coordinating our larynx, pharynx, tongue, palate, jaw, and lips. We possess a lexicon: thousands of vocabulary words. And we possess syntactic skills: the grammar rules of our native language, which govern how words and morphemes can be combined to produce correct sentences. Furthermore, we must practice these skills in situations that are sometimes noisy and distracting, while interacting with other people whose pronunciation, intonation, and speaking rate may be quite different from our own.

A Theoretical Perspective
In the 1960s, MIT linguist Noam Chomsky addressed the question of language acquisition, and his views have influenced the field ever since. Chomsky claimed that a child could not learn everything he or she knows about language from the environment alone. The relatively small number of language examples that a child is exposed to (“the poverty of the stimulus”) simply does not account for the complexity of what the child is able to say and express, or the accurate grammatical generalizations the child is able to make. Can we truthfully say that a child learns a language, when she acquires such a rich knowledge of it without explicit instruction?

Of course, the environment must play some role. A child growing up in a Japanese-speaking household will obviously learn Japanese, not Swahili or English. But children around the world acquire language in strikingly similar ways, despite the fact that their environments vary so widely. How is this possible?

Chomsky theorized that we are born with a genetically determined language faculty: newborns, far from being a blank slate, already possess a universal grammar that determines possible language rules and modes of interaction. According to his theory, the universal grammar works like a network containing a series of switches. Each switch is associated with a language feature or rule, and is set on or off in the course of the child’s interactions with a specific language.

For example, in some languages, verbs precede their objects (as in English, “Mariko ate a pizza.”); in other languages, they follow their objects (as in Japanese, “Mariko a pizza ate.”). According to the theory, one switch in the universal grammar would determine whether such sentences should be constructed with the direct object before or after the verb. Setting the switch requires relatively simple environmental input and proceeds by a more or less biological means of growth. A child exposed to English will set the switch one way, while a child exposed to Japanese will set it another way. A similar process occurs for other grammar rules.

Can a Grammar Grow?
While Chomsky’s ideas are compelling, they can present problems when we try to account for the various stages that children pass through before they acquire adult-like mastery of grammar. If children possess an innate universal grammar, why do they make mistakes as they begin to talk? As many parents know, correcting a child, or providing a child with the right example, does not often produce immediate success. So why don’t children demonstrate mastery of a particular grammatical feature of their native language as soon as they are exposed to it? Cross-linguistic studies have shown that many children pass through a stage in which they don’t use subjects in their sentences (as when a child points to his empty plate and says, “Ate pizza.”). While such sentences are grammatically correct for languages like Spanish, which allow subjects to be dropped, they are incorrect for languages like English, that require subjects.

It’s possible that there is not a single universal grammar, but a sequence of grammars that is available to the growing child at particular points in the process of maturation. Alternatively, mature knowledge of language may rely on the development of other cognitive capabilities such as memory and attention. Some researchers have also pointed out that children do receive some form of direct instruction through ‘Parentese’ – the simplified, sing-song sentences that many parents use with babies.

A Neurobiological Perspective
Chomsky’s theory does not attempt to describe the relationship between universal grammar and physical structures in the brain. Where are the switches of the grammar located, and how do they operate in conjunction with other areas? Ultimately, we cannot fully account for the mysteries of language acquisition unless we also address the issues from a neurobiological perspective.

Research has at least identified certain areas of the adult brain that are typically responsible for specific aspects of language, and these can serve as starting points for understanding children’s brains. The left hemisphere appears to be critical in most right handers and many left handers. As early as the 1860s, neurologist Paul Broca observed that patients who suffer stroke damage on the left side of the brain are far more likely to exhibit language loss than those who suffer damage on the right side. More recent studies suggest that about 97% of the population exhibits left-hemisphere dominance for language.

Broca worked with an adult stroke patient whose speech production was so impaired that the patient repeated a single word (“tan”) over and over – yet the patient’s comprehension abilities remained largely intact. The neurologist identified a lesion on the patient’s left frontal lobe, just above the Sylvian fissure. Later referred to as “Broca’s area,” this part of the brain probably plays an important role in the production of grammatically correct sentences.

Not long afterward, another neurologist named Carl Wernicke presented very different findings from two stroke patients whose speech production remained relatively fluent (albeit characterized by circumlocutions), but whose comprehension was severely impaired. In these patients, he identified lesions just below the Sylvian fissure, in a location now known as “Wernicke’s area” that is considered to play a role in the comprehension and production of meaning.

Lesions to the right hemisphere are not usually associated with language loss, but there is evidence that the right hemisphere plays a role in emotion. Cognitive scientist Sheila Blumstein and her colleagues, for example, demonstrated that the right hemisphere appears to be dominant for processing linguistic intonation (in order to distinguish a question from a statement, for example). Scientists have also hypothesized that the right hemisphere has the potential to assume some language functions if the left hemisphere is damaged.

Can the Adult Tell Us About the Child?
Our data about the left and right hemispheres comes primarily from adults. But what changes does the brain undergo as the process of language acquisition proceeds from birth to adulthood? To some degree, the brain expects, and perhaps even depends upon, interaction with the environment in order to develop and reach maturation. Babbling may assist in the development of language capabilities, just as playing with objects assists in development of motor skills. These first years of life may be a critical or sensitive period for language acquisition, an idea introduced in 1967 by Eric Lenneberg. He observed that around puberty, children begin to lose the ability to fully regain language abilities after a stroke – the same time when they begin to lose the ability to speak a second language without an accent.

But the idea that language is programmed to “grow” in some biological fashion within the brain is challenged by the fact that both adults and children can indeed learn new languages, with or without an accent, just as they learn about other bodies of knowledge. As the linguist James McCawley wrote, “If one is given appropriate exposure to French, Flemish, and German, one develops command of all three languages but does not develop three larynxes or three pairs of ears.”

A Methodological Perspective
Because ethical concerns limit experimentation with human subjects, our techniques for direct investigation are few. Broca and Wernicke both localized their eponymous areas of the brain by working with adult stroke patients and dissecting their brains after death. While it’s not possible to control for the site and size of human brain lesions, research with this population still allows us to isolate important variables and make correlations between language breakdown and brain structures.

Studying language breakdown, however, is not the same as understanding language. We must also investigate people with normal language capabilities, and one way to do this is by taking pictures of the brain. Several different imaging techniques, such as Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET), allow us to view live brain tissue from different angles. Scientists can also measure electrical activity from different points on the scalp. In an experiment described in the book The Scientist in the Crib, researchers used this technique to measure young babies’ reaction to speech sounds, and found similar patterns of brain activity for both native and foreign language sounds. A few months later, however, the same babies demonstrated unique patterns of electrical activity in the left hemisphere when they heard sounds from their own language. Findings like this are a long way from confirming or denying the existence of a universal grammar, but they do provide a starting point from which to consider Chomsky’s notion of an innate language faculty.

Can Behavior Tell Us About the Brain?
Techniques for studying language in a behavioral manner can also help us to uncover aspects of how the child represents language in her mind. In one type of task, the experimenter says a sentence and asks the child to repeat it. This task, called “elicited imitation”, is based on the assumption that the child does not really memorize the sentence, but actively reconstructs its meaning and expresses it in her own way. In “truth value judgment” tasks, the experimenter depicts the meaning of a sentence with toys, and the child gives a yes or no response indicating whether or not he thinks the experimenter performed an accurate depiction of the sentence. Studies of pre-linguistic infants usually measure the rate at which babies suck on pacifiers in response to stimuli, or observe how babies turn their heads.

Some researchers have also performed analyses on language samples collected in natural settings, such as mother-child interactions. Carnegie Mellon University’s CHILDES database, for example, contains transcriptions of child language samples from various settings and provides tools for analyzing them. But a child’s language behavior, either in a laboratory or in a natural setting, does not necessarily reflect everything that the child knows about language and may be affected by non-linguistic factors, such as attention span.

Research with animals may also provide insights. Recent studies have provided evidence of the monkey brain’s ability to re-organize itself in response to new sensory experience, even in adulthood. On the other hand, monkeys do not possess language, and they appear to lack the capability for vocal mimicry that is critical for human language acquisition. Songbirds, who do possess mimicry skills, appear to require both visual and social cues to learn songs. One experimenter found that zebra finches seek out tutors who look like themselves, but don’t necessarily sound like themselves: they will mimic the song of another zebra finch who has learned the alien tune of the Bengalese finch, for example. Zebra finches also prefer to learn interactively, and readily learn alien tunes from tutors of another species if another zebra finch is unavailable for social interaction. Exactly how these findings apply to our own species, however, is an open question.

Perspectives For the Future
In thinking about language acquisition, we ask ourselves more questions than we can answer. Do we understand our own brains well enough to compare them with those of monkeys and birds? Can we fully describe a universal grammar – or, for that matter, can we fully describe English grammar? Can we systematically correlate behavioral data from speakers of different languages with physiological data about the brain? Can we study the brain in a way that allows us to analyze vast numbers of neurons and neural structures in an organized fashion? When, and if, we know the answers to these questions will depend upon the collaboration of thinkers from many different fields of inquiry.

Anne Pycha is a writer who focuses on topics in neuroscience and language. She earned her bachelor’s degree from Brown University and is pursuing a graduate degree in linguistics at the University of Chicago.

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