Language acquisition

 

Language acquisition is:

the process by which humans acquire the capacity to perceive, produce and use words to understand and communicate. This capacity involves the picking up of diverse capacities including syntax, phonetics, and an extensive vocabulary. This language might be vocal as with speech or manual as in sign. Language acquisition usually refers to first language acquisition, which studies infants' acquisition of their native language, rather than second language acquisition, which deals with acquisition (in both children and adults) of additional languages.

The capacity to acquire and use language is a key aspect that distinguishes humans from other organisms. While many forms of animal communication exist, they have a limited range of nonsyntactically structured vocabulary tokens that lack cross cultural variation between groups

 

Language acquisition is one of the central topics in cognitive science. Every theory of cognition has tried to explain it; probably no other topic has aroused such controversy. Possessing a language is the quintessentially human trait: all normal humans speak, no nonhuman animal does. Language is the main vehicle by which we know about other people's thoughts, and the two must be intimately related. Every time we speak we are revealing something about language, so the facts of language structure are easy to come by; these data hint at a system of extraordinary complexity. Nonetheless, learning a first language is something every child does successfully, in a matter of a few years and without the need for formal lessons. With language so close to the core of what it means to be human, it is not surprising that children's acquisition of language has received so much attention. Anyone with strong views about the human mind would like to show that children's first few steps are steps in the right direction. 

Language acquisition is not only inherently interesting; studying it is one way to look for concrete answers to questions that permeate cognitive science:

Modularity. Do children learn language using a "mental organ," some of whose principles of organization are not shared with other cognitive systems such as perception, motor control, and reasoning (Chomsky, 1975, 1991; Fodor, 1983)? Or is language acquisition just another problem to be solved by general intelligence, in this case, the problem of how to communicate with other humans over the auditory channel 

 

Human Uniqueness. A related question is whether language is unique to humans. At first glance the answer seems obvious. Other animals communication with a fixed repertoire of symbols, or with analogue variation like the mercury in a thermometer. But none appears to have the combinatorial rule system of human language, in which symbols are permuted into an unlimited set of combinations, each with a determinate meaning. On the other hand, many other claims about human uniqueness, such as that humans were the only animals to use tools or to fabricate them, have turned out to be false. Some researchers have thought that apes have the capacity for language but never profited from a humanlike cultural milieu in which language was taught, and they have thus tried to teach apes language-like systems. Whether they have succeeded, and whether human children are really "taught" language themselves, are questions we will soon come to. 

Language and Thought. Is language simply grafted on top of cognition as a way of sticking communicable labels onto thoughts (Fodor, 1975; Piaget, 1926)? Or does learning a language somehow mean learning to think in that language? A famous hypothesis, outlined by Benjamin Whorf (1956), asserts that the categories and relations that we use to understand the world come from our particular language, so that speakers of different languages conceptualize the world in different ways. Language acquisition, then, would be learning to think, not just learning to talk. 

This is an intriguing hypothesis, but virtually all modern cognitive scientists believe it is false (see Pinker, 1994a). Babies can think before they can talk (Chapter X). Cognitive psychology has shown that people think not just in words but in images (see Chapter X) and abstract logical propositions (see the chapter by Larson). And linguistics has shown that human languages are too ambiguous and schematic to use as a medium of internal computation: when people think about "spring," surely they are not confused as to whether they are thinking about a season or something that goes "boing" -- and if one word can correspond to two thoughts, thoughts can't be words.

But language acquisition has a unique contribution to make to this issue. As we shall see, it is virtually impossible to show how children could learn a language unless you assume they have a considerable amount of nonlinguistic cognitive machinery in place before they start. 

Learning and Innateness. All humans talk but no house pets or house plants do, no matter how pampered, so heredity must be involved in language. But a child growing up in Japan speaks Japanese whereas the same child brought up in California would speak English, so the environment is also crucial. Thus there is no question about whether heredity or environment is involved in language, or even whether one or the other is "more important." Instead, language acquisition might be our best hope of finding out how heredity and environment interact. We know that adult language is intricately complex, and we know that children become adults. Therefore something in the child's mind must be capable of attaining that complexity. Any theory that posits too little innate structure, so that its hypothetical child ends up speaking something less than a real language, must be false. The same is true for any theory that posits too much innate structure, so that the hypothetical child can acquire English but not, say, Bantu or Vietnamese. 

And not only do we know about the output of language acquisition, we know a fair amount about the input to it, namely, parent's speech to their children. So even if language acquisition, like all cognitive processes, is essentially a "black box," we know enough about its input and output to be able to make precise guesses about its contents. 

 

The scientific study of language acquisition began around the same time as the birth of cognitive science, in the late 1950's. We can see now why that is not a coincidence. The historical catalyst was Noam Chomsky's review of Skinner's Verbal Behavior (Chomsky, 1959). At that time, Anglo-American natural science, social science, and philosophy had come to a virtual consensus about the answers to the questions listed above. The mind consisted of sensorimotor abilities plus a few simple laws of learning governing gradual changes in an organism's behavioral repertoire. Therefore language must be learned, it cannot be a module, and thinking must be a form of verbal behavior, since verbal behavior is the prime manifestation of "thought" that can be observed externally. Chomsky argued that language acquisition falsified these beliefs in a single stroke: children learn languages that are governed by highly subtle and abstract principles, and they do so without explicit instruction or any other environmental clues to the nature of such principles. Hence language acquisition depends on an innate, species-specific module that is distinct from general intelligence. Much of the debate in language acquisition has attempted to test this once-revolutionary, and still controversial, collection of ideas. The implications extend to the rest of human cognition.

2 The Biology of Language Acquisition

Human language is made possible by special adaptations of the human mind and body that occurred in the course of human evolution, and which are put to use by children in acquiring their mother tongue. 

2.1 Evolution of Language

Most obviously, the shape of the human vocal tract seems to have been modified in evolution for the demands of speech. Our larynxes are low in our throats, and our vocal tracts have a sharp right angle bend that creates two independently-modifiable resonant cavities (the mouth and the pharynx or throat) that defines a large two-dimensional range of vowel sounds (see the chapter by Liberman). But it comes at a sacrifice of efficiency for breathing, swallowing, and chewing (Lieberman, 1984). Before the invention of the Heimlich maneuver, choking on food was a common cause of accidental death in humans, causing 6,000 deaths a year in the United States. The evolutionary selective advantages for language must have been very large to outweigh such a disadvantage. 

It is tempting to think that if language evolved by gradual Darwinian natural selection, we must be able to find some precursor of it in our closest relatives, the chimpanzees. In several famous and controversial demonstrations, chimpanzees have been taught some hand-signs based on American Sign Language, to manipulate colored switches or tokens, and to understand some spoken commands (Gardner & Gardner, 1969; Premack & Premack, 1983; Savage-Rumbaugh, 1991). Whether one wants to call their abilities "language" is not really a scientific question, but a matter of definition: how far we are willing to stretch the meaning of the word "language".

The scientific question is whether the chimps' abilities are homologous to human language -- that is, whether the two systems show the same basic organization owing to descent from a single system in their common ancestor. For example, biologists don't debate whether the wing-like structures of gliding rodents may be called "genuine wings" or something else (a boring question of definitions). It's clear that these structures are not homologous to the wings of bats, because they have a fundamentally different anatomical plan, reflecting a different evolutionary history. Bats' wings are modifications of the hands of the common mammalian ancestor; flying squirrels' wings are modifications of its rib cage. The two structures are merely analogous: similar in function. 

 

Though artificial chimp signaling systems have some analogies to human language (e.g., use in communication, combinations of more basic signals), it seems unlikely that they are homologous. Chimpanzees require massive regimented teaching sequences contrived by humans to acquire quite rudimentary abilities, mostly limited to a small number of signs, strung together in repetitive, quasi-random sequences, used with the intent of requesting food or tickling (Terrace, Petitto, Sanders, & Bever, 1979; Seidenberg & Petitto, 1979, 1987; Seidenberg, 1986; Wallman, 1992; Pinker, 1994a). This contrasts sharply with human children, who pick up thousands of words spontaneously, combine them in structured sequences where every word has a determinate role, respect the word order of the adult language, and use sentences for a variety of purposes such as commenting on interesting objects.

This lack of homology does not, by the way, cast doubt on a gradualistic Darwinian account of language evolution. Humans did not evolve directly from chimpanzees. Both derived from common ancestor, probably around 6-7 million years ago. This leaves about 300,000 generations in which language could have evolved gradually in the lineage leading to humans, after it split off from the lineage leading to chimpanzees. Presumably language evolved in the human lineage for two reasons: our ancestors developed technology and knowledge of the local environment in their lifetimes, and were involved in extensive reciprocal cooperation. This allowed them to benefit by sharing hard-won knowledge with their kin and exchanging it with their neighbors (Pinker & Bloom, 1990).

2.2 Dissociations between Language and General Intelligence

Humans evolved brain circuitry, mostly in the left hemisphere surrounding the sylvian fissure, that appears to be designed for language, though how exactly their internal wiring gives rise to rules of language is unknown (see the Chapter by Zurif). The brain mechanisms underlying language are not just those allowing us to be smart in general. Strokes often leave adults with catastrophic losses in language (see the Chapter by Zurif, and Pinker, 1994a), though not necessarily impaired in other aspects of intelligence, such as those measured on the nonverbal parts of IQ tests. Similarly, there is an inherited set of syndromes called Specific Language Impairment (Gopnik and Crago, 1993; Tallal, Ross, & Curtiss, 1989) which is marked by delayed onset of language, difficulties in articulation in childhood, and lasting difficulties in understanding, producing, and judging grammatical sentences. By definition, Specifically Language Impaired people show such deficits despite the absence of cognitive problems like retardation, sensory problems like hearing loss, or social problems like autism. 

 

More interestingly, there are syndromes showing the opposite dissociation, where intact language coexists with severe retardation. These cases show that language development does not depend on fully functioning general intelligence. One example comes from children with Spina Bifida, a malformation of the vertebrae that leaves the spinal cord unprotected, often resulting in hydrocephalus, an increase in pressure in the cerebrospinal fluid filling the ventricles (large cavities) of the brain, distending the brain from within. Hydrocephalic children occasionally end up significantly retarded but can carry on long, articulate, and fully grammatical conversations, in which they earnestly recount vivid events that are, in fact, products of their imaginations (Cromer, 1992; Curtiss, 1989; Pinker, 1994a). Another example is Williams Syndrome, an inherited condition involving physical abnormalities, significant retardation (the average IQ is about 50), incompetence at simple everyday tasks (tying shoelaces, finding one's way, adding two numbers, and retrieving items from a cupboard), social warmth and gregariousness, and fluent, articulate language abilities 

 2.3 Maturation of the Language System

 As the chapter by Newport and Gleitman suggests, the maturation of language circuits during a child's early years may be a driving force underlying the course of language acquisition (Pinker, 1994, Chapter 9; Bates, Thal, & Janowsky, 1992; Locke, 1992; Huttenlocher, 1990). Before birth, virtually all the neurons (nerve cells) are formed, and they migrate into their proper locations in the brain. But head size, brain weight, and thickness of the cerebral cortex (gray matter), where the synapses (junctions) subserving mental computation take place, continue to increase rapidly in the year after birth. Long-distance connections (white matter) are not complete until nine months, and they continue to grow their speed-inducing myelin insulation throughout childhood. Synapses continue to develop, peaking in number between nine months and two years (depending on the brain region), at which point the child has 50% more synapses than the adult. Metabolic activity in the brain reaches adult levels by nine to ten months, and soon exceeds it, peaking around the age of four. In addition, huge numbers of neurons die in utero, and the dying continues during the first two years before leveling off at age seven. Synapses wither from the age of two through the rest of childhood and into adolescence, when the brain's metabolic rate falls back to adult levels. Perhaps linguistic milestones like babbling, first words, and grammar require minimum levels of brain size, long-distance connections, or extra synapses, particularly in the language centers of the brain.

Similarly, one can conjecture that these changes are responsible for the decline in the ability to learn a language over the lifespan. The language learning circuitry of the brain is more plastic in childhood; children learn or recover language when the left hemisphere of the brain is damaged or even surgically removed (though not quite at normal levels), but comparable damage in an adult usually leads to permanent aphasia (Curtiss, 1989; Lenneberg, 1967). Most adults never master a foreign language, especially the phonology, giving rise to what we call a "foreign accent." Their development often fossilizes into permanent error patterns that no teaching or correction can undo. There are great individual differences, which depend on effort, attitudes, amount of exposure, quality of teaching, and plain talent. 

 

Many explanations have been advanced for children's superiority: they can exploit the special ways that their mothers talk them, they make errors unself-consciously, they are more motivated to communicate, they like to conform, they are not xenophobic or set in their ways, and they have no first language to interfere. But some of these accounts are unlikely, based on what we learn about how language acquisition works later in this chapter. For example, children can learn a language without the special indulgent speech from their mothers; they make few errors; and they get no feedback for the errors they do make. And it can't be an across-the-board decline in learning. There is no evidence, for example, that learning words (as opposed to phonology or grammar) declines in adulthood.

The chapter by Newport and Gleitman shows how sheer age seems to play an important role. Successful acquisition of language typically happens by 4 (as we shall see in the next section), is guaranteed for children up to the age of six, is steadily compromised from then until shortly after puberty, and is rare thereafter. Maturational changes in the brain, such as the decline in metabolic rate and number of neurons during the early school age years, and the bottoming out of the number of synapses and metabolic rate around puberty, are plausible causes. Thus, there may be a neurologically-determined "critical period" for successful language acquisition, analogous to the critical periods documented in visual development in mammals and in the acquisition of songs by some birds. 

 

3 The Course of Language Acquisition 

Although scholars have kept diaries of their children's speech for over a century (Charles Darwin was one of the first), it was only after portable tape-recorders became available in the late 1950's that children's spontaneous speech began to be analyzed systematically within developmental psychology. These naturalistic studies of children's spontaneous speech have become even more accessible now that they can be put into computer files and can be disseminated and analyzed automatically (MacWhinney & Snow, 1985, 1990; MacWhinney, 1991). They are complemented by experimental methods. In production tasks, children utter sentences to describe pictures or scenes, in response to questions, or to imitate target sentences. In comprehension tasks, they listen to sentences and then point to pictures or act out events with toys. In judgement tasks, they indicate whether or which sentences provided by an experimenter sound "silly" to them.

As the chapter by Werker shows, language acquisition begins very early in the human lifespan, and begins, logically enough, with the acquisition of a language's sound patterns. The main linguistic accomplishments during the first year of life are control of the speech musculature and sensitivity to the phonetic distinctions used in the parents' language. Interestingly, babies achieve these feats before they produce or understand words, so their learning cannot depend on correlating sound with meaning. That is, they cannot be listening for the difference in sound between a word they think means bit and a word they think means beet, because they have learned neither word. They must be sorting the sounds directly, somehow tuning their speech analysis module to deliver the phonemes used in their language (Kuhl, et al., 1992). The module can then serve as the front end of the system that learns words and grammar.

Shortly before their first birthday, babies begin to understand words, and around that birthday, they start to produce them (see Clark, 1993; Ingram, 1989). Words are usually produced in isolation; this one-word stage can last from two months to a year. Children's first words are similar all over the planet. About half the words are for objects: food (juice, cookie, body parts (eye, nose), clothing (diaper, sock), vehicles (car, boat), toys (doll, block), household items (bottle, light, animals (dog, kitty), and people (dada, baby). There are words for actions, motions, and routines, like (up, off, open, peekaboo, eat, and go, and modifiers, like hot, allgone, more, dirty, and cold. Finally, there are routines used in social interaction, like yes, no, want, bye-bye, and hi -- a few of which, like look at that and what is that, are words in the sense of memorized chunks, though they are not single words for the adult. Children differ in how much they name objects or engage in social interaction using memorized routines, though all children do both.

Around 18 months, language changes in two ways. Vocabulary growth increases; the child begins to learn words at a rate of one every two waking hours, and will keep learning that rate or faster through adolescence (Clark, 1993; Pinker, 1994). And primitive syntax begins, with two-word strings like the following: 

All dry. All messy. All wet. I sit. I shut. No bed.

No pee. See baby. See pretty.