Many animals communicate by means of sound, and some (humans and songbirds are examples) learn these vocalizations. There are, in fact, provocative similarities in the development of human language and birdsong (Box B). Most animal vocalizations, like alarm calls in mammals and birds, are innate, and require no experience to be correctly produced. For example, quails raised in isolation or deafened at birth so that they never hear conspecifics nonetheless produce the full repertoire of species-specific vocalizations. In contrast, humans obviously require extensive postnatal experience to produce and decode speech sounds that are the basis of language.
Importantly, this linguistic experience, to be effective, must occur in early life. The requirement for hearing and practicing during a critical period is apparent in studies of language acquisition in congenitally deaf children. Whereas most babies begin producing speechlike sounds at about 7 months (babbling), congenitally deaf infants show obvious deficits in their early vocalizations, and such individuals fail to develop language if not provided with an alternative form of symbolic expression (such as sign language; see Chapter 27). If, however, these deaf children are exposed to sign language at an early age (from approximately six months onward), they begin to “babble” with their hands just as a hearing infant babbles audibly. This suggests that, regardless of the modality, early experience shapes language behavior (Figure 24.1). Children who have acquired speech but subsequently lose their hearing before puberty also suffer a substantial decline in spoken language, presumably because they are unable to hear themselves talk and thus lose the opportunity to refine their speech by auditory feedback.
Figure 24.1
Manual “babbling” in deaf infants raised by deaf, signing parents compared to manual babble in hearing infants. Babbling was judged by scoring hand positions and shapes that showed some resemblance to the components of American Sign Language. (more...)
Examples of pathological situations in which normal children were never exposed to a significant amount of language make much the same point. In one well-documented case, a girl was raised by deranged parents until the age of 13 under conditions of almost total language deprivation. Despite intense subsequent training, she never learned more than a rudimentary level of communication. This and other examples of so-called “feral children” starkly define the importance of early experience. In contrast to the devastating effects of deprivation on children, adults retain their ability to speak and comprehend language even if decades pass without exposure or speaking. In short, the normal acquisition of human speech is subject to a critical period: The process is sensitive to experience or deprivation during a restricted period of life (before puberty) and is refractory to similar experience or deprivations in adulthood.
On a more subtle level, the phonetic structure of the language an individual hears during early life shapes both the perception and production of speech. Many of the thousands of human languages and dialects use appreciably different repertoires of speech elements called phonemes to produce spoken words (examples are the phonemes “ba” and “pa” in English). Very young human infants can perceive and discriminate between differences in all human speech sounds, and are not innately biased towards the phonemes characteristic of any particular language. However, this universal appreciation does not persist. For example, adult Japanese speakers cannot reliably distinguish between the /r/ and /l/ sounds in English, presumably because this phonemic distinction is not present in Japanese. Nonetheless, 4-month-old Japanese infants can make this discrimination as reliably as 4-month-olds raised in English-speaking households (as indicated by increased suckling frequency or head turning in the presence of a novel stimulus). By 6 months of age, however, infants show preferences for phonemes in their native language over those in foreign languages, and by the end of their first year no longer respond to phonetic elements peculiar to non-native languages. The ability to perceive these phonemic contrasts evidently persists for several more years, as evidenced by the fact that children can learn to speak a second language without accent and with fluent grammar until about age 7 or 8. After this age, however, performance gradually declines no matter what the extent of practice or exposure (Figure 24.2).
Figure 24.2
A critical period for learning language is shown by the decline in language ability (fluency) of non-native speakers of English as a function of their age upon arrival in the United States. The ability to score well on tests of English grammar and vocabulary (more...)
A number of changes in the developing brain could explain these observations. One possibility is that experience acts selectively to preserve the circuits in the brain that perceive phonemes and phonetic distinctions. The absence of exposure to non-native phonemes would then result in a gradual atrophy of the connections representing those sounds, accompanied by a declining ability to distinguish between them. In this formulation, circuits that are used are retained, whereas those that are unused get weaker (and eventually disappear). Alternatively, experience could promote the growth of rudimentary circuitry pertinent to the experienced sounds.
The reality, however, is considerably more complex than either of these scenarios suggest. Experiments by Patricia Kuhl and her colleagues have demonstrated that as a second language is acquired, the brain gradually groups sounds according to their similarity with phonemes in the native language. For example, when asked to categorize a continuous spectrum of artificial phonemes between /r/ and /l/, native English speakers, but not Japanese speakers, tend to perceive sounds as all sounding like either /r/ or /l/, a phenomenon that Kuhl has likened to a “perceptual magnet.” Related but varying sounds (defined by their audiographic spectrum) are evidently grouped together and eventually perceived as representing the same phoneme. Without ongoing experience during the critical period, this process fails to occur. Interestingly, the “baby-talk” or “parentese” used by adults speaking to young children actually emphasizes these phonetic distinctions compared to normal speech among adults. Thus, learning language during the critical period for its development entails an amplification and reshaping of innate biases by appropriate postnatal experience.