Copyright © 1999 Massachusetts Institute of Technology
The acquisition of PHONOLOGY -- or rather its development -- may be divided into two fields: SPEECH PERCEPTION and speech production. There are two reasons why the development of perception is prior to the development of production. One is that although the human ear is almost completely formed when the fetus is 7 months, the oral cavity of a human at birth is very different from the adult's oral cavity. The second reason is that in order to produce the sounds of a given language, a child must be exposed to the relevant linguistic experience. Babbling infants, in fact, produce all sorts of linguistic sounds, even ones they have never heard in their linguistic environment. Although the first reason why perception is prior to production holds exclusively for the acquisition of the mother tongue (L1), the second holds both for the acquisition of L1 and of whichever language comes after L1 (L2).
A child comes into life well equipped to hear subtle differences in sounds (Eisenberg 1976). Though it is conceivable that some speech perception development starts before birth, because newborns discriminate the mother language from a foreign one (Mehler et al. 1988), it is generally assumed that the development of language perception starts at birth.
The most widely accepted theory of speech perception is the innatist theory, first proposed by JAKOBSON and much influenced by the work of Chomsky and other generative linguists. A mechanism, called the LANGUAGE ACQUISITION device (LAD), is assumed to be responsible for the ability humans have to analyze linguistic inputs and to construct grammars that generate them (see GENERATIVE GRAMMAR). Given that some properties of language are common to all languages of the world, it may be assumed that these are the consequences of the innate human endowment. The development of a specific language is achieved through the setting of the values of a set of parameters on the basis of the linguistic data one is exposed to.
According to the innatist hypothesis, a newborn can discriminate between pairs of segments (consonant and vowels) attested in at least one language of the world, even if they are not distinctive in the language they are exposed to (Jakobson 1941). Testing what infants hear has become possible only in the last twenty years. The methodology to test young infants' perception is the nonnutritive (or high amplitude) sucking method (Eimas et al. 1971). When infants are 6 months or older, the preferential headturn procedure is commonly used to test sound discrimination (Moore, Wilson, and Thomson 1977). It has been shown that, indeed, for the newborn, the ability to discriminate does not appear to be related to the language he or she is exposed to (Streeter 1976). At a later stage of development, around 10 months, infants start losing the ability to discriminate sounds that are not distinctive in the language(s) they are exposed to (Werker and Tees 1984; Kuhl et al. 1992). This is in line with the learning by selection theory of neurological development (Changeux, Heidmann, and Patte 1984). It has also been shown that already from 1 month, infants represent linguistic sounds categorically, that is, different acoustic variants of a sound are identified with one category (Eimas, Miller, and Jusczyk 1987; Kuhl 1993).
Perceiving phonological distinctions is not only relevant to the acquisition of a phonological system but is also essential to the development of both LEXICON and SYNTAX. Newborns are in a position similar to that of adults when they hear a language unrelated to any other language of which they have some experience. One of the problems in constructing a lexicon is segmenting a continuous stream of sounds. In order to build a lexicon, a child must come to understand where each word ends and the next one begins. Because newborns are sensitive to edges of phonological or prosodic constituents (Christophe et al. 1994) and to LINGUISTIC STRESS (Sansavini et al. 1995), it is conceivable that they use these prosodic cues to segment the continuous input (cf. PROSODY AND INTONATION).
According to the theory of language initialization known as prosodic bootstrapping, the prosody of a language also provides a cue to its syntactic structure (Gerken, Jusczyk, and Mandel 1994). Given its sensitivity to prosody, an infant should thus be capable of setting certain syntactic parameters long before the period in which he or she shows some knowledge of the lexicon (Mazuka 1996; Nespor, Guasti, and Christophe 1996). The early setting of parameters responsible for word order accounts for the fact that the monolingual child hardly makes any mistakes in the relative order of words when he or she starts combining them into small sentences.
Speech production starts with babbling, the first approximation to language. The vocal apparatus approaches an adult state at about 6 months of age, so it is only from this period that it is safe to talk about babbling. Though the segmental categories in babbling do not resemble those of the adult language (de Boysson-Bardies 1993), even at this first stage, speech production is influenced by speech perception: babbling does not develop normally without an auditory input (Oller and Eilers 1988). In auditorily unimpaired infants, suprasegmentals (i.e., rhythm and intonation) are acquired before segments are, as early as at 6 months (Whalen, Levitt, and Wang 1991). Around the first year of age, both the vowel quality and the syllabic structure of auditorily unimpaired babbling infants is much influenced by that of the adult language (de Boysson-Bardies et al. 1989; Vihman 1992). The first syllable type produced throughout the languages of the world is one formed by a consonant (C) followed by a vowel (V). This is also the only syllable type universally present. The segmental content of the first syllables is such that C and V are as far apart as possible in sonority -- that is, C is pronounced with high air pressure in the oral cavity compared to the external air pressure, as in [p], whereas in the pronunciation of V, the internal pressure is similar to the external one, as in [a]. Subsequently different CV combinations develop and different parameters are set to give the full range of adult syllables such as whether a prevocalic consonant is obligatory or not or whether a postvocalic consonant is allowed or not. PHONOLOGICAL RULES AND PROCESSES also develop with time. For example, the centralization to schwa of unstressed vowels in English is not part of early productions.
The acquisition of the phonology (and of grammar in general) of L1 appears to be impaired after the fifth year of life. This claim is based on the experience of humans who have not been in contact with a speaking community and have been found at age 5 or more. The acquisition of the phonology of L2 appears to be impaired after puberty, as witnessed by the foreign-accent phenomenon, responsible for the fact that we can distinguish a native speaker from a nonnative one. Interestingly, the acquisition of syntax (i.e., the computational system) is not so impaired.
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Copyright © 1999 Massachusetts Institute of Technology