Greg Detre
Wednesday, 11 October, 2000
Almost every child learns to speak almost effortlessly given sufficient opportunity, yet adults find it extremely difficult to learn a second language. Similarly, those who have to learn language in later life through brain injury of lack of human contact never attain the accentless fluency that every child manages, sometimes in more than one language. This led Lenneberg (1967) to postulate a critical window of language-learning, ending before the onset of puberty, after which our ability to acquire language has markedly diminished. This is a problem of speech perception as much as speech production.
Eimas (1971) observed signs of decline in our ability to recognise perceptual categories with his voice onset time experiments on infants. The phonemes /ba/ and /pa/ are differentiated by the length of time between movement of the vocal folds and air pressure being produced against them. This lag time is known as the voice onset time (VOT). For example, /ba/ has a VOT of 0-20ms (some people even pre-voice), while /pa/ has a VOT of 40-120ms. Speech perception is partly the business of categorising continua, like VOT, in order to distinguish different phonemes and so recognise words.
As would be expected, native adult speakers of English are very good at distinguishing between /ba/ and /pa/, since they are two functionally separate phonemes in English, i.e. being able to distinguish between their acoustic properties is critical in determining the meaning of a syllable. Minimal pairs are examples of syllables whose meaning hinges on a single, slight phonemic contrast, e.g. �bat� and �pat�. On the other hand, if two sounds are not categorised as functionally separate phonemes in a language, then it seems reasonable to predict that speakers of that language will find it more difficult to distinguish those sounds, because they will be unused to separating the contexts in which they hear them, or may rarely hear them at all. After all, although individual speakers of a language may indeed pronounce every phoneme slightly differently, those differences do not affect the meaning of their speech in any way. Such phonemic variations which are categorised together and do not affect meaning are known as �allophones�. It is now apparent why developmental changes in our ability to detect acoustic differences play such a crucial role in learning to speak a language flawlessly. If we cannot detect these subtle differences, we will never attain the proficiency of a native speaker. When an individual speaker�s allophones are different enough from the norm to be noticeable by another, they have an accent.
Given that adult speakers� ability to detect phonemic contrasts is limited by the phonemic inventory of their native tongue, it was interesting to find that infants are born able to discriminate between the acoustic nuances employed in any of the world�s human languages, not just in the language they have been so far exposed to. This prompted the hypothesis that there might be some critical window of experience necessary to maintain our ability to perceive phonemic contrasts. If brought up in a wholly English-speaking background, a child�s ability to distinguish between, say, a dental /t/ and a retroflex /T/ in Hindi would be lost by a certain age. Eimas� original experiment observed infants of between 1 and 4 months, whereas later experiments with children of 12 and 8 showed that this critical window of universal acoustic perceptual ability is even narrower than for language learning in general.
The Maintenance-Loss model was developed, which assumed that we were all born with universal category perception which decayed over time due to lack of exposure to non-native phonemes. Two empirical observations proved problematic for this model.
Firstly, 4-year old children perform worse than adults, and thus significantly worse than infants, in recognising non-native phonemic contrasts. It makes little sense for our ability to recognise phonemic contrasts to improve again after the age of four, given that we are not exposed to such non-native sounds any more frequently, the explanation offered by the Maintenance-Loss model for our decline in perceptual sensitivity.
Secondly, though infants are certainly more flexible than adults in terms of the range phonemes they can distinguish, they do not in fact possess �universal� acoustic perception. Infants may be able to distinguish phonemic contrasts employed by any of the world�s human languages, but they cannot detect equal acoustic variations which don�t cross the artificial boundaries of the perceptual categories we employ in our languages. We can conclude then, that an infant�s ability to perceive speech sounds beyond those of any given adult native speaker cannot simply be ascribed to a universal acoustic sensitivity. Rather, the comparatively broad range of infants� speech perception is still restricted to the domain of speech sounds occurring as part of human language. This points to an interesting convergence between the speech sounds our bodies have evolved to produce, and the phonemes employed in any human language you care to mention. Within this universal set of phonemes that humans can produce and perceive, different languages delineate their own repertoire of phonemes from which legitimate syllables can be formed. Phonotactics is the study of the systematic rules governing which phonemes can be legitimately combined to form syllables in a given language. English contains around 40 phonemes, compared to 11 in Polynesian and 141 in Khoisan (�Bushman�).
The Functional Reorganisation Hypothesis explained our developmental changes in perceptual sensitivity relative to the language-learning functions we require at such times in our lives. For instance, infants� ease in learning to perceive �non-native� sounds is necessary because being a native in a language is determined by environment rather than being born of native speakers. As infants, we may be born to parents speaking any of the world�s languages, and so we need to be flexible enough in our speech perception to pick up the particular phonemic contrasts relevant to us. Losing this flexibility over the next few early years facilitates the infant�s attempts to separate out words from the jumbled stream of a foreign language into the tokenised units of meaning that our �phonetic sixth sense� (Pinker) feeds us. By the time we reach the age of 4, when we are at our least sensitive to acoustic distinctions, arguably we are in the midst of our �fast-mapping� (Carey) vocabulary spurt, and are best served by focusing on native phonemes. It would be interesting to observe whether infants� general language learning is negatively affected at this age, even though it falls well within the �critical window� of pre-puberty. Adults, on the other hand, remain easily able to discriminate the phonemes of their native language but seem to lose the ability to discriminate phoneme contrasts employed by languages besides their own. However, with concentration, a little practice and an understanding of what they are listening for, adults are able to make headway towards learning non-native phonemic distinctions, as we need to do when learning a second language.
Thus, the Functional Reorganisation Hypothesis neatly and plausibly explains the observed developmental changes in acoustic sensitivity by reference to the requirements of the stage of language learning we are at.