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There’s another part of the mental grammar that hearing babies start to learn well before they can speak. Remember from a previous chapter that the phonology of each language is specific to that language: the patterns of which features and segments contrast with each other and which are simply allophones is different in each language of the world. So, for example, we know from that in English, aspirated [pʰ] and unaspirated [p] are both allophones of a single phoneme. But in Thai, these two segments contrast with each other and are two different phonemes. The phonetic difference is the same, but how that difference is organized in the mental grammar is different in the two languages. Phoneme contrasts are a classic example of unconscious linguistic knowledge: in all likelihood, nobody ever had to teach you that [k] and [b] are different sounds and that the words cat and bat refer to two different animals! But if no one ever taught it to you, how did you learn it? How did the phonology of your first language come to be in your mental grammar? We saw one kind of habituation technique in the previous unit. Researchers use a different habituation method for slightly older babies.
This method works on the same kind of logic as the high-amplitude sucking method, but instead of measuring sucking strength, the researchers observe where the child looks. When the sound first starts to play from a speaker, the baby usually looks towards the speaker. Once they habituate to that sound, they get bored and look away. If the next sound that’s played is the same as the first one, they stay habituated. But if the next sound is different and they notice the difference, they look back towards the speaker. So just like with the sucking method, if we observe that head-turn, we conclude that they noticed a difference between the two sounds.
Using this technique, linguists and psychologists have learned that babies are very good at noticing phonetic differences, and they can tell the difference between all kinds of different sounds from many different languages. But this ability changes within the first year of life. Janet Werker, at the University of British Columbia, looked at children and adults’ ability to notice the phonetic difference between three different pairs of syllables: the English contrast /ba/ and /da/, the Hindi contrast between a retroflex stop /ʈa/ and a dental stop /t̪a/, and a Nłeʔkepmxcín contrast between glottalized velar /kʼi/ and uvular /qʼi/ stops (Werker & Tees, 1984). Each of these pairs differs in place of articulation, and within each language, each pair is contrastive. The researchers played a series of syllables and asked English-speaking adults to press a button when the syllables switched from one segment to the other. As you might expect, the English-speaking adults were perfect at the English contrast but did extremely poorly on the Hindi and Nłeʔkepmxcín contrasts.
Then Werker tested babies’ ability to notice these three phonetic differences, using the head-turn method. These babies were growing up in monolingual English-speaking homes. At age six months, the English-learning babies were about 80-90% successful at noticing the differences in English, in Hindi and in Nłeʔkepmxcín. But by age ten months, their success rate had dropped to about 50-60%, and by the time they were one year old, they were only about 10-20% successful at hearing the phonetic differences in Hindi and Nłeʔkepmxcín. So these kids are only one year old, they’ve been hearing English spoken for only one year, and they’re not even really speaking it themselves yet, but already their performance on this task is matching that of English-speaking adults. The difference between retroflex [ʈa] and dental [t̪a] is not contrastive in English, so the mental grammar of the English-learning baby has already categorized both those sounds as just unusual-sounding allophones of English alveolar /ta/. Likewise, the difference between a velar and a uvular stop, which is contrastive in Nłeʔkepmxcín, is not meaningful in English, so the baby’s mind has already learned to treat a uvular stop as an allophone of the velar stop, not as a separate phoneme.
The research on babies acquiring signed language came later, but it showed exactly the same pattern (Baker Palmer et al., 2012). Babies younger than six months noticed the difference between phonemically contrastive ASL handshapes even without any exposure to ASL, just like the babies without Hindi experience noticed the Hindi spoken contrast. By age 14 months, ASL-acquiring babies had retained their ability to recognize phonemically contrastive handshapes, but the English-learning babies without ASL experience had lost that sensitivity.
So for either sign language or vocal language, a child’s mind has built up phoneme categories in their mental grammar by about age one, according to the contrasts that they have experience with in their language environment. One thing to note here is that this doesn’t mean that it’s impossible to learn new phonemic contrasts in a new language, but that later learning will be shaped by the learning that has happened in the first year. More on that in the next chapter!
References
Baker Palmer, S., Fais, L., Golinkoff, R. M., & Werker, J. F. (2012). Perceptual Narrowing of Linguistic Sign Occurs in the 1st Year of Life: Perceptual Narrowing of Linguistic Sign. Child Development, 543–553.
Werker, J. F., & Tees, R. C. (1984). Cross-language speech perception: Evidence for perceptual reorganization during the first year of life. Infant Behavior and Development, 7(1), 49–63.
