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In Chapter 11 we learned that babies have set up the phoneme categories of their L1 by the time they’re only twelve months old. So by the time you’re an adult learning a new language, those phoneme categories are pretty deeply embedded in you mental grammar. Psycholinguist Catherine Best has proposed a theory, called the Perceptual Assimilation Model (Best, McRoberts & Goodell, 2001), that predicts which phoneme contrasts will be hard for adult language learners, and which will be easy. The theory centres around the concepts of phonemes and allophones.
Best predicts positive transfer for phoneme contrasts happens if the new language has a phoneme contrast that maps onto a contrast in the adult learner’s L1. In this case, the new phoneme contrast should be easy to learn. She also predicts that it should be easy to learn a new contrast in the case where there’s no transfer at all: if the new language has contrastive phones that don’t exist at all in the learner’s L1, then the learner can set up two new phoneme categories from scratch.
The situation that leads to negative transfer arises when two contrasting phonemes in the later language map onto a single phoneme category in the learner’s L1. In this case, the learner will have spent a lifetime treating the phonetic difference as allophonic variation, and not a meaningful contrast, so it’s a challenge to learn to pay attention to the difference as meaningful.
Best and her colleagues (Best, McRoberts & Sithole, 1988) tested this theory by investigating how English-speaking adults learn phonemic contrasts in Zulu. Zulu is a language that has about 27 million speakers, most of them in South Africa. First, researchers asked the English-speakers to notice the difference between voiced and voiceless lateral fricatives [ɬ and ɮ] in Zulu. English doesn’t have lateral fricatives, but English does have lots of pairs of fricatives that contrast in their voicing, so the theory predicts that it should be easy for English listeners to map the voicing difference between the Zulu fricatives onto those English voicing contrasts and recognize this phonetic difference. And that prediction was upheld: The English listeners were about 95% correct.
Then they asked the English speakers to tell the difference between three Zulu clicks: a dental [ǀ], an alveolar [ǃ], and a palato-alveolar [ǂ] click. English doesn’t have any clicks at all, so the English listeners should be able to simply pay attention to the phonetic differences between these segments, without any interference from their English phonology. The English listeners were about 80% correct at these sounds.
Last, they asked the English listeners to notice the difference between two different kinds of bilabial stops in Zulu: the plosive stop is similar to the English [b] sound. The other is an implosive [ɓ], which is made by obstructing airflow at the lips, but when the stop is released, air flows into the mouth instead of out of the mouth. The English adults were not much better than chance at hearing this difference.
These results are consistent the predictions of the Perceptual Assimilation Model: Because the English listeners have only one phoneme category for voiced bilabial stops, their mental grammar simply treats the implosive as an allophone of the plosive. It’s difficult for the adult learners to hear the phonetic difference between the two sounds in their new language because the mental grammar of their L1 categorizes both segments as members of the same phoneme.
References
Best, C. T., McRoberts, G. W., & Goodell, E. (2001). Discrimination of non-native consonant contrasts varying in perceptual assimilation to the listener’s native phonological system. The Journal of the Acoustical Society of America, 109(2), 775–794.
Best, C. T., McRoberts, G. W., & Sithole, N. M. (1988). Examination of perceptual reorganization for nonnative speech contrasts: Zulu click discrimination by English-speaking adults and infants. Journal of Experimental Psychology: Human Perception and Performance, 14(3), 345–360.
Best, C. T., & Tyler, M. D. (2007). Nonnative and second-language speech perception: Commonalities and complementarities. In M. J. Munro & O.-S. Bohn (Eds.), Second language speech learning: The role of language experience in speech perception and production (pp. 13–34). John Benjamins.
