Phonetic learning as a pathway to language: new data and native language magnet theory expanded (NLM-e)

Abstract

Infants' speech perception skills show a dual change towards the end of the first year of life. Not only does non-native speech perception decline, as often shown, but native language speech perception skills show improvement, reflecting a facilitative effect of experience with native language. The mechanism underlying change at this point in development, and the relationship between the change in native and non-native speech perception, is of theoretical interest. As shown in new data presented here, at the cusp of this developmental change, infants' native and non-native phonetic perception skills predict later language ability, but in opposite directions. Better native language skill at 7.5 months of age predicts faster language advancement, whereas better non-native language skill predicts slower advancement. We suggest that native language phonetic performance is indicative of neural commitment to the native language, while non-native phonetic performance reveals uncommitted neural circuitry. This paper has three goals: (i) to review existing models of phonetic perception development, (ii) to present new event-related potential data showing that native and non-native phonetic perception at 7.5 months of age predicts language growth over the next 2 years, and (iii) to describe a revised version of our previous model, the native language magnet model, expanded (NLM-e). NLM-e incorporates five new principles. Specific testable predictions for future research programmes are described.

Figure 1

Universal timeline of infants' perception and production of speech in the first year of life. Modified from Kuhl (2004).

Figure 2

The effects of age on speech perception performance in a cross-language study of the perception of American English /r–l/ sounds by American and Japanese infants. From Kuhl et al. (2006).

Figure 3

Scatter plots show significant correlations between infants' phonetic discrimination measures at 7.5 months for (a) native as opposed to (b) non-native phonetic contrasts and their later language abilities. Better performance on speech discrimination, as measured by the MMN, is indicated by a more negative value, producing a negative correlation between native speech discrimination and later language and a positive correlation between non-native speech discrimination and later language (closed square, Mandarin; open square, Spanish).

Figure 4

A median split of infants whose MMNs indicate better versus poorer discrimination of (a) native and (b) non-native phonetic contrasts is shown along with their corresponding longitudinal growth curve functions for the number of words produced between 14 and 30 months of age.

Figure 5

NLM-e is shown in four phases (see text for description). The representations of native language input for vowels and consonants are drawn roughly to reflect existing data for Swedish (Fant 1973; Lacerda in preparation), English (Dalston 1975; Flege et al. 1995; Hillenbrand et al. 1995) and Japanese (Iverson et al. 2003; Lotto et al. 2004).