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 nonnative 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.

In this article, we present a summary of recent research linking speech perception in infancy to later language development, as well as a new empirical study examining that linkage. Infant phonetic discrimination is initially language universal, but a decline in phonetic discrimination occurs for nonnative phonemes by the end of the 1st year. Exploiting this transition in phonetic perception between 6 and 12 months of age, we tested the hypothesis that the decline in nonnative phonetic discrimination is associated with native-language phonetic learning. Using a standard behavioral measure of speech discrimination in infants at 7 months and measures of their language abilities at 14, 18, 24, and 30 months, we show (a) a negative correlation between infants' early native and nonnative phonetic discrimination skills and (b) that native-and nonnative-phonetic discrimination skills at 7 months differentially predict future language ability. Better native-language discrimination at 7 months predicts accelerated later language abilities, whereas better nonnative-language discrimination at 7 months predicts reduced later language abilities. The discussion focuses on (a) the theoretical connection between speech perception and language development and (b) the implications of these findings for the putative "critical period" for phonetic learning.Work in my laboratory has recently been focused on two fundamental questions and their theoretical intersect. The first is the role that infant speech perception plays in the acquisition of language. The second is whether early speech perception can reveal the mechanism underlying the putative "critical period" in language acquisition.A theoretical position that links the two has been offered for debate and discussion (Kuhl, 2002(Kuhl, , 2004. The proposed theory, Native Language Magnet, argues that early phonetic learning alters perception and changes future learning abilities. The underlying mechanism is "neural commitment" to the acoustic and statistical properties of native language phonetic units, a process that has bidirectional effects. Native language neural commitment (NLNC) enhances native-language learning while not supporting alternate phonetic patterns. NLNC may provide a clue to the mechanisms underlying a "critical period" at the phonetic level for language.The goal of this article is twofold: (a) to discuss these issues and the NLNC hypothesis in the context of broader work in neurobiology on animal learning in vision and learning of species-typical communication systems and (b) to present new empirical data that support the NLNC hypothesis for human speech learning. LINKING SPEECH TO LANGUAGEResearchers focused on speech perception and language acquisition have traditionally worked in parallel, aware of their counterparts'data and theorizing, but not linking the two. New data have begun to explain how infants'early phonetic discrimination skills could affect the young child's ability to acquire words, morphology, and syntax. The data suggest that infants'abilities...

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Studies using the English and Spanish MacArthur Communicative Development Inventories demonstrated that the grammatical abilities of 20-30-month-old bilingual children were related more strongly to same-language vocabulary development than to broader lexical-conceptual development or maturation. First, proportions of different word types in each language varied with same-language vocabulary size. Second, individual changes in predicate and closed class word proportion scores were linked to growth in same-language vocabulary but not to total conceptual vocabulary. Third, increases in English utterance length and English and Spanish sentence complexity were related to growth in same-language vocabulary but not to growth in conceptual vocabulary. Increases in Spanish utterance length were linked to growth in both Spanish vocabulary and conceptual vocabulary. Possible mechanisms underlying these patterns are considered.