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Linguistic experience alters an individual's perception of speech. We here provide evidence of the effects of language experience at the neural level from two magnetoencephalography (MEG) studies that compare adult American and Japanese listeners' phonetic processing. The experimental stimuli were American English /ra/ and /la/ syllables, phonemic in English but not in Japanese. In Experiment 1, the control stimuli were /ba/ and /wa/ syllables, phonemic in both languages; in Experiment 2, they were non-speech replicas of /ra/ and /la/. The behavioral and neuromagnetic results showed that Japanese listeners were less sensitive to the phonemic /r-l/ difference than American listeners. Furthermore, processing non-native speech sounds recruited significantly greater brain resources in both hemispheres and required a significantly longer period of brain activation in two regions, the superior temporal area and the inferior parietal area. The control stimuli showed no significant differences except that the duration effect in the superior temporal cortex also applied to the non-speech replicas. We argue that early exposure to a particular language produces a "neural commitment" to the acoustic properties of that language and that this neural commitment interferes with foreign language processing, making it less efficient.