There is considerable debate about whether the early processing of sounds depends on whether they form part of speech. Proponents of such speech specificity postulate the existence of language-dependent memory traces, which are activated in the processing of speech but not when equally complex, acoustic non-speech stimuli are processed. Here we report the existence of these traces in the human brain. We presented to Finnish subjects the Finnish phoneme prototype /e/ as the frequent stimulus, and other Finnish phoneme prototypes or a non-prototype (the Estonian prototype /õ/) as the infrequent stimulus. We found that the brain's automatic change-detection response, reflected electrically as the mismatch negativity (MMN), was enhanced when the infrequent, deviant stimulus was a prototype (the Finnish /ö/) relative to when it was a non-prototype (the Estonian /õ/). These phonemic traces, revealed by MMN, are language-specific, as /õ/ caused enhancement of MMN in Estonians. Whole-head magnetic recordings located the source of this native-language, phoneme-related response enhancement, and thus the language-specific memory traces, in the auditory cortex of the left hemisphere.

Studies using behavioral methods, such as head-turning experiments, in which children are conditioned to turn their heads toward the sound source when they detect a change in the sound, have shown that environment has an important effect on how infants perceive language 1-4 . Young infants are able to discriminate almost all phonetic contrasts, whereas older infants discriminate better between phonemes that occur in the language that they normally hear, rather than foreign-language phonemes. Here we demonstrate the development of language-specific 'memory traces' in the brains of the same group of infants between six months and one year of age.The Estonian and Finnish languages, which are closely related to each other, have very similar vowel structures 5 . For example, the vowels /e/ and /ö/, which differ only in the second-formant (F2) frequency, exist in both languages. However, only Estonian has the vowel /õ/, which is approximately between /ö/ and /o/ (Fig. 1). We used these phonemes to investigate the development of language-specific memory traces in infants. This was done by analyzing mismatch negativity (MMN), an electric brain response automatically elicited by infrequent changes (deviant stimulus) in a repetitive (standard) stimulus 6-12 . The MMN amplitude increases with an increasing acoustic difference between deviant and standard stimuli 9 . Thus, when the phoneme /e/ is the standard, the MMN amplitude should be larger when the deviant stimulus

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Discriminative responses to tones, harmonics, and syllables in the left hemisphere were measured with magnetoencephalography in neonates, 6-month-old infants, and 12-month-old infants using the oddball paradigm. Real-time head position tracking, signal space separation, and head position standardization were applied to secure quality data for source localization. Minimum current estimates were calculated to characterize infants' cortical activities for detecting sound changes. The activation patterns observed in the superior temporal and inferior frontal regions provide initial evidence for the developmental emergence early in life of a perceptual-motor link for speech perception that may depend on experience.

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For decades, behavioral methods, such as the head-turning or sucking paradigms, have been the primary tools to investigate speech perception and learning of a language in infancy. Recently, however, new methods provided by event-related potentials have emerged. These are called mismatch negativity (MMN) and late discriminative negativity (LDN). MMN, the brain’s automatic change-detection response in audition, has been intensively used in adults in both basic and clinical studies for longer than 20 years. LDN, on the other hand, was only recently discovered. There seem to be many differences between these two responses. MMN is developmentally quite stable and can be obtained even from preterm infants. LDN, however, can be obtained most reliably from young children, and its amplitude decreases as a function of age. New data suggest that both of these responses have a special role in language processing, although both of them can also be elicited by nonspeech stimuli.

It is not yet clear whether humans are able to learn while they are sleeping. Here we show that full-term human newborns can be taught to discriminate between similar vowel sounds when they are fast asleep. It is possible that such sleep training soon after birth could find application in clinical or educational situations.

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