Manuela Friedrich

N400-like Semantic Incongruity Effect in 19-Month-Olds: Processing Known Words in Picture Contexts

To understand mechanisms of early language acquisition, it is important to know whether the childs brain acts in an adult-like manner when processing words in meaningful contexts. The N400, a negative component in the event-related potential (ERP) of adults, is a sensitive index of semantic processing reflecting neural mechanisms of semantic integration into context. In the present study, we investigated whether the mechanisms indexed by the N400 are already working during early language acquisition. While 19-month-olds were looking at sequentially presented pictures, they were acoustically presented with words that were either congruous or incongruous to the picture content. The ERP averaged across the group of 55 children revealed an N400-like semantic incongruity effect in addition to an early phonologicallexical priming effect. The results suggest that both lexical expectations facilitating early phonological processing and mechanisms of semantic priming facilitating integration into semantic context are already present in 19-month-olds. The childs specific comprehension abilities are reflected in strength, latency, and hemispheric differences of the semantic incongruity effect. Spatio-temporal differences in that effect, thus, indicate changes in the organization of brain activity correlated with the childs behavioral development.

Neural manifestation of cognitive and precognitive mismatch detection in early infancy

We recorded event-related potentials (ERPs) in 2-month-old infants in two different states of alertness: awake and asleep. Syllables varying in vowel duration (long vs short) were presented in an oddball paradigm, known to elicit a mismatch brain response. ERPs of both groups showed a mismatch response reflected in a positivity followed by a frontal negativity. While the positivity was present as a function of the stimulus type (present for long deviants only), the negativity varied as a function of the state of alertness (present for awake infants only). These data indicate a functional separation between precognitive and cognitive aspects of duration mismatch essential for the distinction between long and short vowels during early infancy.

Brain Responses in 4-Month-Old Infants Are Already Language Specific

Language is the most important faculty that distinguishes humans from other animals. Infants learn their native language fast and effortlessly during the first years of life, as a function of the linguistic input in their environment. Behavioral studies reported the discrimination of melodic contours [1] and stress patterns [2, 3] in 1-4-month-olds. Behavioral [4, 5] and brain measures [6-8] have shown language-independent discrimination of phonetic contrasts at that age. Language-specific discrimination, however, has been reported for phonetic contrasts only for 6-12-month-olds [9-12]. Here we demonstrate language-specific discrimination of stress patterns in 4-month-old German and French infants by using electrophysiological brain measures. We compare the processing of disyllabic words differing in their rhythmic structure, mimicking German words being stressed on the first syllable, e.g., pápa/daddy[13], and French ones being stressed on the second syllable, e.g., papá/daddy. Event-related brain potentials reveal that experience with German and French differentially affects the brain responses of 4-month-old infants, with each language group displaying a processing advantage for the rhythmic structure typical in its native language. These data indicate language-specific neural representations of word forms in the infant brain as early as 4 months of age.

Phonotactic Knowledge and Lexical–Semantic Processing in One-year-olds: Brain Responses to Words and Nonsense Words in Picture Contexts

During their first year of life, infants not only acquire probabilistic knowledge about the phonetic, prosodic, and phonotactic organization of their native language, but also begin to establish first lexical-semantic representations. The present study investigated the sensitivity to phonotactic regularities and its impact on semantic processing in 1-year-olds. We applied the method of event-related brain potentials to 12-and 19-month-old children and to an adult control group. While looking at pictures of known objects, subjects listened to spoken nonsense words that were phonotactically legal (pseudowords) or had phonotactically illegal word onsets (nonwords), or to real words that were either congruous or incongruous to the picture contents. In 19-month-olds and in adults, incongruous words and pseudowords, but not non-words, elicited an N400 known to reflect mechanisms of semantic integration. For congruous words, the N400 was attenuated by semantic priming. In contrast, 12-month-olds did not show an N400 difference, neither between pseudo-and nonwords nor between incongruous and congruous words. Both 1-year-old groups and adults additionally displayed a lexical priming effect for congruous words, that is, a negativity starting around 100 msec after words onset. One-year-olds, moreover, displayed a phonotactic familiarity effect, that is, a widely distributed negativity starting around 250 msec in 19-month-olds but occurring later in 12-month-olds. The results imply that both lexical priming and phonotactic familiarity already affect the processing of acoustic stimuli in children at 12 months of age. In 19-month-olds, adult-like mechanisms of semantic integration are present in response to phonotactically legal, but not to phonotactically illegal, nonsense words, indicating that children at this age treat pseudo-words, but not nonwords, as potential word candidates.

Lexical priming and semantic integration reflected in the event-related potential of 14-month-olds.

This study investigates by means of the event-related brain potential whether mechanisms of lexical priming and semantic integration are already developed in 14-month-olds. While looking at coloured pictures of known objects children were presented with basic-level words that were either congruous or incongruous to the pictures. The event-related potential of 14-month-olds revealed an early negativity in the lateral frontal brain region for congruous words, and a later N400-like negativity for incongruous words. These results indicate that both lexical priming and semantic integration are already present as early as 14 months.

The processing of prosody: Evidence of interhemispheric specialization at the age of four

Beyond its multiple functions in language comprehension and emotional shaping, prosodic cues play a pivotal role for the infants amazingly rapid acquisition of language. However, cortical correlates of prosodic processing are largely controversial, even in adults, and functional imaging data in children are sparse. We here use an approach which allows to experimentally determine brain activations correlating to the perception and processing of sentence prosody during childhood. In 4-year-olds, we measured focal brain activation using near-infrared spectroscopy and demonstrate that processing prosody in isolation elicits a larger right fronto-temporal activation whereas a larger left hemispheric activation is elicited by the perception of normal language with full linguistic content. Hypothesized by the dual-pathway-model, the present data provide experimental evidence that in children specific language processes rely on interhemispheric specialization with a left hemispheric dominance for processing segmental (i.e. phonological) and a right hemispheric dominance for processing suprasegmental (i.e. prosodic) information. Generally in accordance with the imaging data reported in adults, our finding underlines the notion that interhemispheric specialization is a continuous process during the development of language.

Generalization of word meanings during infant sleep

Sleep consolidates memory and promotes generalization in adults, but it is still unknown to what extent the rapidly growing infant memory benefits from sleep. Here we show that during sleep the infant brain reorganizes recent memories and creates semantic knowledge from individual episodic experiences. Infants aged between 9 and 16 months were given the opportunity to encode both objects as specific word meanings and categories as general word meanings. Event-related potentials indicate that, initially, infants acquire only the specific but not the general word meanings. About 1.5 h later, infants who napped during the retention period, but not infants who stayed awake, remember the specific word meanings and, moreover, successfully generalize words to novel category exemplars. Independently of age, the semantic generalization effect is correlated with sleep spindle activity during the nap, suggesting that sleep spindles are involved in infant sleep-dependent brain plasticity.

Neural Correlates of Syntactic Processing in Two-Year-Olds

Event-related brain potential (ERP) studies of sentence processing in adults have shown that phrase-structure violations are associated with two ERP components: an early left anterior negativity (ELAN) and a late, centro-parietal positivity (P600). Although the ELAN reflects highly automatic first-pass sentence parsing, the P600 has been interpreted to reflect later, more controlled processes. The present ERP study investigates the processing of phrase-structure violations in children below three years of age. Both children (mean age of 2.8 years) and adults passively listened to short active sentences that were either correct or syntactically incorrect. Adults displayed an ELAN that was followed by a P600 to the syntactic violation. Children also demonstrated a biphasic ERP pattern consisting of an early left hemispheric negativity and a late positivity. Both components, however, started later and persisted longer than those observed in adults. The left lateralization of the childrens negativity suggests that this component can be interpreted as a child-specific precursor to the ELAN observed in adults. The appearance of the early negativity indicates that the neural mechanisms of syntactic parsing are present, in principle, during early language development.

Neurophysiological correlates of online word learning in 14-month-old infants

Between 12 and 14 months infants switch from slow to fast word learning mode. The neural processes involved in this development are largely unknown. This study explored the brain activity related to the fast learning of object–word mappings in 14-month-old infants. After four repetitions of eight object–word pairs, two priming effects known from earlier infant event-related potential studies were observed: word form priming indexed by the fronto-lateral negativity in the 200–500 ms range and semantic priming indexed by the parietal N400. These neurophysiological correlates suggest that infants have learned object–word mappings by four presentations. In a test phase applied at least 1 day later, the N400 differentiated between trained congruous and incongruous pairings, which indicates that this newly established referential knowledge has been consolidated in memory.

Word learning in 6-month-olds: Fast encoding-weak retention

There has been general consensus that initial word learning during early infancy is a slow and time-consuming process that requires very frequent exposure, whereas later in development, infants are able to quickly learn a novel word for a novel meaning. From the perspective of memory maturation, this shift in behavioral development might represent a shift from slow procedural to fast declarative memory formation. Alternatively, it might be caused by the maturation of specific brain structures within the declarative memory system that may support lexical mapping from the very first. Here, we used the neurophysiological method of ERPs to watch the brain activity of 6-month-old infants, when repeatedly presented with object–word pairs in a cross-modal learning paradigm. We report first evidence that infants as young as 6 months are able to associate objects and words after only very few exposures. A memory test 1 day later showed that infants did not fully forget this newly acquired knowledge, although the ERP effects indicated it to be less stable than immediately after encoding. The combined results suggest that already at 6 months the encoding process of word learning is based on fast declarative memory formation, but limitations in the consolidation of declarative memory diminish the long lasting effect in lexical-semantic memory at that age.