Brain mechanisms in early language acquisition

Abstract

The last decade has produced an explosion in neuroscience research examining young children's early processing of language. Noninvasive, safe functional brain measurements have now been proven feasible for use with children starting at birth. The phonetic level of language is especially accessible to experimental studies that document the innate state and the effect of learning on the brain. The neural signatures of learning at the phonetic level can be documented at a remarkably early point in development. Continuity in linguistic development from infants' earliest brain responses to phonetic stimuli is reflected in their language and prereading abilities in the second, third, and fifth year of life, a finding with theoretical and clinical impact. There is evidence that early mastery of the phonetic units of language requires learning in a social context. Neuroscience on early language learning is beginning to reveal the multiple brain systems that underlie the human language faculty.

Figure 1

Four techniques now used extensively with infants and young children to examine their responses to linguistic signals (From Kuhl and Rivera-Gaxiola, 2008).

Figure 2

The relationship between age of acquisition of a second language and language skill (adapted from Johnson and Newport, 1989).

Figure 3

Effects of age on discrimination of the American English /ra-la/ phonetic contrast by American and Japanese infants at 6–8 and 10–12 months of age. Mean percent correct scores are shown with standard errors indicated (adapted from Kuhl et al., 2006).

Figure 4

(A) A 7.5-month-old infant wearing an ERP electrocap. (B) Infant ERP waveforms at one sensor location (CZ) for one infant are shown in response to a native (English) and nonnative (Mandarin) phonetic contrast at 7.5 months. The mismatch negativity (MMN) is obtained by subtracting the standard waveform (black) from the deviant waveform (English = red; Mandarin = blue). This infant’s response suggests that native-language learning has begun because the MMN negativity in response to the native English contrast is considerably stronger than that to the nonnative contrast. (C) Hierarchical linear growth modeling of vocabulary growth between 14 and 30 months for MMN values of +1SD and −1SD on the native contrast at 7.5 months (C, left) and vocabulary growth for MMN values of +1SD and −1SD on the nonnative contrast at 7.5 months (C, right) (adapted from Kuhl et al., 2008).

Figure 5

The need for social interaction in language acquisition is shown by foreign-language learning experiments. Nine-month-old infants experienced 12 sessions of Mandarin Chinese through (A) natural interaction with a Chinese speaker (left) or the identical linguistic information delivered via television (right) or audiotape (not shown). (B) Natural interaction resulted in significant learning of Mandarin phonemes when compared with a control group who participated in interaction using English (left). No learning occurred from television or audiotaped presentations (middle). Data for age-matched Chinese and American infants learning their native languages are shown for comparison (right) (adapted from Kuhl et al., 2003).

Figure 6

(A) Nine-month-old infants experienced 12 sessions of Spanish through natural interaction with a Spanish speaker. (B) The neural response to the Spanish phonetic contrast (d-t) and the proportion of gaze shifts during Spanish sessions were significantly correlated (from Conboy et al., submitted).

Figure 7

(A) Neuromagnetic signals were recorded in newborns, 6-month-old (shown), and 12-month-old infants in the MEG machine while listening to speech and nonspeech auditory signals. (B) Brain activation in response to speech recorded in auditory (B, top row) and motor (B, bottom row) brain regions showed no activation in the motor speech areas in the newborn in response to auditory speech, but increasing activity that was temporally synchronized between the auditory and motor brain regions in 6- and 12-month-old infants (from Imada et al., 2006).