IN THE FIRST 8 MONTHS: talking to children early, reading to them early, and interacting socially with children around language and literacy activities creates the milieu in which plasticity during the critical period (birth to age 3) can be maximized for all children.
Early Language Learning Predicts Later Language Skills
Early language learning is a complex process. Our working hypothesis is the following: Infants computational skills, modulated by social interaction, open a window of increased plasticity at about 8 months of life. Between 8 and 10 months monolingual infants show an increase in native language phonetic perception, a decrease in nonnative phonetic perception, and remain open to phonetic learning from a new language that can be induced by social experience with a speaker of that language (though not via a standard TV experience). The complexity of learning in this early phase is not trivial, and that complexity might explain why our laboratory studies show wide individual differences in the early phonetic transition. An important question, especially for practice, was suggested by these data: Is an individual child’s success at this early transition toward language indicative of future language skills or literacy?
We began studies to determine whether the variability observed in measures of early phonetic learning predicted children’s language skills measured at later points in development. We recognized that it was possible that the variability we observed was simply “noise,” in other words, random variation in a child’s skill on the particular day that we measured that child in the laboratory. We were therefore pleased when our first studies demonstrated that infants’ discrimination of two simple vowels at 6 months of age was significantly correlated with their language skills at 13, 16, and 24 months of age (Tsao, Liu, and Kuhl, 2004). Later studies confirmed the connection between early speech perception and later language skills using both brain (Rivera-Gaxiola et al., 2005b; Kuhl et al., 2008) and behavioral (Kuhl et al., 2005a) measures on monolingual infants, and with bilingual infants using brain measures (Garcia-Sierra et al., in press). Other laboratories also produced data that indicated strong links between the speed of speech processing and later language function (Fernald, Perfors, and Marchman, 2006) and between various measures of statistical learning and later language measures (Newman, Ratner, Jusczyk, Jusczyk,and Dow, 2006).
Recent data from our laboratory indicate long-term associations between early measures of infants’ phonetic perception and future language and reading skills. The new work measures vowel perception at 7 and 11 months and shows that the trajectory of learning between those two ages predicts the children’s language abilities and pre-literacy skills at the age of 5 years—the association holds regardless of socio-economic status, as well as the level of children’s language skills at 18 and 24 months of age (Cardillo Lebedeva and Kuhl, 2009).
Infants tested at 7 and 11 months of age show three patterns of speech perception development: (1) infants who show excellent native discrimination at 7 months and maintain that ability at 11 months, the high-high group, (2) infants who show poor abilities at 7 months but excellent performance at 11 months, the low-high group, and (3) infants who show poor abilities to discriminate at both 7 and 11 months of age, the low-low group. We followed these children until the age of 5, assessing language skills at 18 months, 24 months, and 5 years of age. Strong relationships were observed between infants’ early speech perception performance and their later language skills at 18 and 24 months. At 5 years of age, significant relationships were shown between infants’ early speech perception performance and both their language skills and the phonological awareness skills associated with success in learning to read. In all cases, the earlier in development that infants showed excellent skills in detecting phonetic differences in native language sounds, the better their later performance in measures of language and pre-literacy skills (Cardillo Lebedeva and Kuhl, 2009).
These results are theoretically interesting and also highly relevant to early learning practice. These data show that the initial steps that infants take toward language learning are important to their development of language and literacy years later. Our data suggest as well that these early differences in performance are strongly related to experience. Our studies reveal that these early measures of speech discrimination, which predict future language and literacy, are strongly correlated to experience with “motherese” early in development (Liu, Kuhl, and Tsao, 2003). Motherese exaggerates the critical acoustic cues in speech (Kuhl et al., 1997; Werker et al., 2007), and infants’ social interest in speech is, we believe, important to the social learning process. Thus, talking to children early in life, reading to them early in life, and interacting socially with children around language and literacy activities creates the milieu in which plasticity during the critical period can be maximized for all children.
There is increasing evidence that children raised in families with lower socio-economic status (SES) show deficits in language measured either behaviorally or in brain studies (for extensive review, see Raizada and Kishiyama, 2010). In one of the first studies of 5-year-old children combining behavioral and brain measures, Raizada et al. (2008) examined the associations between standardized test scores of language, social cognition, intelligence, SES, and fMRI-measured brain activity as the 5-year-old children worked on a rhyming task. The results showed correlations between SES, language performance, and the degree of hemispheric specialization in Broca’s area, as measured by left-minus-right fMRI activation (Figure 6). The SES-Broca’s link remained highly significant after the effects of the language scores were removed, indicating the relationship cannot be attributed to both measures’ correlations with the language scores. The study shows a correlational link, which of course we cannot assume to be causal.
The authors concluded that fMRI is a more sensitive measure of the development of Broca’s area than any of the behavioral tests; each behavioral score is a compound function of perception, cognition, attention and motor control, whereas fMRI probes Broca’s more directly. Thus, neuroimaging studies, especially early in development, may be able to provide us with highly sensitive measures of competence.
We assumed that SES is not itself the variable driving these effects on the brain—SES is likely a proxy for the opportunity to learn. We learned in a follow-up study that SES could be removed from the equation if language input itself was measured. The complexity of language input is the more direct factor influencing development of brain areas that code language. When measures of the complexity of maternal language were assessed across the entire sample of children in the study, we observed a correlation with structural measures of the brain in Broca’s area. These measures indicated that greater grey matter in the left hemisphere language areas was related to the complexity of maternal language in conversations between the mothers and their 5-year-old children.
In summary, our results suggested that language input to the child—its complexity and diversity—was the factor affecting brain development in the language areas, not SES per se. The implication is that children’s brains literally depend on input for development. Though these results are correlational, we believe that the connection between experience with language and brain development is potentially causal and that further research will allow us to develop causal explanations.