It is well established that variation in caregivers' speech is associated with language outcomes, yet little is known about the learning principles that mediate these effects. This longitudinal study (n = 27) explores whether Spanish-learning children's early experiences with language predict efficiency in real-time comprehension and vocabulary learning. Measures of mothers' speech at 18 months were examined in relation to children's speech processing efficiency and reported vocabulary at 18 and 24 months. Children of mothers who provided more input at 18 months knew more words and were faster in word recognition at 24 months. Moreover, multiple regression analyses indicated that the influences of caregiver speech on speed of word recognition and vocabulary were largely overlapping. This study provides the first evidence that input shapes children's lexical processing efficiency and that vocabulary growth and increasing facility in spoken word comprehension work together to support the uptake of the information that rich input affords the young language learner.

Research using online comprehension measures with monolingual children shows that speed and accuracy of spoken word recognition are correlated with lexical development. Here we examined speech processing efficiency in relation to vocabulary development in bilingual children learning both Spanish and English (n=26; 2;6 yrs). Between-language associations were weak: vocabulary size in Spanish was uncorrelated with vocabulary in English, and children’s facility in online comprehension in Spanish was unrelated to their facility in English. Instead, efficiency of online processing in one language was significantly related to vocabulary size in that language, after controlling for processing speed and vocabulary size in the other language. These links between efficiency of lexical access and vocabulary knowledge in bilinguals parallel those previously reported for Spanish and English monolinguals, suggesting that children’s ability to abstract information from the input in building a working lexicon relates fundamentally to mechanisms underlying the construction of language.

In research on language development by bilingual children, the early language environment is commonly characterized in terms of the relative amount of exposure a child gets to each language based on parent report. Little is known about how absolute measures of child-directed speech in two languages relate to language growth. In this study of 3-year-old Spanish-English bilinguals (n = 18), traditional parent-report estimates of exposure were compared to measures of the number of Spanish and English words children heard during naturalistic audio recordings. While the two estimates were moderately correlated, observed numbers of child-directed words were more consistently predictive of children's processing speed and standardized test performance, even when controlling for reported proportion of exposure. These findings highlight the importance of caregiver engagement in bilingual children's language outcomes in both of the languages they are learning.

In child-directed speech (CDS), adults often use utterances with very few words; many include short, frequently used sentence frames, while others consist of a single word in isolation. Do such features of CDS provide perceptual advantages for the child? Based on descriptive analyses of parental speech, some researchers argue that isolated words should help infants in word recognition by facilitating segmentation, while others predict no advantage. To address this question directly, we used online measures of speech processing in a looking-while-listening procedure. In two experiments, 18-month-olds were presented with familiar object names in isolation and in a sentence frame. Infants were 120 ms slower to interpret target words in isolation than when the same words were preceded by a familiar carrier phrase, suggesting that the sentence frame facilitated word recognition. Familiar frames may enable the infant to 'listen ahead' more efficiently for the focused word at the end of the sentence.

Research with monolingual children has shown that early efficiency in real-time word recognition predicts later language and cognitive outcomes. In parallel research with young bilingual children, processing ability and vocabulary size are closely related within each language, although not across the two languages. For children in dual-language environments, one source of variation in patterns of language learning is differences in the degree to which they are exposed to each of their languages. In a longitudinal study of Spanish/English bilingual children observed at 30 and 36 months, we asked whether the relative amount of exposure to Spanish vs. English in daily interactions predicts children's relative efficiency in real-time language processing in each language. Moreover, to what extent does early exposure and speed of lexical comprehension predict later expressive and receptive vocabulary outcomes in Spanish vs. English? Results suggest that processing skill and language experience each promote vocabulary development, but also that experience with a particular language provides opportunities for practice in real-time comprehension in that language, sharpening processing skills that are critical for learning.