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Why do infants learn some words earlier than others? Many theories of early word learning focus on explaining how infants map labels onto concrete objects. However, words that are more abstract than object nouns, such as "uh-oh," "hi," "more," "up," and "all-gone," are typically among the first to appear in infants' vocabularies. We combined a behavioral experiment with naturalistic observational research to explore how infants learn and represent this understudied category of high-frequency, routine-based non-nouns, which we term "everyday words." In Study 1, we found that a conventional eye-tracking measure of comprehension was insufficient to capture U.S.-based English-learning 10- to 16-month-old infants' emerging understanding of everyday words. In Study 2, we analyzed the visual and social scenes surrounding caregivers' and infants' use of everyday words in a naturalistic video corpus. This ecologically motivated research revealed that everyday words rarely co-occurred with consistent visual referents, making their early learnability difficult to reconcile with dominant word learning theories. Our findings instead point to complex patterns in the types of situations associated with everyday words that could contribute to their early representation in infants' vocabularies. By leveraging both experimental and observational methods, this investigation underscores the value of using naturalistic data to broaden theories of early learning.

Perception is not an independent, in-the-moment event. Instead, perceiving involves integrating prior expectations with current observations. How does this ability develop from infancy through adulthood? We examined how prior visual experience shapes visual perception in infants, children, and adults. Using an identical task across age groups, we exposed participants to pairs of colorful stimuli and implicitly measured their ability to discriminate relative saturation levels. Results showed that adult participants were biased by previously experienced exemplars, and exhibited weakened in-the-moment discrimination between different levels of saturation. In contrast, infants and children showed less influence of memory in their perception, and they actually outperformed adults in discriminating between current levels of saturation. Our findings suggest that as humans develop, their perception relies more on prior experience and less on current observation.

Picture books inherently contain many parallel dimensions of information and serve as a rich source of input for children. However, studies of children's learning from picture books tend to focus on a single type of information (e.g., novel words). To better understand the learning-related potential of shared book reading, we examined 4.5- to 5.5-year-old children's simultaneous learning of novel words, moral lessons, and story details from a reading interaction with a parent. Results showed that children successfully learned new words, extracted a moral lesson, and recalled story details from the picture book. Contrary to expectations, children's learning was equally strong regardless of whether or not parents were prompted to focus on learning as the key purpose of book reading. This research demonstrates that children learn diverse information presented across different time scales from picture books.

Adults typically struggle to perceive non-native sound contrasts, especially those that conflict with their first language. Do the same challenges persist when the sound contrasts overlap but do not conflict? To address this question, we explored the acquisition of lexical tones. While tonal variations are present in many languages, they are only used contrastively in tonal languages. We investigated the perception of Mandarin tones by adults with differing experience with Mandarin, including naïve listeners, classroom learners, and native speakers. Naïve listeners discriminated Mandarin tones at above-chance levels, and performance significantly improved after just one month of classroom exposure. Additional evidence for plasticity came from advanced classroom learners, whose tonemic perception was indistinguishable from that of native speakers. The results suggest that unlike many other non-native contrasts, adults studying a language in the classroom can readily acquire the perceptual skills needed to discriminate Mandarin tones.

In bilingual language environments, infants and toddlers listen to two separate languages during the same key years that monolingual children listen to just one and bilinguals rarely learn each of their two languages at the same rate. Learning to understand language requires them to cope with challenges not found in monolingual input, notably the use of two languages within the same utterance (e.g., Do you like the "perro"? or ¿"Te gusta el doggy"?). For bilinguals of all ages, switching between two languages can reduce the efficiency in real-time language processing. But language switching is a dynamic phenomenon in bilingual environments, presenting the young learner with many junctures where comprehension can be derailed or even supported. In this study, we tested 20 Spanish-English bilingual toddlers (18- to 30-months) who varied substantially in language dominance. Toddlers' eye movements were monitored as they looked at familiar objects and listened to single-language and mixed-language sentences in both of their languages. We found asymmetrical switch costs when toddlers were tested in their dominant versus non-dominant language, and critically, they benefited from hearing nouns produced in their dominant language, independent of switching. While bilingualism does present unique challenges, our results suggest a united picture of early monolingual and bilingual learning. Just like monolinguals, experience shapes bilingual toddlers' word knowledge, and with more robust representations, toddlers are better able to recognize words in diverse sentences.

Learning always happens from input that contains multiple structures and multiple sources of variability. Though infants possess learning mechanisms to locate structure in the world, lab-based experiments have rarely probed how infants contend with input that contains many different structures and cues. Two experiments explored infants' use of two naturally occurring sources of variability--different sounds and different people--to detect regularities in language. Monolingual infants (9-10 months) heard a male and female talker produce two different speech streams, one of which followed a deterministic pattern (e.g., AAB, "le-le-di") and one of which did not. For half of the infants, each speaker produced only one of the streams; for the other half of the infants, each speaker produced 50% of each stream. In Experiment 1, each stream consisted of distinct sounds, and infants successfully demonstrated learning regardless of the correspondence between speaker and stream. In Experiment 2, each stream consisted of the same sounds, and infants failed to show learning, even when speakers provided a perfect cue for separating each stream. Thus, monolingual infants can learn in the presence of multiple speech streams, but these experiments suggest that infants may rely more on sound-based rather than speaker-based distinctions when breaking into the structure of incoming information. This selective use of some cues over others highlights infants' ability to adaptively focus on distinctions that are most likely to be useful as they sort through their inherently multidimensional surroundings.

Recent research has begun to explore individual differences in statistical learning, and how those differences may be related to other cognitive abilities, particularly their effects on language learning. In this research, we explored a different type of relationship between language learning and statistical learning: the possibility that learning a new language may also influence statistical learning by changing the regularities to which learners are sensitive. We tested two groups of participants, Mandarin Learners and Naïve Controls, at two time points, 6 months apart. At each time point, participants performed two different statistical learning tasks: an artificial tonal language statistical learning task and a visual statistical learning task. Only the Mandarin-learning group showed significant improvement on the linguistic task, whereas both groups improved equally on the visual task. These results support the view that there are multiple influences on statistical learning. Domain-relevant experiences may affect the regularities that learners can discover when presented with novel stimuli.