At its core, word learning and recognition concerns a relation between a low-level acoustic signal and a high-level semantic representation. Fundamental to the study of the early lexical-semantic system is the manner and degree to which the acoustic signal activates the semantic representation, and how meanings associated with words relate to one another. An implicit notion in the developmental literature is that upon hearing a word, young children activate the corresponding semantic representation in a dichotomous fashion – i.e., the semantic representation is either activated resulting in word recognition or not activated resulting in lack of recognition. Further, it is not entirely known the degree to which very young children appreciate that word referents are meaningfully related.

To date, models of early auditory-semantic processing are primarily based on studies of meaningful relations between words and referents. However, lexical information (i.e., words) is not the only type of auditory input that is meaningful – environmental sounds (e.g., dog barking or pen scribbling) are nonverbal yet complex sounds that carry deep semantic associations with a corresponding referent. Therefore, a thorough investigation into the fundamental relation between acoustic signals and meaning requires an understanding of how meaning is associated with both lexical and non-lexical sounds.

A series of four studies with toddlers and adults are presented to investigate three related issues in the study of early auditory-semantic development: 1.) Is lexical knowledge dichotomous or continuous? 2.) How is lexical information semantically processed and organized? 3.) To what degree is such processing specific to language?

In the first set of studies, Chapters 2 and 3 present behavioral evidence that early word recognition is not binary, but instead graded, and partial knowledge plays a role in future learning. In the second set of studies, Chapters 4 and 5 replicate and extend findings that suggest the lexical-semantic system is organized as an interconnected semantic network both early in life and into adulthood. Further, these chapters present evidence that the electrophysiological markers of semantic processing are present during environmental sound processing in toddlers and adults, however words and environmental sounds appear to be organized somewhat differently, with a more consistent fine-grained structure for words compared to environmental sounds. Together, these studies provide behavioral and electrophysiological evidence to further our understanding of the nature of the early auditory-semantic system.

Vocabulary knowledge and speed of lexical access are early components of linguistic skill that co-develop rapidly in the first two years of life. Evidence suggests that these components are foundational for downstream linguistic and cognitive skills and that early individual differences set the stage for a developmental cascade throughout the lifespan. The three studies in this dissertation seek to clarify how early, decontextualized vocabulary predicts later development, whether it has utility in individual prediction, and how vocabulary and speed of lexical access support language development together and separately.

Chapter 1 reviews the current literature on vocabulary development and speed of lexical access, exploring how these skills are interrelated and their relation to other domain-general learning mechanisms, closing with extant evidence on predictive utility. Chapter 2 follows from recent findings that early vocabulary predicts later literacy but that it accounts for a modest amount of variance in outcomes. Chapter 2 uses an assessment of decontextualized vocabulary and shows that early, decontextualized vocabulary predicts later vocabulary and kindergarten readiness at age four, explaining additional variance above and beyond a parent report assessment of early vocabulary. In a similar vein, results from Chapter 3 reveal decontextualized vocabulary to be a robust predictor of language abilities at age three and present preliminary evidence for its utility as a screening measure. Finally, Chapter 4 presents preliminary evidence for the hypothesis that although vocabulary knowledge and lexical access are related, they are somewhat distinct in the extent to which they depend on long-term learning vs. processing efficiency. Results tentatively support this conclusion by revealing dissociation in prediction between these two components of early language. All three chapters examine these effects across groups differing linguistically and geographically and explore whether patterns across samples suggest generalization of effects across languages or reveal linguistic differences. Practically, the goal of this research is to elucidate the processes of early language development, improve identification of children at risk for language delay and inform intervention strategies

The studies included in this dissertation explore lexical-semantic development within early toddlerhood across monolingual and bilingual language learners. That is, when and how do children map words with their respective meanings when they are learning one or two languages? Chapter 1 presents a brief overview of the seminal questions that guided this dissertation research. Chapter 2 presents a comprehensive review of the empirical literature examining lexical-semantic development in young monolingual and bilingual children followed by a discussion of theoretical models and their ability to account for the available evidence. Importantly, the chapter ends drawing attention to several gaps in our understanding of bilingual lexical development that are the foci of Chapters 3 through 5. Specifically, Chapter 3 presents a methodological tool for operationalizing language exposure in dual language learning contexts. Chapter 4 examines the influence of vocabulary size on speed of auditory word recognition using haptic responses in 16 to 22 month old monolingual and bilingual toddlers. The final set of studies (Chapters 5 and 6) report on monolinguals and bilinguals, respectively, extend the previous work on word learning to the development of connections between words using gaze responses to assess lexical-semantic networks in the second year of life. Here we investigate how single or dual language exposure and vocabulary size influence the emergence of lexical-semantic priming. Throughout this dissertation, we consistently examine how theoretical models can guide predictions and be revised to account for early development. Together these investigations advance our understanding of bilingual language representation in the semantic domain.