Language delay and contextual diversity : individual and environmental differences in early word learning

The production of the child’s first words is the result of mapping the words heard to the correct referent in the world as well as learning the words’ semantic characteristics. Experiencing the word in different linguistic contexts facilitates the accomplishment of these two learning tasks. The amount of different linguistic contexts associated with a word is known as the word’s contextual diversity. Individual differences in exploiting contextual diversity might result in differences in the type of words that young children acquire. At the same time, environmental differences might affect the contextual diversity quality of the words in the speech that children hear. Altogether, the complexity of the interaction of internal and external factors related to contextual diversity might lead to differences in the child’s language outcome. The general expectation is that poor ability coupled with a poor environment would be associated with early language delay. This thesis investigates this hypothesis by four means: 1) examining young children’s lexical profiles, 2) understanding the role of contextual diversity in word learning relative to other contextual measures, 3) investigating the underlying mechanisms involved in word learning through contextual diversity, and 4) evaluating the semantic richness of child-directed speech of parents whose children are late talkers, typical talkers or late bloomers. Chapter 1 introduces the topic and presents an outline of the thesis. Chapter 2 investigates whether the lexical composition of late talkers and children with ASD, who also have language delay, differ from that of typical talkers. The most striking finding was that late talkers and children with ASD showed higher proportions of verbs than typical talkers, which might explain the weaker noun bias they presented. Most differences appear to reflect the extent of the language delay between the groups. However, children with ASD produced fewer high-social verbs than neurotypical children, a difference that might be associated with ASD features. Chapter 3 explores the preverbal semantic maturation of words and its influence on early lexical learning. Study 1 compares the vocabularies of a large sample of late and typical talkers. Late talkers showed lower contextual diversity in their noun vocabularies but higher in their verb vocabularies. Study 2 explains these differences by comparing three computational models finding that language delay is associated with lower semantic maturation of words. Chapter 4 asks whether the structural quality of child-directed speech is associated with late language onset and whether its network-structure is reflected in the structural properties of the children’s vocabularies. The study examines the vocabulary trajectories of 63 children as well the child-directed speech that they received at home during a 6-months period. The language environment experienced by the three groups of children with different lexical growth rates (i.e., typical talkers, late talkers, and late bloomers) differed in their structural-network properties, with the environment of late talkers being less semantically rich and less well-connected. The semantic associations between words that late talkers learned from their environments led to the structure of their expressive vocabularies to have a weaker network structure. Further, late talkers understood a different set of words which made their receptive lexical networks to be less well-connected. The results suggested an association between the quality of the language environment and the children’s lexical abilities as well as the semantic network-structure of their vocabularies, which, as I discuss in Chapter 4, might have consequences in how children process language. Chapter 5 investigates the role of two opposing but related contextual features in facilitating early word acquisition: contextual diversity and contextual distinctiveness. Hierarchical regression analysis confirmed the importance of both word features for word acquisition. In contrast to previous findings, words with an intermediate degree of contextual distinctiveness were found to be the earliest to be acquired. In addition, the best measure of contextual diversity considers the immediate context (words surrounding the word within speech), suggesting that the main role of contextual diversity is the semantic enrichment of the word’s concept in the child’s mind. Finally, Chapter 6 discusses the theoretical and practical implications of the findings. Contextual diversity is an important word feature whose main role appears to be to aid a word’s semantic maturation in the child’s mental lexicon. The ability of parents to modulate the contextual diversity in their speech seems to be reflected in the structure of children’s vocabularies and appears to be associated with early language delay in the case where the language input is semantically poor