Understanding individual differences in learning and consolidating new vocabulary

Sleep plays an active role in consolidating new words into vocabulary, in line with Complementary Learning Systems models that describe "offline" integration of rapidly acquired memories into longer-term stores. However, these models describe average learning processes, and do not account for individual variability. In this thesis, it is proposed that existing vocabulary knowledge may be one source of variation in supporting rapid integration of new words into memory, based on extant evidence that existing vocabulary knowledge predicts overnight improvements in new word memory. To test causal hypotheses of this relationship, eight experiments manipulated the extent to which trained pseudowords were similar to existing lexical items. Semantic and word-form lexical similarity to existing English words both influenced new learning, regardless of whether this learning took place in explicit teaching or incidental learning contexts. Adults showed long-lasting benefits of lexical similarity, whereas children received greater benefits from offline consolidation that enabled memory for lexically dissimilar items to catch up. These greater offline improvements for children relative to adults were consistent across experiments, supporting the claim that the developing brain may benefit from richer sleep in learning novel information. Standardised measures of vocabulary knowledge strongly predicted overall performance in all five of the experiments that incorporated them, but showed limited relationships with lexical similarity or offline improvements. In a ninth experiment, children’s memory was tracked over equivalent periods of wake and sleep, finding that sleep soon after learning had long-term benefits for new word knowledge. However, only when a day’s wake intervened between learning and sleep was overnight consolidation predicted by existing vocabulary knowledge. To conclude, prior knowledge supports vocabulary consolidation in some but not all learning contexts, and perhaps influences what is later consolidated rather than the consolidation process itself. Implications for optimising word learning in those with vocabulary weaknesses are discussed.