Effects of Sleep on Memory Consolidation and Automaticity in Word Learning

Compared to sleep studies on memory consolidation, there have been limited studies investigating effects of sleep on memory integration. Memory integration occurs when long-term associations between new and existing items are formed, leading to generalization of item knowledge across new contexts. Using semantic distance and size congruity effects as measures of automatic semantic access in integrated word representations, this thesis aimed to investigate the effects of sleep on declarative knowledge integration. By examining relationships between behavioural data and sleep architecture, this thesis also aimed to investigate if sleep plays an active role in knowledge integration. Experiment 1 revealed that participants who slept after learning exhibited significantly stronger semantic distance and size congruity effects for new second language words compared to participants who remained awake. Analysis of sleep polysomnography data also revealed that the strength of semantic distance and congruity effects were positively correlated with slow-wave sleep and sleep spindles respectively. Experiments 2 and 3 examined if selective factors such as training performance and encoding strength would modulate effects of sleep-dependent memory integration. Experiment 2 found that nap-associated gains in automaticity were stronger in participants who performed poorly during training. Experiment 3 also revealed that daytime-napping selectively benefitted integration of items that were more difficult to process. Experiment 4 explored the time-course required for automatic semantic access to emerge. Results highlighted that semantic distance effects emerge rapidly, whereas, size congruity effects that were considered a purer measure of automaticity only emerged after overnight consolidation. Finally, Experiment 5 studied if the sleep-associated findings from Experiments 1-4 would be similar for numeral learning. Compared to word learning, integration of new numerals required a shorter time-course and lower levels of sleep dependence. The thesis findings were evaluated against models of memory consolidation to gain theoretical understanding on the neural changes relating to knowledge integration.