The role of sleep in memory consolidation

Field: Neuroscience (Life Sciences)

Sleep benefits both learning and memory, with offline memory consolidation leading to the reactivation and integration of new information into the long-term store. Previous literature further demonstrates an influence of sleep on memory for related but unseen information, termed false memories. However, current findings provide mixed results regarding sleep’s role in the formation of these false memories within the Deese-Roediger-McDermott (DRM) paradigm, with differing memory effects suggested to be due to various task differences. For word recognition, the reactivation of memories during sleep promotes access to the long-term store in the left hemisphere, whereas false memories may lead to equal spread of activation across the two hemispheres. Whether hemispheric processing of memories after sleep affects memories lateralised at encoding, rather than at retrieval, is however unknown. The emotionality of to-be-remembered information also enhances the consolidation effect, with greater veridical memory performance for both negative and positive stimuli. However, the effect of sleep on false memories varying in emotionality is yet unknown. This thesis therefore presents a series of experiments that examine several factors, namely lateralisation and emotionality, that may influence the role of sleep in learning and memory consolidation. In Chapter 2, we conduct a meta-analysis to establish the influence of sleep on veridical and false memories within the DRM paradigm literature. Although results suggest no overall effect of sleep, whether participants took part in a recall or recognition task, and the number of words within a DRM word list, moderated the effect. The role of sleep in DRM memory consolidation is therefore dependent on specific task features. Chapter 3 exposes participants to DRM word lists to the left or right hemisphere at encoding. The results demonstrate no hemispheric difference in memory performance after sleep for either veridical or false memories, suggesting that lateralisation effects after sleep are specific to retrieval processes. In Chapters 4 and 5, we use behavioural testing (Chapter 4) and polysomnography (PSG; Chapter 5) to investigate sleep-dependent veridical and false memory for emotionally negative, positive, and neutral DRM word lists. Findings demonstrate that negative emotion, compared to neutral, enhances veridical memory performance after sleep, whereas wake supports positive memory. Interestingly, sleep spindles during slow wave sleep (SWS) were found to correlate with increased memory performance for emotional words. In Chapter 6, we investigate the effect of sleep on learning of and memory for novel metaphorical word pairs of either negative, positive, or neutral valence. Findings demonstrate increased memory for emotional word pairs after sleep than wake. These results help clarify the role sleep plays in the formation of emotional memories, and highlights factors that modulate the effect of sleep on both veridical and false memories. We provide evidence that lateralisation effects are specific to retrieval processes, and suggest that sleep boosts the consolidation of emotional information, indicating a potential role of sleep spindles specific to SWS in the consolidation of emotional memories.