The Role of Sleep in the Formation of Semantic Memory

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Abstract

Semantic memory represents our general knowledge about the world. The formation of semantic representations requires three key computational challenges: i) integrating information in a time- and modality-invariant fashion, ii) abstracting statistical regularities, and iii) assimilating new information into existing semantic networks. All three processes have been suggested to benefit from sleep-dependent memory consolidation. During sleep, memories are repeatedly reactivated, which is assumed to cause a reorganisation, such that memories are integrated into long-term memory and qualitatively altered to become decontextualised, schema-like representations. Although memory processing during sleep and the mechanisms involved in the formation of semantic memory are obviously related, this relationship has hardly been considered in research so far. The work described in this thesis utilised polysomnography (PSG), behavioural memory testing, and functional magnetic resonance imaging (fMRI) to investigate sleep-related memory reorganisation with regard to the three computational processes that are thought to be involved in the formation of semantic memory. In Chapter 2 we assessed whether time- and sleep-dependent consolidation facilitates cross-modal category learning. We found that offline consolidation had a beneficial effect on category learning, but surprisingly this benefit was specific to consolidation across wake, but not sleep. These results suggest that the integration of information from different sensory modalities may preferentially occur during wakefulness. Together with other findings in the literature our results emphasise the question of what determines whether memories are processed during sleep. In Chapter 3 we explored whether the beneficial effect of sleep on the extraction of statistical regularities could be enhanced by cued memory reactivation. Interestingly, our manipulation interfered with the abstraction of the underlying pattern. These findings raise important questions about the underlying mechanisms of statistical abstraction during sleep. Lastly, Chapters 4 and 5 addressed whether sleep plays a role in the assimilation of newly learned information into pre-existing semantic networks. In Chapter 4 sleep-dependent differences in the consolidation of information that either related to prior knowledge or was completely unrelated were investigated. Our findings suggest that sleep spindle density marks the process of assimilating new information into long-term memory, reflected in enhanced memory retention and decreased hippocampal engagement. In Chapter 5 we replicated the association between sleep spindle density and the development of the schema effect across time and showed that a weak link to pre-existing knowledge was sufficient in triggering the schema benefit.In conclusion, our results provide new insights to the role of sleep in memory reorganisation. We have provided evidence that cued reactivation during sleep can influence the extraction of statistical regularities and that sleep spindles are associated with the assimilation of new information into semantic networks.

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