Declarative Memory - Declarative Memory Consolidation During Sleep

Declarative Memory Consolidation During Sleep

It is believed that sleep plays an active role in consolidation of declarative memory. Specifically, sleep’s unique properties enhance memory consolidation, such as the reactivation of newly learned memories during sleep. For example, it has been suggested that the central mechanism for consolidation of declarative memory during sleep is the reactivation of hippocampal memory representations. This reactivation transfers information to neocortical networks where it is integrated into long-term representations. Studies on rats involving maze learning found that hippocampal neuronal assemblies that are used in the encoding of spatial information are reactivated in the same temporal order. Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus in slow-wave sleep (SWS) after spatial learning. Together these studies show that newly learned memories are reactivated during sleep and through this process new memory traces are consolidated. In addition, researchers have identified three types of sleep (SWS, sleep spindle and REM) in which declarative memory is consolidated.

Slow-Wave Sleep, often referred to as deep sleep, plays the most important role in consolidation of declarative memory and there is a large amount of evidence to support this claim. One study found that the first 3.5 hours of sleep offer the greatest performance enhancement on memory recall tasks because the first couple of hours are dominated by SWS. Additional hours of sleep do not add to the initial level of performance. Thus this study suggests that full sleep may not be important for optimal performance of memory. Another study shows that people who experience SWS during the first half of their sleep cycle compared to subjects who did not, showed better recall of information. However this is not the case for subjects who were tested for the second half of their sleep cycle, as they experience less SWS.

Another key piece of evidence regarding SWS’s involvement in declarative memory consolidation is a finding that people with pathological conditions of sleep, such as insomnia, exhibit both reduction in Slow-Wave Sleep and also have impaired consolidation of declarative memory during sleep. Another study found that middle aged people compared to young group had a worse retrieval of memories. This in turn indicated that SWS is associated with poor declarative memory consolidation but not with age itself.

Some researchers suggest that sleep spindle, a burst of brain activity occurring during stage 2 sleep, plays a role in boosting consolidation of declarative memories. Critics point out that spindle activity is positively correlated with intelligence. In contrast, Schabus and Gruber point out that sleep spindle activity only relates to performance on newly learned memories and not to absolute performance. This supports the hypothesis that sleep spindle helps to consolidate recent memory traces but not memory performance in general. The relationship between sleep spindles and declarative memory consolidation is not yet fully understood.

There is a relatively small body of evidence that supports the idea that REM sleep helps consolidate highly emotional declarative memories. For instance Wagner, et al. compared memory retention for emotional versus neutral text over two instances; early sleep that is dominated by SWS and late sleep that is dominated by REM phase. This study found that sleep improved memory retention of emotional text only during late sleep phase, which was primarily REM. Similarly, Hu & Stylos-Allen, et al. performed a study with emotional versus neutral pictures and concluded that REM sleep facilitates consolidation of emotional declarative memories.

The view that sleep plays an active role in declarative memory consolidation is not shared by all researchers. For instance Ellenbogen, et al. argue that sleep actively protects declarative memory from associative interference. Furthermore, Wixted believes that the sole role of sleep in declarative memory consolidation is nothing more but creating ideal conditions for memory consolidation. For example, when awake, people are bombarded with mental activity which interferes with effective consolidation. However, during sleep, when interference is minimal, memories can be consolidated without associative interference. More research is needed to make a definite statement whether sleep creates favourable conditions for consolidation or it actively enhances declarative memory consolidation.