The role of memory strength for processes of sleep-associated memory consolidation

While recent findings demonstrate that processes of sleep-associated memory consolidation can stabilize memories, it is unclear whether such consolidation varies with one of the most basic features of memory representations, i.e., memories` initial strength. The aim of this project is to fill this empirical gap by examining the role of memory strength for memory consolidation, behaviorally and neuroscientifically. Across 13 experiments, memory strength will be manipulated using intentional and incidental encoding procedures; intentional manipulations of strength will be examined in the paired associate task and the list learning task and will be implemented by study-induced and retrieval-induced strengthening; incidental manipulations of strength will be investigated in the list learning task by using different orienting tasks. Behaviorally, effects of memory consolidation will be measured via investigations of time-dependent forgetting and memories` interference susceptibility. Neuronally, electroencephalography (EEG) will be used to study how manipulations of encoding strength change the neural correlates of initial memory formation and change retrieval processing right after encoding as well as following 12 hours of sleep or wakefulness. In addition, sleep EEG will be employed to examine sleep mechanisms believed to be directly involved in the reactivation and restructuring of newly formed memories. The results will provide a first overall picture on the role of memory strength for sleep-associated memory consolidation.

It has long been known that sleep plays an important role for memory. Over sleep, newly learned memory contents are stabilized by reactivation or replay, so that these are less susceptible to time-dependent forgetting. In addition, new memories are integrated into existing memory systems during this offline period. In this way, access to new memory entries is simplified and long-term storage is ensured. Previous research, however, suggests that sleep does not promote all newly acquired memory, but that it exerts its positive effect on specific contents. For example, it has been shown that sleep particularly supports the recollection of emotional contents or those of future relevance for the learner. Based on these findings, the present project examined in detail the extent to which the positive effect of sleep on memory depends on the initial strength of newly acquired content. For this purpose the initial memory strength was manipulated over several studies by different methods (e.g., seeing word pairs twice, thrice or needing to retrieve them before falling asleep). The group around Professor Karl-Heinz Bäuml (Regensburg, Germany) investigated the influence of the memory strength on memory processes in sleep on behavioral level, while the group around Prof. Manuel Schabus focused on the (i) role of various brain oscillations when forming new memories or (ii) concentrated on certain reinforcing patterns during sleep. The results show that sleep benefits the consolidation of newly acquired material and that it can practically eliminate forgetting over the night (as opposed to periods of awakening). In addition, the results suggest that information that is not merely learned but also retrieved before sleep is better remembered. Sleep, however, appears to exert its beneficial effects overnight particularly on those contents which were learned but not retrieved before falling asleep. Last but not least, recent findings show that sleep does not selectively consolidate weakly or deeply encoded information over the night, but that it rather supports consolidation of every newly learned (and relevant) material. For a society under increasing pressure and stress, it therefore should be emphasized that natural sleep, with its positive effects on mood, attention and memory, is not a luxury that can be neglected, but that it is a central aspect of well-being, health and optimal performance for any human being.