Sleep is assumed to serve different functions, particularly in playing a major role in the consolidation of memories. Short daytime sleep intervals (“naps”) have as well been shown to benefit memory retention. Certain neurophysiological components such as spindles are thought to be essential for memory consolidation during sleep. Recently it has been shown that selection processes might occur during sleep given that not all learnt information is retrieved equally well after sleep. Motivational relevant memories seem to be consolidated most preferentially during sleep. The aim of the present thesis was to investigate the role of naps on recognition memory processes behaviorally and with electrophysiological measures. Further, it was aimed to link this to physiological parameters occurring during sleep. Finally, it was tested whether motivational cues at encoding impact a subsequent nap as well as memory retention post-sleep. The aim of the first experiment was to test whether and how sleep influences recognition memory. According to the dual-process theory it is assumed that recognition memory is comprised of two distinct processes. Familiarity is assumed to be context-independent; eliciting a feeling of knowing something. Recollection is assumed to be context-dependent, concrete details and associations can be remembered, and it is described as a hippocampus-dependent process. Both processes have also been associated with distinct event-related potential (ERP) old/new effects. An early mid-frontal old/new effect has been associated with familiarity while a late parietal old/new effect has been linked to recollection. In the first experiment, participants learnt single words and word-pairs before performing an item memory (IM) and an associative memory (AM) test. One group was subsequently allowed to nap while the other watched DVDs (control group). Afterwards, both groups performed a final IM- and AM-test for the learned stimuli. IM performance decreased for both groups, whereas AM performance decreased for the control group but endured for the nap group. ERP old/new effects did not differ between groups. In an additional ERP analysis taking the associative discrimination ability into account, however, group differences were found. Participants of the nap group showed larger ERP effects which are linked to a process of recollection. Positive correlations were observed between spindle density during SWS and AM posttest performance as well as between spindle density during non-REM (NREM) sleep and AM baseline performance. It was thus questioned whether a general superior learning before sleep impacts spindle density in a subsequent nap, i.e. that better learners show more spindles. Alternatively, it was assumed that spindle density might be related to selective memory performance for items which are associated with high future values as recent findings show that sleep seems to selectively benefit memories that are relevant for the future. The second experiment therefore investigated whether the processing of different reward cues at encoding is associated with changes in electrophysiological measures and sleep physiology as well as memory retention. Participants’ memory was tested after learning a list of non-associated word-pairs both before and after taking a 90-minute nap. During learning, word-pairs were preceded by a cue indicating either a high or a low reward for correct memory performance at test. As expected, memory declined to a greater extent from pre- to post-sleep for low rewarded than for high rewarded word-pairs what was also reflected in differential ERP correlates of recollection. Positive correlations between spindle density during NREM sleep and general memory performance pre- and post-sleep were found. In addition to this, however, a selective positive relationship between memory performance for highly rewarded word-pairs at post-sleep and spindle density during NREM sleep was also observed. Further, a tendency of a positive relationship between ERPs to high reward cues at encoding and spindle density was found. These results support the view that motivationally salient memories are preferentially consolidated and that sleep spindles may be an important underlying mechanism for selective consolidation.Taken together, the results of the present thesis show that nap sleep benefits memory retention in an associative memory paradigm what is also reflected in ERP correlates of recollection. Additionally, memory retention is linked to density of sleep spindles both before and after sleep. The present dissertation extends previous research by showing distinct effects of sleep and wake on ERPs related to recollection in the ability of associative memory discrimination. Additionally, by finding a link between sleep spindles and post-sleep memory performance for highly relevant information, recent assumptions of a selective influence of sleep on memory retention can be supported.
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