Tagging of memories during encoding may be realized through changes in arousal-related neuromodulators, such as norepinephrine and cortisol, and functional brain connectivity ( Kim and Payne, 2020). It appears that certain salience cues, such as emotional valence or personal relevance, present during the peri-encoding period can act as behavioral “tags” that indicate which memories should be consolidated during sleep ( Payne et al., 2008b, 2012, 2015 Fischer and Born, 2009 Wilhelm et al., 2011). Some evidence suggests that this process is selective, with certain memories being prioritized for retention over others ( Payne et al., 2008b Diekelmann et al., 2009 Stickgold and Walker, 2013 Payne and Kensinger, 2018). Sleep aids in the consolidation of memories ( Stickgold, 2005 Payne et al., 2008a Klinzing et al., 2019). This provides new evidence for preferential sleep-based consolidation and elucidates a physiological correlate of this benefit. We extend this to show that fast spindles were uniquely associated with the consolidation of weakly encoded memories. Fast sleep spindles, a hallmark oscillation of non-rapid eye movement sleep, mediate consolidation processes. This suggests that sleep helps attenuate the forgetting of weak memory traces. Here, we found that, compared with staying awake, sleep was associated with better memory for weakly encoded information. SIGNIFICANCE STATEMENT Given the countless pieces of information we encode each day, how does the brain select which memories to commit to long-term storage? Sleep is known to aid in memory consolidation, and it appears that certain memories are prioritized to receive this benefit. This suggests that sleep spindles facilitate memory consolidation, guided in part by memory strength. These relationships were unique to weakly encoded items, with spindles not correlating with memory for intermediate or strong items. Moreover, sleep spindles that were coupled to slow oscillations predicted the consolidation of weak memories independently of uncoupled sleep spindles. Within the nap group, consolidation of weakly encoded items was associated with fast sleep spindle density during non-rapid eye movement sleep. In the nap group, however, this effect was attenuated, with forgetting rates equivalent across encoding strengths. Results showed that, across 6 h awake, weakly encoded memories deteriorated the fastest. Thirty-six had a 2 h nap opportunity following learning, while the remaining 18 remained awake throughout. Here, 54 human participants (76% female) studied pairs of words to three distinct encoding strengths, with recall being tested immediately following learning and again 6 h later. However, the role of consolidation-mediating oscillations, such as sleep spindles and slow oscillations, in this preferential consolidation has not been explored. Initial strength of a memory appears to be an important boundary condition in determining which memories are consolidated during sleep. Sleep has been shown to be critical for memory consolidation, with some research suggesting that certain memories are prioritized for consolidation.
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