Abstract
Objective: In recognition of the mixed associations between traditionally scored slow wave sleep and memory, we sought to explore the relationships between slow wave sleep, electroencephalographic (EEG) power spectra during sleep and overnight verbal memory retention in older adults.Design, Setting, Participants, and Measurements: Participants were 101 adults without dementia (52% female, mean age 70.3 years). Delayed verbal memory was first tested in the evening prior to overnight polysomnography (PSG). The following morning, subjects were asked to recall as many items as possible from the same List (overnight memory retention; OMR). Partial correlation analyses examined the associations of delayed verbal memory and OMR with slow wave sleep (SWS) and two physiologic EEG slow wave activity (SWA) power spectral bands (0.5–1 Hz slow oscillations vs. 1–4 Hz delta activity).Results: In subjects displaying SWS, SWS was associated with enhanced delayed verbal memory, but not with OMR. Interestingly, among participants that did not show SWS, OMR was significantly associated with a higher slow oscillation relative power, during NREM sleep in the first ultradian cycle, with medium effect size.Conclusions: These findings suggest a complex relationship between SWS and memory and illustrate that even in the absence of scorable SWS, older adults demonstrate substantial slow wave activity. Further, these slow oscillations (0.5–1 Hz), in the first ultradian cycle, are positively associated with OMR, but only in those without SWS. Our findings raise the possibility that precise features of slow wave activity play key roles in maintaining memory function in healthy aging. Further, our results underscore that conventional methods of sleep evaluation may not be sufficiently sensitive to detect associations between SWA and memory in older adults.
Highlights
Sleep is profoundly important for optimizing brain function and cognition
Since the duration of sleep is highly variable, to better control for this, we focused on power spectral analysis of NREM sleep during the first 177 epochs (88.5 min) after sleep onset, when participants finished the first ultradian cycle of SWA (0.5–4 Hz, combining slow oscillation and the delta band)
We compared absolute and relative power for the slow wave sleep (SWS) (–) group with the SWS (+) group during NREM sleep in the first ultradian cycle of sleep
Summary
Sleep is profoundly important for optimizing brain function and cognition. The rapidly advancing age of the global population highlights the urgent need to understand the dynamics between aging, sleep, and cognitive performance (Scullin and Bliwise, 2015). Since the publication of these seminal findings, a growing body of research has demonstrated that the retention of verbal, visual, motor, and spatial information is improved after sleep (Plihal and Born, 1997; Walker et al, 2002, 2003; Tucker et al, 2006; Rasch and Born, 2007; Rasch et al, 2007; Lahl et al, 2008). Using functional magnetic resonance imaging, Takashima and colleagues suggested that the association of SWS with memory might reflect the transitioning of memories from the hippocampus to the ventral medial pre-frontal cortex (Takashima et al, 2006). Rasch et al cued new memories during sleep by presenting an odor that had been presented as context during pre-sleep learning (Rasch et al, 2007) and found that re-exposure to the odor during SWS improved the retention of declarative memories (Rasch et al, 2007). The growing literature on SWS and memory underscores a potential role of SWS as integral to memory retention across a broad range of memory domains (Walker, 2009; Walker and Stickgold, 2010; Goerke et al, 2017)
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