Abstract
BackgroundLearning followed by a period of sleep, even as little as a nap, promotes memory consolidation. It is now generally recognized that sleep facilitates the stabilization of information acquired prior to sleep. However, the temporal nature of the effect of sleep on retention of declarative memory is yet to be understood. We examined the impact of a delayed nap onset on the recognition of neutral pictorial stimuli with an added spatial component.Methodology/Principal FindingsParticipants completed an initial study session involving 150 neutral pictures of people, places, and objects. Immediately following the picture presentation, participants were asked to make recognition judgments on a subset of “old”, previously seen, pictures versus intermixed “new” pictures. Participants were then divided into one of four groups who either took a 90-minute nap immediately, 2 hours, or 4 hours after learning, or remained awake for the duration of the experiment. 6 hours after initial learning, participants were again tested on the remaining “old” pictures, with “new” pictures intermixed.Conclusions/SignificanceInterestingly, we found a stabilizing benefit of sleep on the memory trace reflected as a significant negative correlation between the average time elapsed before napping and decline in performance from test to retest (p = .001). We found a significant interaction between the groups and their performance from test to retest (p = .010), with the 4-hour delay group performing significantly better than both those who slept immediately and those who remained awake (p = .044, p = .010, respectively). Analysis of sleep data revealed a significant positive correlation between amount of slow wave sleep (SWS) achieved and length of the delay before sleep onset (p = .048). The findings add to the understanding of memory processing in humans, suggesting that factors such as waking processing and homeostatic increases in need for sleep over time modulate the importance of sleep to consolidation of neutral declarative memories.
Highlights
There is a substantial body of evidence supporting the idea that sleep facilitates the consolidation of newly formed memories
Change in performance between these testing phases was compared between the different conditions (n = 9 per group), groups that napped at intervals (Immediate, 2-Hour, 4Hour) as well as the control wake group (Wake)
We investigated the temporal relationship between learning and memory of a spatial and recognition task by using a staggered nap design with a waking control to assess the effects of delaying sleep onset
Summary
There is a substantial body of evidence supporting the idea that sleep facilitates the consolidation of newly formed memories. In the case of hippocampal-dependent declarative memories, non-rapid eye movement sleep (NREM, stages 1-4), slow wave sleep (SWS, combined stages 3 & 4), is thought to facilitate the shift of the burden of memory reactivation from short term dependence on hippocampal areas to long term stores in the neocortex, a process known as systems consolidation [9,10]. Neurons that were most recently fired during waking, such as those representing declarative memory, are thought to be reactivated by sharp-wave ripples and the theorized resulting long-term potentiation (LTP), the predominant candidate as a mechanistic explanation for synaptic consolidation within the network, gives rise to further strengthening between involved synapses due to structural modifications of the pre- and post-synaptic cells [12,13,14]. We examined the impact of a delayed nap onset on the recognition of neutral pictorial stimuli with an added spatial component
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