Introduction to Sleep and Memory
What is the purpose of sleep? Though scholarly speculation about sleep dates back to the Greeks and Romans, scientific theories based on experimental data have only recently emerged. In recent years a new hypothesis has received increasing attention: Sleep actively
functions to consolidate recently-formed memories.

Early research concluded that the memory benefits of sleep were either a mistake (Ebbinghaus, 1885) or that sleep passively protects individuals’ memory from interference (Jenkins & Dallenbach, 1924). However, with the development of polysomnography and the discovery of various stages of sleep, sleep could no longer be viewed as a passive and invariant state of brain activity and consciousness.  In the 1960-70s, Bruce Ekstrand and  colleagues observed that different stages of sleep were associated with
differential effects on memory. However, such findings were not fully appreciated until several decades later when Wilson and McNaughton (1994, Science) discovered in an animal model that memories are “replayed” in the hippocampus during slow wave sleep (slow wave sleep, captured using EEG, is indicated to the right). This hippocampal reactivation, which has now been elegantly demonstrated in humans (e.g., Rasch et al., 2007, Science), is causally related to future memory enhancements. The classic theoretical explanation for such findings is that memories are intermediately stored in the hippocampus but eventually must be either discarded or transferred to long-term neocortical memory stores. This “consolidation” process promotes long-term recollection and also frees the hippocampus to encode new information.

Sleep, Memory, and Aging Research
Most sleep-dependent memory consolidation research has examined sleep and memory in college-aged adults, ignoring how
consolidation may change with aging and disease. Separate research literatures have reported substantial age-related sleep disturbances (including declines in slow wave sleep) and profound memory declines in older adults. These literatures converge on an intriguing question: Can older adults effectively consolidate memories during sleep? This question is especially pertinent because recent animal studies have shown that chronic sleep deficiencies cause amyloid plaque buildup, which is a hallmark of Alzheimer’s disease. In a recent study (figure on right), I found that sleep, and particularly slow-wave sleep, was strongly associated with benefits to the retention of episodic memories encoded prior to sleeping in healthy younger adults. However, such sleep-dependent memory benefits were absent in healthy older adults (Scullin, 2013, Psychology & Aging). This research was covered by Fox News.


Sleep and Cognitive "Training" Research
 

Many resea
rchers and clinicians are interested in whether cognitive abilities can be trained. Our research suggests that high quality sleep following an initial "training" phase might facilitate training on a working memory task (Scullin, Trotti, Wilson, Greer, & Bliwise, 2012, Brain). We observed that dopamine-treated Parkinson's disease patients  demonstrated "offline" gains in a working memory task after a night of sleep. Such sleep-dependent improvements, as illustrated to the right, were strongest when patients had a lot of slow wave sleep and when they showed no signs of sleep apnea. These results provide clear evidence that the type of sleep obtained during a training interval can be associated with either the facilitation or the blocking of cognitive training.

You can read the news release for this research here. For more information on my research on sleep and cognitive training see this powerpoint presentation.




Sleep and Prospective Memory Research

Sleep has been demonstrated to actively benefit memory for previously learned information ("retrospective memory") in college-aged adults, but there has been little research on whether sleep also benefits prospective memory, which is the memory you use for initiating future events. In a previous project (Scullin & McDaniel, 2010, Psychological Science) we tested prospective memory performance across delay intervals that included nocturnal sleep or daytime wake and found that sleep following forming a prospective memory intention led to greater prospective remembering during a test context that was originally temporally associated with the prospective memory intention. In a sense, sleep reinforced in memory a context in which the prospective memory goal was to be executed. A critical, unanswered question concerns which aspect of sleep (e.g., slow wave sleep) may be associated with benefits to prospective memory, and particularly with the reinforcement of the context--intention link.

You can read the news release for this research here. For more information on my research on sleep and prospective memory, see this powerpoint presentation.