Sleep, learning, and memory are complex phenomena that are not entirely understood. However, human studies suggest that the quantity and quality of sleep have a profound impact on learning and memory. Research suggests that sleep helps learning and memory in two distinct ways (Peigneux, Laureys, Delbeuck, Maquet,& 2001; Born,& Wilhelm, 2012; Ellenbogen, Payne,& Stickgold, 2006). First, a sleep-deprived person cannot focus attention optimally and therefore cannot learn efficiently. Second, sleep itself has a role in the consolidation of memory, which is essential for learning new information.
Learning and memory are often described in terms of three functions. Acquisition refers to the introduction of new information into the brain. Consolidation represents the processes by which a memory becomes stable. Recall refers to the ability to access the after it has been stored.
Each of these steps is necessary for proper memory function. Acquisition and recall occur only during wakefulness, but research (Rauchs, Desgranges, Foret,& Eustache, 2005; Peigneux et al., 2001) states that memory consolidation takes place during sleep through the strengthening of the neural connections that form our memories. Although there is no consensus about how sleep makes this process possible, many researchers proposed that specific characteristics of brainwaves during different stages of sleep are associated with the formation of particular types of memory (Nishida,& Walker, 2007; Tucker et al, 2006; Schabus et al., 2004; Ficca,& Salzarulo, 2004).
Sleep researchers study the role of sleep in learning and memory formation in three ways. The first approach looks at the different stages of sleep in response to learning a variety of new tasks (Nishida,& Walker, 2007; Ficca,& Salzarulo, 2004; Rieth, Cai, McDevitt,& Mednick, 2010).
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The second approach examines how sleep deprivation affects learning (Rauchs et al., 2005; Peigneux et al., 2001). Third is based on changes in sleep-spindles activity during the night sleep (Shabus et al. 2004; Rieth et al. 2010).
Modern working model of sleep and memory
According to Rauchs et al. (2005), human memory encompasses a short-term memory system and four systems of long-term memory: procedural memory as a system dedicated to the encoding, storage and retrieval of the information; the perceptual representation system, which is involved in nonconscious expressions of memory; semantic memory, with encompasses knowledge about the world regardless of the context of acquisition and episodic memory, that stores events located in time and space.
Next is sleep a unitary process, but is composed of at least two substrates (Rauchs et. al. 2005). One is characterized by the presence of rapid eye movements and often referred to as REM sleep. The other main sleep type is known as non REM (NREM) sleep. In humans, NREM sleep is divided into several stages, corresponding to increasing sleep depth, they are, stage 2 with K complexes and sleep spindles and slow wave sleep (SWS) which is a combination of stage 3 and 4. All through the night, the NREM and REM sleep periods alternate following an ultradian cycle, with dominance of SWS.
According to Ellenbogen et al.(2006),the search for mechanisms underlying sleep-dependent consolidation of declarative memories in humans must rely on examination of specific properties of sleep physiology. By showing that a specific form of memory consolidation crucially depends on a brain property unique to sleep, the connection can be proving.
Sleep deprivation based studies. Recent attempts have been made connect the sleep deprivation studies and effect of nonsleep on memory consolidation.
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For example, works of Rauchs et al. (2005) and Peigneux et al. (2001), used sleep deprivation approach to link sleep and consolidation process. Rauchs et al. (2005) work proved that SWS and REM sleep, as well as stage NREM sleep, may all be involved in memory consolidation. Moreover, it was demonstrated that procedural memory consolidation relies mainly on NREM sleep whereas REM sleep may have a beneficial effect on consolidation of cognitive skills, semantic memory and the perceptual representation system. Consolidation of sensory-perceptual skills and episodic memory appears to rely more consistently on a combination of SWS and REM sleep. Pigneux et al. paper in 2001 shown that, REM sleep deprivation has a profound effect on short term memory tasks and prefrontal cortex. In addition, episodic memory is depended on SWS. Therefore, lack of sleep could simply affect the recall of the learned information.
Based on Pigneux et al. paper (2001) it was possible to determine parts of the brain and their connection with memory. For example, hippocampal formation selectively supports episodic memory while the surrounding entorhinal, perirhinal and parahippocampal cortices play the main role in semantic memory. In addition, the striatum is important for habit formation and interacts with the cerebellum for motor-based skill learning, while modality-specific neocortical regions mediate modality-specific perceptual priming and the critical role in unconscious emotional learning is played by the amygdala.
Learning based studies. Other studies which are based on approach of learning new tasks. Overall researches found quiet controversial results. In Born and Wilhelm (2011) and Ficca and Salzarulo (2004) research, both are proved a two-stage model of memory consolidation. The key concept is that events experienced during wakefulness are initially encoded in parallel in neocortical networks and in the hippocampus during SWS.
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The reactivation and redistribution of memories during SWS are regulated in a dialogue between neocortex and hippocampus that is under feed-forward control of the slow oscillation. This oscillation process synchronizes neuronal in the neocortex and in hippocampus. The hippocampo-to-neocortical transfer of reactivated memory information is reaching neocortical circuitry in the up-state and in synchrony with thalamocortical spindles. Based on this concept it was found that the level of local field potentials, slow oscillations, spindle activity and hippocampal ripple activity are increased during sleep after a learning experience, and there is also evidence that these increases are linked to an improved retention of the learned memories.
In addition, research of Ficca and Salzarulo in 2004, states that either no difference between REM and NREM or a positive effect of REM sleep in declarative memory tasks and it was ascertained that motor tasks are actually improved by stage 2, NREM sleep.
In other studies, in addition to learning, scientists used daytime nap periods as a form of night sleep. In two papers of Tucker et al. (2006), and Nishida and Walker (2007), both used different task approaches. Tucker et al. (2006) study showed that the effects of NREM
sleep demonstrated the improvement on a paired associateaa‚¬a„?s declarative memory task. It was greater for subjects that took an early afternoon nap than for subjects that remained awake
during the baseline-retest interval. This difference in improvement was not observed for the procedural memory task, which lends that NREM sleep plays a special role in the processing of declarative, but not procedural, memories. Another study conducted by Nishida and Walker (2007), states that NRAM sleep can improve motor memory consolidation. It was demonstrated that increase in sleep-spindle activity in central regions of the learning hemisphere, relative to more non-specific activity in the non-learning hemisphere.
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Sleep spindles based studies. Sleep spindles are another possible mechanism for sleep to directly enhance memory. Schabus et al. paper (2004), were provided evidence for the involvement of sleep spindle activity in declarative memory task consolidation. The results suggest that the relationship between memory performance and spindle activity is not an indirect effect of sleep-stage durations. Even when all sleep stages are controlled, the correlation between memory performance and spindle-activity changes remains significant.
Lastly in Rieth et al. (2010) study, contrary to prior studies reporting benefits from sleep, they found that no sleep-specific improvements were fall on pursuit motor performance compared with a quiet rest control even when using an implicit motor task. For all pattern and spacing conditions, performance either did not change between sessions, or changed identically for sleeping and resting participants.
In conclusion, based on literature review it was demonstrated that connection between sleep and memory exists. Moreover according to sleep deprivation studies, it was found that NREM sleep supports perceptual motor skills consolidation, REM sleep support cognitive skills, semantic memory, perceptual representation system and short-term memory, and both SWS and REM helps to consolidate information which connected to sensory-perceptional.
Based on learning task approach, researchers found that, memory-consolidation is a two-stage model, and NRAM sleep supports motor memory and declarative memory consolidation.
Present studies based on sleep spindles states that there are no changes in any memory tasks improvement during sleep spindle activity.
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