Sleep-Dependent Memory Consolidation

Memory consolidation is a time window in which new memories are stabilized after initial acquisition. During this process, memories become resistant to interference and are converted into long-lasting optimally integrated memories. The memories afterwards are less prone This wiki analyzes the relationship between sleep and memory consolidation. Following a brief introduction into the mechanics of sleep and memory, there will be a focus on the contemporary theories concerning the role of sleep in memory consolidation.

Sleep and Memory: An Introduction

Sleep in mammals is characterized primarily by behavioral inactivity together with distinct electrophysiological changes in brain activity. Despite controversies surrounding the function of sleep in general, the last two decades has seen an upsurge in literature supporting the importance of sleep for memory consolidation. (4)

Memory encompasses the stages of acquisition, consolidation, and retrieval. Following the initial encoding of a memory, a series of alterations develop over time that stabilize and enhance the initial memory representation. These fragile memory states transform into more permanent ones, available for continued reactivation recall over extended periods of time. The extent to which sleep contributes to this process is unresolved, and its complexity lies in the multiple memory systems underlying the processes.

Sleep Cycles

Sleep progresses in five distinct cycles, prompted by natural cycles of activity in the brain. The cycles consists of two basic states: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, which consists of Stages 1-4. When measuring the brain activity of each cycle, researchers generally rely on electroencephalograms (EEG) to differentiate between the brain waves exhibited during each stage. Typically, each stage lasts from 5-15 minutes, with a full consisting of a progression from stages 1-4 before REM sleep. In humans, together the NREM and REM sleep alternate throughout the night every 90 minutes.

Stage 1: The first stage of sleep is characterized by theta waves, which lats roughly 10 minutes. In this stage, a person can be easily woken, but generally experiences deep levels of relaxation.

Stage 2: Stage 2 also contains theta waves and is still considered light sleep. This stage is characterized strongly by physiological changes as well as random short bursts of increased frequency, or sleep spindles. Specifically, body temperature, heart rate, and breathing slow down alongside brain activity.

Stage 3: Stage 3 is considered the beginning of deep sleep, also known as slow wave sleep (SWS). During this stage, brain waves transition to delta waves, slower in frequency than theta.

Stage 4: This stage is characterized by the deepest sleep of the night. Brain waves continue to show delta waves and, as such, it is quite difficult to wake someone during this stage.

Stage 5: Also referred to as REM sleep, this stage is a period when most dreaming occurs. During REM sleep, blood flow, breathing, and brain activity increases, a highly interesting phenomenon considering its similarity to the first stages of sleep. This stage of sleep is accompanied by the a sudden and dramatic loss of muscle tone and a paralyzed skeletal tone. (1)

Memory

There is no clear consensus at this time on how many memory systems there are, and how they should be defined, either in terms of information content or brain structures involved in their storage. The most widely accepted taxonomy divides human memories first into declarative and non declarative, based on their accessibility to “conscious recall” and then into finer subdivisions.

Declarative memories are further divided into episodic memories, that is, memories of specific events, and semantic memories, memories of general information. Current theories of declarative memory emphasize the critical importance of structures in the medial temporal lobe, most especially the hippocampus. Non-declarative memories are also divided into subcategories, such as procedural skills, conditioning, and implicit memory. (2)

The difficulty in distinguishing among these categories lies in their conjoined role in real life scenarios. In learning languages, a combination of memory sources, ranging from non-declarative memory for procedural motor programs to articulate speech, to memory of grammatical rules and structure, through to aspects of declarative memory for the source of word selection is involved.(3) The range of equal involvement clearly demonstrates that memory cannot be treated as a set of categories, each with its own set role, instead possessing a range of mechanisms that contribute to a task.

Consolidation

At this time, the processes taken after initial encoding, and especially the specifics as to the role of consolidation. Classically, the term memory consolidation refers to a process, as mentioned above, whereby memory becomes increasingly resistant to interference in the form of either competing factor or decay through time. Recently, the definition has extended to include enhancing the properties of the memory. In this modern interpretation, the stabilizing/resistance stage appears to be sorely limited to waking hours. The second, the enhancement stage, appears to occur primarily during sleep. (3)

Studies of Sleep and Memory

As a portion of the steps in consolidation occur exclusively during the sleep cycle, researchers are now focusing on identifying those types types of memory, and, for each, the individual steps that indicate strong sleep-dependent activation.

Specific stages of sleep appear to be critical for discrete steps in the consolidation of various forms of memory. Stabilization of some forms of procedural memory, for instance, can develop across 3-6 hours of wake. In contrast, the enhancement of procedural sensory and motor memories has been discovered to depend on overnight sleep. Consolidation of motor skills has also been connected to the sleep cycle, most specifically to NREM sleep stages, stage 2 in some cases and slow wave sleep in others. (3) These results leave little doubt that sleep contributes to the consolidation of memories, especially their enhancement. But, the details of these procedures remain unclear. In the following sections, let us explore the relation between various motor and cognitive tasks and their connection to sleep.

Spatial Retrieval

Several studies have supported the sleep dependent enhancement of a spatial discrimination task. Moreover, reinstating a learned concept during slow-wave sleep enhances retrieval of spatial information learned in that context. In this task, subjects were taught to associate each of 50 unique object images with a location on a computer screen prior to a nap. Each object was paired with a characteristic sound delivered over a speaker. For the entirety of the nap, white noise was presented at an unobtrusive intensity (~62dB sound pressure level), and during non-REM sleep the sounds for 25 of the objects were presented, with white-noise intensity lowered so that overall levels were approximately constant.

Upon waking, subjects viewed all 50 objects and were instructed to position each one on its original location. Absolute distance measurements indicated that object placements were more accurate for objects that were cued by their sounds during sleep that for those not cued. EEG recordings further provided information for determining sleep stages. (5)

Visual Perception Learning

Karni et al. demonstrated that learning on a visual texture discrimination task improves significantly following a night of sleep. They further established that selective disruption of REM but not NREM sleep resulted in a loss of these performance gains. (6)

In another study, Gais et al. Selectively deprived subjects of early sleep (SWS) or late night sleep (REM cycle) and concluded that consolidation was initiated by SWS related processes, while REM sleep then promoted additional enhancement. (7) Through a similar experiment, Stickgold illustrated that these enhancements are specifically sleep and not time dependent and are correlated positively with the amount of both early night SWS and late REM sleep. Importantly they also showed that these performance benefits were dependent on the first night of sleep following acquisition. (8)

Biological Studies

Over the last several years, as evidence of sleep’s role in learning and memory consolidation has grown, several researchers have debated the existence and reliability of this relationship. However, behavioral studies on animals as well as genetic screens has attempted to bridge the gap between the subjectivity found in human subjects and unknown aspects on the role of sleep on memory consolidation.

At the molecular level, significant number of genes appear to be up-regulated specifically in brain tissue during sleep, and at least one immediate early gene related to synaptic plasticity, zif-286, is up-regulated during REM sleep expressly in response to environmental or direct electrical stimulation of the hippocampus. In rats, patterns of neuronal activation expressed during waking exploration reappear during subsequent sleep, and, in humans, patterns of regional brain activation seen during daytime task training are repeated during subsequent REM sleep. (3)

At the electrophysiological level, studies in rats have shown that retention of learning of a shuttle box avoidance task increases subsequent P wave density and is strongly correlated with this increase, while in humans, spindle density increases following training on a declarative memory task, and, again, this increase correlates with subsequent improvement on the task. (3)

At the behavioral level, animal studies have found robust increases in REM sleep following task training and decrements in performance after REM deprivation, even when retesting is delayed until a week after the end of deprivation. In contrast, several animal studies have failed to find evidence of either increased REM sleep or deterioration following deprivation. Most likely this reflects a combination of methodological problems and conditions under which consolidation is, in fact, not sleep dependent. Similarly, human studies have provided examples where increases in REM sleep are seen following training, where REM, SWS, or stage 2 NREM deprivation diminishes subsequent performance, and where overnight improvement correlates with REM, SWS, or stage 2 NREM sleep. (3)

Conclusion

Several questions remain on the role of sleep on memory consolidation, most importantly: what types of memory are consolidated during sleep? Although there are some studies indicating evidence for some forms of procedural learning being consolidated and more for forms of declarative memory, the rules determining which are consolidates remain unclear.

In the end, the debate lies not in whether sleep mediates learning and memory consolidation, but instead, how it does so. As such, a more comprehensive understanding of the influence of sleep in memory processes is necessary to reveal the underlying mechanisms of sleep in memory consolidation.

References

1. http://web.mst.edu/~psyworld/sleep_stages.htm Stages of Sleep

2. http://www.nature.com/nature/journal/v437/n7063/pdf/nature04286.pdf Robert Stickgold Sleep-Dependent Memory Consolidation Nature Vol. 437 (2005)

3. http://www.sciencedirect.com/science/article/pii/S0896627304005409 Matthew P. Walker Sleep-Dependent Learning and Memory Consolidation Nature Vol.44 (2004)

4. http://inside.bard.edu/~luka/documents/sleepconsolidation.pdf Lisa Marshall and Jan Born The Contribution of Sleep to Hippocampus-Dependent Memory Consolidation TRENDS in Cognitive Sciences Vol.11 No.10

5. John D. Rudoy Strengthening Individual Memories by Reactivating them During Sleep Science (2009).

6. Karni et al. 1994 A. Karni, D. Tanne, B.S. Rubenstein, J.J. Askenasy, D. Sagi Dependence on REM sleep of overnight improvement of a perceptual skill Science, 265 (1994), pp. 679–682

7. Gais et al. 2000 S. Gais, W. Plihal, U. Wagner, J. Born, Early sleep triggers memory for early visual discrimination skills, Nat. Neurosci, 3 (2000), pp. 1335–1339

8. Stickgold et al. 2000a, R. Stickgold, L. James, J.A. Hobson, Visual discrimination learning requires sleep after training, Nat. Neurosci, 3 (2000), pp. 1237–1238 a