Sleep Essential for Learning and Memory Consolidation

What happens in the brain during sleep is still largely an enigma. However, researchers know that it is a vital process, affecting the body and the mind to continue to function. Current research as shown that sleep might play a crucial role in learning and memory consolidation. During sleep, the body cycles through rapid eye movement (REM) and the four stages of non-REM sleep. Slow-wave sleep is the deepest of the non-REM sleep phases and is the furthest removed from the normal brain wave patterns of wakefulness. During slow-wave sleep, brain waves consist of mostly delta waves, oxygen intake decreases, cortisol production ceases, and growth hormone is being released by the anterior pituitary gland. Slow-wave sleep is thought to be critical for consolidating declarative memory, which refers to the type of memory that recalls factual knowledge or experiences.

There are many other types of memory in addition to declarative memory. Plaford (2009) explains the differences between them in his book on sleep. Non-declarative memory refers to the things people learn to do, such as skills or procedures that can be performed without conscious thought. Semantic memory corresponds to knowledge of facts that are independent of space and time. Similarly, episodic memory corresponds to memories of specific moments occurring in a fixed space and time. Episodic memory tends to be easier to recall than semantic memory because of its relation to time and space as helpful cues. Memories begin as short-term memories, which can be stored in the hippocampus for about one to two years. The hippocampus then either discards or transports these memories to other locations in the brain, such as the visual cortex, where they are stored as long-term memories. Different types of memory appear to be consolidated best during specific stages of sleep. For instance, REM sleep is associated with the consolidation of procedural and emotional memories. In addition, REM sleep is also thought to be important for converting experiences into long-term memories. On the other hand, slow-wave sleep appears to be important for episodic memory consolidation in application to both verbal and spatial tasks (Plaford, 2009). Since time and space offer helpful, contextual clues that make memory recall easier, there is the possibility that other external context cues could be helpful for improving memory. Specifically, if these external cues are paired with information during wakeful learning and repeated during sleep, they would help improve memory and allow new information could be learned quickly and more efficiently.

According to Northwestern University’s study conducted by Antony, Gobel, Ottare, Reber, & Paller, (2012), there may be a link between sound stimulation and memory. In this study, sound stimulation was used to enhance learning during waking hours. Research participants learned how to play two artificially generated musical tunes with well-timed key presses. After the learning session, participants took a ninety-minute nap. Researchers played only one of the practiced tunes from the learning session to participants while they were in slow-wave sleep, the sleep stage most linked to the process of cementing memory. Testing done after the nap revealed that participants made fewer errors when pressing the keys to play the melody that they heard while they were asleep in comparison to the melody that was not played during their nap (Antony et al., 2012). These results show how sleep is important for strengthening memory of information that the brain has already learned. It also suggests that sound stimulation during sleep can enhance memory recall of prior information occurring during the following waking hours.

These results were supported by another Northwestern study in 2009, which tested how sound cues affect memory recall for visual information. As explained by Tremmel’s (2009) article, participants of this study were taught to associate each of fifty images with a specific random location on a computer screen. Each image was paired with a corresponding sound that played from a speaker. For instance, an image of a broken wine glass was paired with the sound of breaking glass. The locations for each image were learned by repeated trials until participants were able to place all of the objects in their correct assigned places. Then, forty-five minutes after completing the learning task, each participant was taken to lie down in a quiet, dark room for a nap. Electrodes attached to their scalp measured their brain activity, thus indicating when they were asleep. The same sound cues were played for participants in the experimental group while they were napping, but no sounds were played for the control group. Memory testing conducted after the naps revealed that participants who were played sound cues during sleep were better able to recall associated information while they were awake than those who had no sound cues during sleep. These results indicate that external stimulation, such as sound cues, can play an important role in reactivating memories so they can be stored more efficiently for skill-building during waking hours (Tremmel, 2009). Additionally, these results also suggest that learning a new language or skill can occur faster and easier if sound cues accompany daytime studies.

Sound cues are not the only kids of cues that can affect memory during sleep. A recent study by Rasch, Buchel, Gais, & Born, (2007) has found that smell also plays an influential role in memory. Odors are well known contextual retrieval cues for autobiographical and visuospatial memories (Rasch et al., 2007). According to Glynn (2012), when someone smells a pleasant aroma, they instinctually inhale deeply. In contrast, when someone smells an unpleasant odor, they cut their inhalation short and also might hold their breath. Interestingly, when responding to different smells, the brain acts exactly the same when it is asleep as it does while it is awake (Glynn, 2012). Additionally, an individual’s sense of smell is associated with some high brain areas. One such, the hippocampus is well known for its involvement in memory formation and storage (Rasch et al., 2007). In the study by Rasch et al. (2007), certain odors were presented to participants after they heard various tones while they were asleep. The odors caused participants to inhale more deeply even though they were not conscious of the presence of the odor. Pairing a sound with an olfactory stimulation is known as conditioning, and it will also cause people to make deeper inhalations after they hear the same tones even if no odor has been presented. Additionally, participants reacted the same way to the tones when they were awake, even though they were not conscious of their behavior. This experiment suggests a new way of modifying behavior during sleep since participants were able to transition the sensory information pairing to their wakeful states of consciousness (Glynn, 2012). Odor cues during slow-wave sleep activate the hippocampus much more than odor cues during wakefulness. Once an odor becomes associated with the context of learning object locations, the reapplication of the odor during slow-wave sleep will act as a context clue that reactivates the new memories, therefore boosting their consolidation. However, this conditioning did not seem to work if the odor was presented during REM sleep (Rasch et al., 2007). Other research suggests that for learned associations to be transferred from sleep to waking consciousness, the learning has to occur during the non-REM sleep (Glynn, 2012). Therefore, slow-wave sleep might be the key time for processing and consolidating memory.

In addition to external cues, dreams can also influence memory and the rate at which an individual is able to learn a new skill. Neider, Pace-Schott, Forselius, & Pittman, (2011) reported that the state of consciousness during dreams shares similar characteristics with the state of waking consciousness. In both dreaming and wakefulness, individuals are aware of objects, events, and themselves. However, some people have the ability to experience lucid dreams, in which they are completely aware that they are dreaming. Being able to consciously recognize dream states gives individuals the power to control the outcomes of their dreams. It is possible to train the brain to increase the frequency and degree of lucidity of lucid dreams. PET studies show that the executive lateral frontal areas and posterior multimodal association areas are deactivated during typical dreams. The deactivation of these areas may impair working memory by preventing an individual from comparing their ongoing experiences with their recent prior experiences. In contrast, polysomnographic studies, functional neuroimaging, and quantitative EEG show elevated levels of frontal cortical activity during lucid dreams. These ventromedial prefrontal regions support self-related, social, and emotional cognition in addition to facilitating decision making. The study by Neider et al. (2011) tested the frequency of lucid dreams with performance on a variety of assessments. Participants who reported having more lucid dreams scored better on the IOWA Gambling Task, an assessment that tests decision-making abilities (Neider et al., 2011). Neider et al.’s research opens the doors for lots of future studies. If the brain has elevated activity in these regions during lucid dreaming, it might be similar enough for wakefulness for individuals to learn new skills and information by rehearsing it in their dreams.

While there is still a lot of research to be done in order to gain a full understanding of what happens to the brain during sleep, these studies all offer substantial proof that sleep is an important part of learning and memory. The brain's ability to recall information can be improved if external context cues, such as sound, are paired with periods of learning and then repeated during the slow-wave phases of sleep. Additionally, the elevated brain activity during lucid dreams may be similar enough to wakeful consciousness to allow new information to be learned in this state. With sleep complications, the brain may be unable to create long-term memories, would result in impaired cognition.

References

Antony, J., Gobel, E., Ottare, J., Reber, P., & Paller, K. (2012). Cued Memory Reactivation During Sleep Influences Skill Learning. Nature Neuroscience, 15, 1114-1116.

Glynn, S. (2012, August 28). You Can Learn While You Sleep, Says Study. Medical News Today. Retrieved November 14, 2013, from http://www.medicalnewstoday.com/articles/249505.php

Neider, M., Pace-Schott, E. F., Forselius, E., Pittman, B., & Morgan, P. T. (2011). Lucid Dreaming And Ventromedial Versus Dorsolateral Prefrontal Task Performance. Consciousness and Cognition, 20(2), 234-244.

Plaford, G. R. (2009). Sleep and learning: the magic that makes us healthy and smart. Lanham, Md.: Rowman & Littlefield Education.

Rasch, B., Buchel, C., Gais, S., & Born, J. (2007). Odor Cues During Slow-Wave Sleep Prompt Declarative Memory Consolidation. Science, 35(5817), 1426-1429.

Tremmel, P. (2009, November 24). Brain listens, learns while we sleep. Futurity. Retrieved November 14, 2013, from http://www.futurity.org/brain-listens-learns-while-we-sleep/