Memory Consolidation and Efficiency as a Result of Sleep

Jim Wiltsee
Professor Grobstein
5/9/09
NeuroBio & Behavior
Web Paper III: Memory Consolidation and Efficiency as a Result of Sleep

    The two topics that I have previously wrote on were sleep paralysis and the effect of depression on memory.  After learning the fundamentals of memory and sleep, I now wish to learn more about how the two work together in memory retention and consolidation.  This is especially relevant to my life as I often debate whether to stay up late and study or go to bed earlier and wake up and study.  After researching this topic, sleep is important to the consolidation and effectiveness of memory. 

    New memories stay in a fragile state and need to be consolidated over time (Sterpenich).  One of the obscure questions is what exactly happens to memories during sleep.  At the cellular and molecular level, no one has seen a memory yet.  At the same time, nobody has strong evidence for the exact form memories take or how they are produced.  Lastly, how does post-encoding processing lead to the stabilization of memories or how they are integrated into larger networks of memories (Stickgold).  Looking at these three questions, researchers identified three main stages of memory processing: stabilization, which takes around six hours and is vulnerable to being lost, consolidation that occurs during sleep, and lastly, the recall phase where the memory is accessed (McPherson).   

    Recently, researchers at the University of Pennsylvania described how cellular changes in the sleeping brain promote the formation of memories.  Marcos Frank illustrated that the brain during sleep is fundamentally different from the brain while awake.  While the brain is asleep, the synaptic change that occurs, termed cortical plasticity, is the basis for memory formation.  Supporting this is a study that showed that neuronal connections in the visual cortex are remodeled during sleep, in an experiment using cats (McPherson, Jackson).  The making and breaking of connections between neurons formulate memories in response to life experiences.  The main cellular player is the N-methyl D-aspartate receptor, which acts as a listening post and gatekeeper.  Once the brain is triggered to reorganize its neural networks, a cascade of enzymes emerge that reorganize neurons during sleep (Marcos). 

    There is a two-step model of memory consolidation.  The first step involves the embedding of new knowledge in the network of previous knowledge in the hippocampus.  The second step is the initial formation transfer from the hippocampal to the amygdala complexes to neocortical sites during NREM slow wave sleep.  Hippocampal stimulation induces long-term potentiation, which generates sharp-wave ripples that are coupled with sleep spindles.  This coupling could be the method of communication between the hippocampus and neocortex (Clemens).  The integration of this information into existing memory networks in the neocortex occurs during REM sleep (Capellini).  The initial step occurs during high hippocampal gamma activity, when the hippocampus receives high sensory inputs.  The second step occurs during periods of low sensory inputs, which may serve to transfer information to the neocortex (Axmacher).   

    Evolutionary responses seem to support the hypothesis that sleep leads to memory consolidation.  An example of this is that the hippocampus is part of the brain that is highly involved in spatial memory processing and retention.  Studying birds, it is possible to see that hippocampal volumes and neurons are extremely high because birds exhibit the need for spatial memory.  On the other hand, patients with temporal lope epilepsy have been found to display impairments in sleep-associated memory consolidation (Clemens).  The brain structures that are associated to memory consolidation should be positively correlated with sleep duration.  It can be seen that the hippocampus, amygdala, and neocortex are highly active during sleep (Capellini).  Another evolutionary explanation that memory consolidation occurs during sleep is that processes that lead to synaptic decay are more active during sleep.  Therefore it is more energy efficient to consolidate memories during sleep than during periods of wakefulness when a larger amount of information is required to be translated (Axmacher). 

    Many studies also seem to support the hypothesis that sleep supports memory consolidation because sleep deprived individuals perform poorly in learning tasks compared to fully rested individuals (Capellini).  In an arithmetic task, sleep deprived participants performed worse and there was less activity in the prefrontal cortex.  In a verbal learning exercise, activity in the left temporal lobe decreased.  This region of the brain is the language-processing center (McPherson).  In another study that used emotional pictures by Sterpenich, subjects with sleep utilized parts of the frontal cortex, the parietal cortex, the occipital lobe, and the limbic areas around the amygdala and hippocampus.  Sleep deprived patients only recruited the ventral medical prefrontal cortex.  Recent studied on mammalian sleep duration have reported a positive relationships between rapid-eye-movement (REM) sleep and the volume of the brain, which may support the relationship that more sleep leads to a higher cognitive function (Capellini).

    Working memory is understood to be a cognitive system for both temporary storage and manipulation of remembered information.  It is often thought of as short-term memory.  It is regarded as a specific process by which a remembered stimulus is used to guide behavior in the absence of external factors.  It can be seen as your unconscious and your storyteller.  Much of its capacity is determined by its ability to filter out irrelevant information by the prefrontal cortex and basal ganglia.  A study by Kuriyama et. al. suggests that working memory could be based on neural plasticity and training could improve its ability.  They also found that improvements were only acquired across a night of sleep, but not over the same period of wakefulness. 

    A question that may be considered is whether not only sleep duration, but also does sleep intensity play a role in memory retention?  That is, can a more intense sleep have a more beneficial effect over the same time or less time than someone engaged in regular sleep?  Another question relating to memory transfer during low sensory input stages is whether memory transfer could occur during wakefulness.  Axmacher et. al. believes although sleep is the obvious state when there is low sensory inputs, it can occur during periods of rest and wakefulness. 

  Lastly, a question I want to explore whether a memory is fixed.  That is, since information is coded in more persistent molecular or structural formats, it has been assumed that once this process occurs, the memory is permanently "fixed" (McPherson).  I feel as though this cannot be the case because, as we learned in class, memories are stories that can be recalled.  Memories that are more consolidated become less vivid and slower when recalled (Axmacher).  Therefore, memories will be subjective to each person, so if for instance you and a friend remember an old memory and they differ, you both will change your memories so they converge toward one another, ultimately changing the “fixed” memory. 

    After expanding my knowledge of this topic, I feel as if memory consolidation is very similar to depression.  In both cases, there seems to be an emphasis on plasticity of the nerve synapses.  The brains ability to consolidate memories depends on the activity of specific regions of the brain, which studies have shown to be more active during sleep than during a wakeful state.  Without sleep, a person’s mind is unable to function as efficiently because it is unable to recruit the necessary regions of the brain specific to increasing cognitive ability.  Lastly, it seems as if the relationship between the brain and nervous system depends on the ever-dynamic change of firing neurons and synapses in role-specific regions of the brain.

Works Cited
Anon. "The Sleeping Bain." Journal o College Science Teaching 38 (2009): 12.
Axmacher, Haupt, Fernandez, Elger, and Fell. "The Role of Sleep in Declarative Memory Consolidation- Direct Evidence by Intracranial EEG." Cerebral Cortex 18 (2008): 500-08.
Capellini, McNamara, Preston, Nunn, and Barton. "Does Sleep Play a Role in Memory Consolidation? A Comparative Test." PLoS one 4 (2009): E4609.
Clemens, Molle, Eross, Barsi, Halasz, and Born. "Temporal coupling of parahippocampal ripples, sleep, spindles and slow oscillations in humans." Brain 130 (2007): 2868-878.
Kuriyama, Mishima, Suzuki, Aritake, and Uchiyama. "Sleep Accelerates the Improvement in Working Memory Performance." The Journal of Neuroscience 28 (2008): 10145-0150.
McPherson, F. 2003. The role of sleep in memory. http://www.memory-key.com/NatureofMemory/sleep.htm
Sterpenich, and Et. al. "Sleep Promotes the Neural Reorganization of Remote Emotional Memory." The Journal of Neuroscience 29 (2009): 5143-152.
Stickgold, Robert. "Sleep: The Ebb and Flow of Memory Consolidation." Current Biology 18 (2008): R423-425.