Analysis of The Different Ways in Which Sleep Affects Memory

Introduction

Everyday we encounter multiple situations where we rely on past knowledge or a quick understanding of the circumstances to make decisions. The ability to create and access memories is vital to a person’s adaptation to the dynamic world with which we interact. Memory is our ability to encode, store, retain and subsequently recall information and past experiences in the human brain. “If past events could not be remembered, it would be impossible for language, relationships, or personal identity to develop.” There have been many studies over the last few decades which indicate that sleep improves memory retention. Some of these portray, that sleep has a role in strengthening memories by shielding them from interfering stimuli, Whereas others emphasize on the role of sleep in the consolidation of memories to make them long-lasting. Through this paper, we will be studying and analysing the different ways in which sleep affects memory.

Stages of Memory

Memory functions primarily consist of three sub-processes, i.e. Encoding, Consolidation, and Recovery. Encoding is the process through which our brain processes all the stimuli we encounter. It is thought that each new stimulus that we encounter, results in the formation of a new memory trace. A memory trace is a unit of cognitive information inside the brain, theorized to be the means by which memories are stored. These memory traces formed during encoding haven’t been stabilised and are volatile. If they are not given the opportunity to consolidate, they may be lost, which results in “forgetting of these memories”. At this stage the memory traces are very susceptible to be disrupted easily, either due to external influences or proactive interference by older memories.

Next, Consolidation is the process by which the memory traces generated during encoding are stabilised. They are slowly integrated, by waves of consolidation processes, into the long-term memory and become available as resources Consolidation is usually considered to consist of two sub-processes, synaptic consolidation and system consolidation. Synaptic consolidation occurs in the time immediately after the memory trace is formed in encoding. Through system consolidation memories in the hippocampus are slowly made independent of it in the time that follows. In consolidation, the strength of the synapse (connection between 2 neurons) increases, through potentiation. This happens when a pathway is used repeatedly, thus making it more likely that the connected neurons will fire together. “At the neuronal level, memory formation is thought to be based on the change in the strength of synaptic connections in the network representing the memory.”

It is presumed that the waking brain is configured for acute external input processing involving the encoding of new information and memory retrieval, Thus when we are awake, our senses are constantly occupied due to the abundance of stimuli and encoding and retrieval are given priority. “Sleep is defined as a natural and reversible state of reduced responsiveness to external stimuli and relative inactivity, accompanied by a loss of consciousness.” The reduction in input while we are asleep is crucial to the consolidation of memory. This effect of sleep, protects the memory from retroactive interference due to the lack of formation of new memories. Retrieval refers to the access and recall of already stored memories.

The main concern with memory is that we have a limited capacity and therefore seem to forget or misremember things that we learn. There are several theories which speculate why 'forgetting' or loss of memory occurs. One of the earliest studies which indicated the effect of sleep on memory, is the “forgetting curve” experiment done by Ebbinghaus on himself. It was observed that his retention ability was increased when he slept in the retention period. Over time, several studies were conducted which indicated the direct impact sleep had on memory, and several theories were made. Among these the Interference theory helps explain the effect in more detail. Interference theory proposes that newer memories hamper older memories and vice versa, which thereby result in memories not being able to be recalled. Jenkins and Dallenbach conducted a famous experiment involving sleep where participants’ retention capability was tested in the state of being awake and asleep. The results of this study showed that in the state of wakefulness, the amount of forgetting is increased due to the new stimuli that we perceive, which is a fundamental part of Interference theory.

Multiple experiments have been done which show the positive effect of sleep on memory, thereby examining also longer retention intervals in the short term. Studies such as (Ref. 2 and Ref 3.) have shown that sleep definitively provides a huge benefit in retention for at least a day. “The underlying concept was that sleep acts as a “temporary shelter” that simply postpones the effect of interference and, thereby, passively maintains the memory traces.” It is also observed that the interference effects are prominent in the time shortly after learning. The longer the trace waits to be consolidated, the more likely that it will be disrupted. In the next section we will look at the effect of different types of sleep on memory. Sleep is of two types REM and SWS. REM (Rapid Eye Movement) sleep is the type of sleep in which dreaming and other bodily movement occur. SWS (Slow Wave Sleep) refers to deepest phase of our sleep. It is the phase during which we are least active.

Memory is majorly classified into two types. The first is the Declarative memory, which primarily consists of facts and the stimuli we perceive. Non Declarative memory is the other major division of memory. It refers to the Procedural Memory, that we recall subconsciously. An example of Non Declarative memory is muscle memory. We will discuss Dual Process and Sequential Hypotheses of sleep in memory.

According to the dual processes hypothesis, different sleep stages help in consolidating different types of memories. It is thought that declarative memory profits from SWS, whereas the consolidation of nondeclarative memory occurs during REM sleep. Experiments for this theory are done by testing participants on different things after different types of sleep. 'Originated from Ekstrand and co-workers, who created the two types of sleep by dividing the participants into two groups, the early-sleep and late-sleep condition.' In the early-sleep condition, the participants, performed learning tasks in the evening and then slept for 3 hours and then their retention was measured. This type of sleep is associated with SWS and consolidation of Declarative memory. The late sleep condition is associated with REM sleep. The participants are asked to sleep for 3 hrs, then perform the learning tasks and then sleep again. The second sleep is more likely to be REM and thus helps in the consolidation of Non declarative memory.

According to the Sequential hypothesis memories acquired during wakefulness are processed during sleep in two successive steps respectively occurring during SWS and REM sleep. The first step acts as a filter. The memories to be remembered and the traces to be strengthened are chosen and consolidated. All the other information that we take in when we are awake are removed and disregarded in this step. The neuronal system decides which of the facts are important and which are not. The respective memory traces and pathways are strengthened as a part of consolidation. The second step of this process involves REM sleep. The processed and consolidated memories from before, are further consolidated and slowly added to the long term memory in this step. Basically the REM stage acts like a reinforced storage for the memories consolidated during the SWS stage. Experiments on these theories consist of tasks consist of comparing retention ability before sleep, after SWS (which occurs in the first quarter of sleep) and finally to retention ability after REM sleep.

Conclusion

From all of the studies and experiments conducted, is clear that sleep plays an integral part in the storing of memory. It does this in two ways, passively and also with an active role. In its passive role, due to the loss of consciousness, it reduces the encoding of new stimuli, thereby preventing them from retroactively affecting memories which are still being stabilised. The active role of sleep is associated with the stabilisation of memories. Studies indicate that sleep acts as a consolidation mechanism for memories and with several experiments and theories being proposed regarding the exact method in which it happens, with Dual process and Sequential Hypothesis being among the most popular.

References

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Citations

1. Eysenck, Michael (2012). Attention and Arousal : Cognition and Performance. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 978-3-642-68390-9. OCLC 858929786.
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4. Ebbinghaus H. Über das Gedächtnis. Untersuchungen zur experimentellen Psychologie . Darmstadt, Germany: Wiss. Buchges., 1992.
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