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

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.

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. 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. (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. File:MemoryCategories.png 

Declarative memories are further divided into episodic memories, that is, memories of specific events, and semantic memories, memories of general information. 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)

References

1. [1] Stages of Sleep

[2]

Robert Stickgold Sleep-Dependent Memory Consolidation Nature Vol. 437 (2005)