Overview of Sleep and Memory

Overview of Sleep and Memory

• Sleep is a naturally recurring state characterized by suspended sensory and motor activity, often with total or partial unconsciousness.
• Sleep differs from wakefulness due to a reduced ability to respond to stimuli, and it's more easily reversible than a coma.
• During sleep, the brain demonstrates distinct patterns of brain wave activity, including dreaming.
• Polysomnography (PSG) is used to examine brain activity during sleep by analyzing EEG (brain activity), EOG (eye movement), and EMG (muscle activity).
• Sleep is not a dormant state; the brain displays oscillatory patterns.

Definition of Sleep

• Sleep is a repeating state of relatively suspended sensory and motor activity, characterized by total or partial unconsciousness.

Brain Activity During Sleep

• Polysomnography (PSG) aids in studying sleep-related brain activity.
• Electroencephalography (EEG) measures brain electrical activity.
• Electrooculography (EOG) detects eye movements.
• Electromyography (EMG) records muscle activity.
• While asleep, the brain exhibits oscillatory patterns.

NREM vs REM Sleep

• N1: Transition from wake to sleep (5-10% of sleep); eye rolling is common during this stage.
• N2: Light sleep stage characterized by sleep spindles and K-complexes (45-60% of sleep).
• N3: Deep sleep or slow-wave sleep (SWS) dominated by slow oscillations and δ waves (1-4 Hz) (20% of 30 seconds, encompassing 15-25% of sleep).
• REM: Characterized by rapid eye movements (REMs), low-amplitude mixed-frequency EEG, and muscle atonia (20-25% of sleep).

Power Spectrum and Brain Oscillations

• Brain activity involves oscillations at various frequencies.
• A power spectrum visualizes the different frequencies in brain activity.
• Wake, NREM, and REM phases have distinct power spectra.

Slow Oscillations (SOs)

• SOs are characterized by widespread, high-amplitude, and low-frequency cortical oscillations.
• The average frequency is 0.8 Hz.
• SOs feature up and down states: positive half-waves (up-states) show high neuronal activity, while negative half-waves (down-states) indicate neuronal silence.

Sleep Spindles

• Sleep spindles are characterized by a waxing and waning waveform.
• They arise from thalamo-cortical loops.
• Frequency range is 12-16 Hz.
• They're usually a hallmark of N2 sleep, but can also occur in SWS.
• Sleep spindles are linked to synaptic plasticity and communication between different brain regions.

Function of Sleep

• Sleep is associated with energy conservation and restoration.
• Memory consolidation is a crucial process during sleep, strengthening newly acquired memories for later access.
• Sleep is linked to avoiding danger and conserving energy.

3 Phases of Memory

• Memory involves three stages: encoding, consolidation, and retrieval.

Memory Systems

• Memories exist on different time scales (short-term and long-term).
• Long-term memories (LTM) are comprised of varied domains often mediated by specific brain areas.

Declarative vs. Non-declarative Memory

• Declarative memory encompasses episodic memory, paired-associate learning, and object location tasks, which can be verbally described.
• Non-declarative memory includes finger tapping, mirror tracing, and other skills that can be demonstrated but not easily put into words.

Forgetting Curve

• Hermann Ebbinghaus' research showed how memory retention declines over time, not due to memory decay but more due to interference, the replacement of old memories with new ones.
• Ebbinghaus studied memory retention using meaningless syllables.

Basic Study Design

• Memory tasks are administered before and after sleep to study the effects of sleep on memory consolidation.
• Memory is assessed both immediately and after a delay to analyze potential changes.

Sleep Study Designs

• Various sleep study designs are used, including 12-hour sleep/wake cycles and sleep deprivation conditions. Control groups are essential in sleep studies.

Improvement vs. Forgetting

• Sleep often leads to improved memory performance compared to wakefulness due to better memory consolidation.
• Sleep also reduces forgetting by minimizing interference with new memories.

Consolidation Theories

• Synaptic homeostasis theory explains how sleep regulates synaptic strength after waking, through either strengthening or pruning synaptic connections, particularly those strengthened due to learning.
• Sleep promotes homeostasis by globally reducing synaptic strengths through mechanisms involving slow-wave activity. This allows for prioritization of important information.

Learning-dependent SWA

• Studying learning impacts on slow-wave activity (SWA) is done via visuomotor tracing tasks.
• Immobilisation of the arm before sleep reduces SWA.
• Increases in SWA in motor areas, predict subsequent improvements in task performance.

SO Enhancement

• Sleep oscillations (SOs) are linked to memory consolidation, although further research is needed.
• Non-invasive brain stimulation can mimic sleep-like brain oscillations, potentially enhancing memory functions.

Pharmacological Manipulation of Spindles

• Research into memory enhancement sometimes uses drugs affecting sleep spindles (e.g., Sodium Oxybate, Zolpidem).

Closed-Loop Auditory Stimulation

• Auditory stimulation synchronized with sleep oscillations could potentially enhance memory encoding.

SOs, Spindles, and Ageing

• Older adults often have decreased sleep quality and SOs, which correlates with decreased memory benefit from sleep.

Reactivation & Replay

• Memory replay/reactivation involves reactivating information during sleep, strengthening memories.
• In rodents, replay is linked to hippocampal ripples.

Contextual Memory Reactivation

• Memory reactivation can be elicited by cues linked to the context when memories were originally formed. This is relevant to externally driving memory reactivation during sleep.

Reactivation During Wakefulness

• Research examines reactivation during wakefulness with potential applications in psychotherapy and learning enhancement.

Multivariate Pattern Analysis (MVPA)

• MVPA analyzes EEG data for patterns related to different cognitive tasks (e.g., object vs scene processing).

MVPA and Memory Reactivation

• Combining MVPA and memory reactivation helps research see how specific brain signals correlate with recall during encoding, potentially revealing how memory is consolidated in the brain.

Sleep and Encoding

• Sleep is sometimes beneficial before encoding, as it may clear the hippocampus, allowing for better storage and encoding of new information. The pre-encoding sleep benefit is noted in enhancing encoding abilities.

Explore the fascinating relationship between sleep and memory in this quiz. Delve into the unique brain activity patterns that occur during sleep, including the roles of polysomnography and various measurement techniques. Test your knowledge on sleep characteristics and its importance for memory consolidation.