There have been multiple studies that focus on the correlation between sleep and memory. We have been taught that poor sleep, especially among older adults, can lead to brain deterioration and extensive memory loss – even Dementia and Alzheimer’s. This month’s topic will focus on brainwaves during sleep and how NREM sleep can support memory strengthening and consolidation.
Slow, continual brainwaves during sleep must sync up at certain times to essentially hit the “save” button in our brains and allow new memories to occur. These oscillations occur hundreds of times per night during stage 2 NREM sleep. Slow oscillations (SO’s) are the synchronized activity of large populations of neurons that consist of alternating (low frequency) active and silent periods.
While young adults are typically able to process these brainwaves, the brainwaves in older adults don’t always make timely contact with spindles (quick, electrical bursts of neural oscillations). Sleep spindles are produced by the TRN (thalamic reticular nucleus) and other thalamic nuclei for approximately .5 seconds at a time. Only when the slow waves and spindles come together can the brain effectively accept new memories.
During slow wave sleep, hippocampal ripples are created. This is when the hippocampus generates high-frequency oscillations while neurons replay preceding activity in a consolidated method. Therefore, these ripples are responsible for reorganizing and strengthening information transfer and memory during resting periods.
Communication among these hippocampal ripples, SO’s and sleep spindles creates a hierarchy of information transfer necessary for memory retention (long-term memory).
As the brain ages, it can’t always provide coordination and synchronicity between the SO’s and spindles. This mistiming leads to memory loss, forgetfulness and impaired memory consolidation. The brain’s failure is typically due to degradation or atrophy of the medial frontal cortex, and deterioration within the frontal lobe disallows coordination between the slow waves and spindles.
The more severe the atrophy is, the more uncoordinated the brainwaves are. The medial frontal cortex is responsible for generating restorative sleep, thus prefrontal deficits result in impaired NREM sleep (not present in younger adults).
Several studies have utilized a combination of memory assessment, MRI, PSG and EEG to monitor electrical brainwave activity. EEG results have shown that spindles tend to peak early in the memory-consolidation cycle, and are therefore unable to sync up with the new SO’s.
New research suggests that a way to strengthen SO’s is to apply electrical brain stimulation to the frontal lobe. Boosting the brainwaves and increasing SO “power” overnight can indirectly harmonize sleep spindle activity and would restore deep sleep in older adults, potentially improving and retaining their memory.
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