Sleep stabilizes and strengthens memory through a phenomenon known as sleep-dependent memory consolidation. The spindles, a type of oscillation typically observed during sleep, have repeatedly proven their importance in consolidating motor skills in young people. Sleep-dependent motor memory consolidation appears to be impaired in the elderly, and spindle dysfunction may be one of the causes. This phenomenon remains largely unexplored. On the other hand, stroke is one of the leading causes of disability in adults, with around two million new strokes each year. Motor deficits, sleep architecture, and oscillation changes are commonly reported after stroke and persist in the chronic stage (~6 months post-stroke and more). Sleep-dependent memory consolidation post-stroke has been little studied and the results obtained diverge. The main reason for these discrepancies is that only a few studies have investigated sleep oscillations in great depth, and none have estimated their contribution to post-stroke memory consolidation. Interestingly, sleep spindle characteristics have been shown to change in populations with motor slowing (e.g., older adults), and with motor deficits, (e.g., individuals who have suffered a stroke); and could hypothetically be at the root of the motor alterations.
Thus, the application of sleep-targeted spindle interventions could open up new treatment avenues for improving motor function in healthy older adults and after stroke. To determine a novel intervention strategy targeting these deficits, we applied spindle-like oscillatory electrical activity using non-invasive open-loop brain stimulation during sleep to improve sleep-dependent motor memory consolidation in healthy older adults and individuals after stroke.
The results showed that spindle stimulation did not significantly influence the consolidation of sleep-dependent motor memory in healthy older adults and individuals after stroke. Sleep oscillations including spindles were not improved by stimulation as well. The third study takes advantage of the lack of effect of stimulation to understand how a stroke alters sleep physiology (e.g., spindles), sleep-dependent motor memory consolidation, and the contribution of sleep oscillations in motor memory consolidation processes. Results showed that people with stroke spend more time in light sleep, and produce as many spindles as older people, but their duration is longer. Motor performance is consolidated differently after a nap or a night's sleep: it decreases after a nap and stabilizes after a night's sleep; similarly, in healthy elderly people and in those who have suffered a stroke. Sleep spindle density helps limit nap-related memory loss in both populations, while sleep-related memory consolidation is associated with different processes. After a stroke, longer spindle durations support memory consolidation and thus appear to be the result of a compensatory physiological mechanism to stabilize memory.
In conclusion, this thesis demonstrates that spindles are involved in sleep-dependent motor memory consolidation in healthy elderly people and in those who have suffered a stroke. The findings extend existing knowledge of sleep-dependent memory consolidation and sleep physiology in normal aging and after a stroke.
