Memories we acquire during our day-to-day lives are malleable, and by no means consolidated forever. The theory of reconsolidation explains that reactivating a memory during wakefulness destabilizes memories. It also assumes that reactivating memories during slow wave sleep, or SWS, consolidates memory.
A study published in the January 23 issue of Nature Neuroscience by Susanne Diekelmann and colleagues reports that while memories reactivated during waking enter into a malleable state, those reactivated during sleep remain stable. In other words, sleep protects memories from being tampered with.
Not surprisingly, reactivating memories in study participants involves different brain regions, depending on whether they are awake or asleep. Reactivation during wakefulness activates areas of the prefrontal cortex, the front-most area of the brain. On the other hand, recalling memories during sleep mainly activates areas in the back of the cerebral cortex, along with the hippocampus, a structure critical to memory, deep inside the brain.
If memories are reactivated, they must be stabilized once again if they are ever to be recalled in the future. Reactivating memories during SWS produces this crucial stability. In their study, Diekelmann and colleagues showed that reactivating memories during sleep produces this renewed stability in synaptic connections, thus protecting memories against interference. They found that destabilization occurs at a much faster rate during sleep than in wakefulness.
In the study, researchers made participants recall memories during waking or sleep states, using auditory or olfactory stimuli. Using this technique, they found that memories did not enter their usual highly malleable state when they were remembered during sleep. Fascinatingly, this stabilizing effect occurred in the absence of subsequent REM (rapid eye movement) sleep, suggesting that REM sleep might not even be necessary for this process.
What causes this difference? Since during wakefulness, subjects are consciously aware of remembering, and are engaged in various activities at the same time, they may have unknowingly created new associations with the memory, explaining the labile phase of memory formation. It is also widely believed that the waking state and sleep state of the brain complement each other to update memories and strengthen memories alternately.
