Memory reactivation during sleep strengthens memories and reduces forgetting. This view is widely accepted in memory research. However, a recent study published in Learning & Memory has shown that memory reactivation, in the context of sleep disruption, can lead to forgetting for reactivated items.
Targeted memory reactivation (TMR) is a procedure that has repeatedly been used to study the mechanisms of memory reactivation. In a typical TMR experiment, a tone or an odor previously associated with a chosen task during awake learning is represented during sleep.
Reactivated memories are typically better remembered than those not reactivated, suggesting that memory reactivation during sleep can strengthen memories. However, it is important to note that TMR procedure should not wake the participants up or cause an arousal response that could disrupt sleep.
To investigate whether memory reactivation would lead to forgetting as a result of sleep disruption, the researchers behind the study conducted a TMR experiment with 24 participants without any sleep disorder recruited from Northwestern University (18–30-year-old, 11 male and 13 female).
Participants learned the location of 75 objects paired with sounds on a noisy background and then performed a memory test where they moved the objects from the centre of the screen to their correct location. Objects were then divided into three sets of 25, with one set cued with loud sounds, one set cued with soft sounds, and one set not cued during sleep. The TMR procedure was delivered during stage 2 and stage 3 of Non-Rapid Eye Movement (NREM) sleep, and it was followed by a second memory test, identical to the first.
The results showed that forgetting was significantly higher for cued objects compared to uncued objects, regardless of whether the reactivation cue was loud or soft. Moreover, the authors examined the effect of arousal during sleep on memory reactivation and showed that forgetting was greater for objects that were cued with arousal than for uncued objects, regardless of sound intensity.
Interestingly, participants’ subjective reports of hearing a sound during sleep did not reveal any differences in memory performance.
The study suggests that uninterrupted sleep is important for memory consolidation, and TMR procedures that don’t avoid sleep arousal may not actually improve memory. While the researchers acknowledge that other factors, such as sleep inertia, might have affected participants’ memory performance after sleep, they argue that reactivated and non-reactivated items should have been equally affected, and this was not the case.
Further research is needed to better understand how, and which factors can affect the impact of TMR-induced sleep disruption.
Overall, the study highlights the importance of uninterrupted sleep for memory consolidation and suggests that TMR procedures should be carefully designed and delivered to avoid disrupting sleep.
The study, “Sleep disruption by memory cues selectively weakens reactivated memories“, was authored by Nathan W. Whitmore and Ken A. Paller.