We know sleep helps consolidate memories. Now a new study sheds light on how your sleeping brain decides what’s worth keeping. The study found that when the information that makes up a memory has a high value—associated with, for example, making more money—the memory is more likely to be rehearsed and consolidated during sleep.
The study involved 60 young adults who learned the unique locations of 72 objects on a screen while hearing characteristic object sounds. Each object was assigned a value indicating the reward that could be earned if remembered later. Recall was tested 45 minutes later, followed by a 90 minute break, during which participants either slept or remained awake. In the sleep condition, low-intensity white noise was played to mask any external sounds. In one condition, 18 of the sound cues associated with low-value objects were also repeatedly presented during the sleep period. In the wake condition, participants either watched a movie or performed a difficult working memory task (during which the sound cues were similarly sometimes presented in the background).
For all groups, at the first memory test, recall accuracy was significantly lower for low-value items compared to high-value (there was not, unsurprisingly, any difference between the groups). But let’s get to the important results. After sleep, in the absence of sound reminders, accuracy declined significantly more for low-value objects than for high-value objects. However, when sound cues had been played during sleep (although participants had no awareness of them), low-value objects were not differentially disadvantaged.
Interestingly, the sound reminders benefited not only those low-value objects which were cued, but all the low-value objects. But, in the wake condition, when sound cues had been softly played in the background, only those objects which had been cued benefited from the reminders.
Also interestingly, two participants who heard the cues during stage 2 sleep rather than slow-wave sleep received the least benefit.
What all this suggests is that covert reactivation may be a major factor in determining what gets chosen for consolidation, and wake and sleep reactivation might play distinct roles in this process - the former helping to strengthen individual, salient memories, and the latter strengthening, while also linking togther, categorically related memories.
The findings provide more weight to the idea that I have propounded before — that it’s worth consciously reviewing the day's memories that you want to keep, just before going to sleep.