How does the brain transform our memories during sleep?
About the author: Liz Siefert
Liz is a 4th year PhD candidate working with Dr. Anna Schapiro and Dr. Brett Foster. She is interested in how our memories change and shift overtime, and the role of arousal states (wake, sleep) in these changes.
or technically,
Memory reactivation during sleep does not act holistically on object memory
[See Original Abstract on Pubmed]
Authors of the study: Elizabeth M. Siefert, Sindhuja Uppuluri, Jianing Mu, Marlie C. Tandoc, James W. Antony, Anna C. Schapiro
Whether you’re a human, dog, or fruit fly, we all sleep. There’s no doubt that sleep is essential, but why we sleep is still hotly debated. Many ideas have centered around the observation that sleep is important for strengthening key memories and putting them into storage. Neuroscientists believe this happens during sleep when the brain replays memories to strengthen them, a process called memory reactivation. Supporting this idea, numerous studies have shown that people tend to do better if they take even a short nap between learning something and being tested on it.
NGG student Elizabeth Siefert was fascinated by the process of memory reactivation during sleep, but wanted to better understand what impact it has on our memories. She noticed that sleep can’t just be improving memory overall, because that’s not how memory works. “As we go about our lives, our memories don’t always continue to improve,” says Siefert. “Often, they get worse, or they change in different ways. So really, across time, memory isn’t just improving, but it’s transforming.” Following on these observations, Siefert designed an experiment to understand whether sleep simply improves memory or rather has the power to transform it by strengthening some aspects of a memory while allowing others to fade.
Studying the sleeping brain
To probe the nature of memory transformations during sleep, Siefert and her team needed a memory test that allowed them to assess different aspects of memory. “Memories have lots of different features, so we wanted to know if memory reactivation has the power to act on different features of our memories in different ways,” says Siefert. She did this by asking participants to learn the identities of several satellites belonging to three groups. The satellites were created by mixing different parts so that the team could control the different features of a memory (Figure 1). Some shared features appeared on multiple satellites within a group, but never on satellites in other groups. Other unique features were specific to just one satellite, allowing the participants to identify it by name. This allowed the team to look at how representations of the individual versus the shared features were transformed in memory during sleep. Siefert asked the participants to learn the satellite names and groups, had them take a nap, and then tested how well they remembered the unique versus shared satellite features.
Just because participants took a nap doesn’t mean they were necessarily going to replay their memories of the satellites. That’s why Siefert stepped in to nudge their sleeping brains to replay the memories she was interested in. She did this using a method called targeted memory reactivation (TMR). During TMR, experimenters measure a participant’s brain activity while they sleep and look for moments when the sleeping brain is most likely to replay a memory. When those moments are identified, the experimenter plays a cue to remind them of a recent memory and encourage the brain to replay that specific memory. The cues are played softly enough that they don’t wake up the participant, and participants don’t know that the cues are being played, so any impacts of TMR are unconscious. “The best method we know in humans for studying memory reactivation in sleep is TMR,” says Siefert. “Our sleeping brain is already prioritizing certain information, and what we’re doing with TMR is biasing the brain to prioritize what we want it to.” In Siefert’s experiment, she played the name of some of the satellites the participant had just learned to encourage reactivation of those memories.
Figure 1. Satellites used in the memory task. Satellites belonged to three groups: alpha, beta, or gamma. The purple and orange boxes highlight the shared and unique features of the satellite volar. The purple boxes show volar’s features that are shared with other satellites in the alpha category, while the orange boxes show the features that are unique to volar.
Clarifying the relationship between sleep and memory
With all the pieces in place, Siefert was ready to ask how memory replay during sleep impacted different features of memory. Before the nap, participants tended to do a better job learning the satellites’ unique features compared to their shared features. In other words, they were better able to identify that a particular feature belonged to an individual satellite than that a particular feature was shared by satellites of a certain group. After a nap with TMR, that divide only widened. Siefert found that TMR increased participants’ memory for unique features while decreasing their memory for shared features. For satellites whose names were not used as cues during sleep, there was no impact on memory. This demonstrates that the effects were likely due to the reactivation encouraged by TMR. “That showed us that memory reactivation during sleep, it’s not just wholistically improving our memory, it has the power to act on specific features of our memory in very specific ways,” says Siefert. “It [can] even impact different features of our memory in different ways, such as improving some even at the cost of others.”
Why were certain features strengthened while others got weaker with reactivation? One possibility is that the cues used to trigger replay during sleep were the satellite names. This may have encouraged the brain to prioritize information about the individuals’ identity rather than their group membership. Future studies could use the group names as a cue instead and see if that nudges the brain to prioritize shared over unique features. Another possibility is the fact that people already tended to learn the unique features better before the nap. That prioritization may have carried over into their sleeping brains. “Your learning strategy and goals before sleep might bias what type of information the sleeping brain wants to reactivate,” says Siefert. “It’s possible that it was those learning strategies that led that information to be benefitted more by sleep.” Importantly, no matter what the explanation, it’s not the case that unique features will always be remembered over shared features following a night of sleep. Instead, a complex combination of learning goals and strategies likely shape exactly how memories are transformed during sleep.
Unbeknownst to the participants, Siefert was also testing how the way she presented the cues during sleep impacted memory. Sometimes she played the same satellite name over and over in a block, and other times she played satellite names intermixed with each other. Importantly, she always played the satellite names the same number of times during sleep, only changing in what order she played them. While both methods led to improved memory for unique over shared features, repeating the same satellite name many times in a row was more effective in transforming memory than playing the names in a random order. Siefert suggests that this may be evidence that reactivating memories in blocks helped the brain differentiate things in memory more than random reactivations.
The lab is already extending their results to understand more about the relationship between sleep and memory. Specifically, they’re interested in how sleep may help us take new information and incorporate it into older memories. “A study that we’ve run in the lab since [mine] is trying to understand what it looks like to learn new information that is aligned with things that you know from the past and how that new information can become integrated into older memories without totally overwhelming those old memories,” says Siefert. It seems that we’re only scratching the surface of what TMR can teach us about the sleeping brain’s relationship to memory.
What does this mean for everyday life?
After learning about these results, you might wonder whether students should give up on wakeful studying and just play their textbooks while they sleep. Unfortunately, it’s still not that easy. “Because [memory reactivation] doesn’t just improve memory and has this transformation component, if you took this device home and used it to play a textbook it’s not clear to me whether it would improve or hurt your memory of that information,” says Siefert. “You might need specific learning goals, you would need to think carefully about when you are delivering the cue, and you need to consider lots of other things.” It may be possible to design a system that could help a student study in their sleep, but we still need to learn a lot more about how TMR works before that will be possible.
Despite its complicated nature, some scientists are optimistic that they may be able to bring TMR to your bedroom. Rather than recording brain activity to target specific moments for reactivation, they aim to use audio recordings of movement during sleep to target the longer periods of sleep when they think replay naturally occurs. “For me in the lab it’s really important to target specific moments so I really know what my cues are doing, but in the real world that might be less important,” says Siefert. TMR is already being used in some clinical settings to help stroke patients relearn how to move their bodies, and Siefert suggested that it could one day be helpful for things like language learning if you’ve already learned some of the basics. For those who may be worried about TMR being used for brain washing or unconscious influence, Siefert says we aren’t capable of that now and may never be. “Your brain is already doing things and we’re just biasing it to little quick moments,” says Siefert. “We aren’t at the point where we can totally change the way that you’re thinking about something.”
Even in the absence of a TMR system on your nightstand, one takeaway is clear: sleep is important. “We don’t have a good understanding of why we’re remembering and forgetting certain things but knowing that the brain is selecting information means that that selection is probably important, and that selection is clearly happening during sleep,” says Siefert. “That means you should be getting good sleep, because you want to allow your brain time to process the information.” So next time you’re faced with the decision to stay up a little later studying for a test or prepping for a meeting, remember that choosing to sleep may be even more important than putting in that extra half hour of work.
About the brief writer: Catrina Hacker
Catrina Hacker is a PhD candidate working in Dr. Nicole Rust’s Lab. She is broadly interested in the neural correlates of cognitive processes and is currently studying how we remember what we see.