Looking back on yesterday, you may remember one conversation or an unexpected moment, while dozens of other details have slipped away. Sleep helps preserve some memories, but the brain may start choosing them while the experience is still happening.
In a study published in PLOS Biology, researchers identified a pattern of brain activity during learning that may act like a “tag,” marking certain information for later processing during sleep. The pattern was associated with what participants remembered after sleeping, a link that did not appear when they stayed awake.
“How the brain decides what is important to remember and what can be forgotten is still a mystery. These new findings help to answer that question. By uncovering for the first time a signature of neural activity which instructs the brain which experiences to process during sleep and form into long-term memories,” the study authors said in a press release.
Testing Memory After Sleep and Wakefulness
The study included 31 healthy adults aged 18 to 23. Each completed two experimental sessions, one with a two-hour nap opportunity and the other with two hours spent awake watching nature documentaries.
Participants studied 160 pairs consisting of an object and a word. To form a connection between them, they imagined a scene combining the two. Someone shown a football and the word “coffee,” for example, might picture a cup balanced on top of a football.
Explore All OptionsSensors placed on the scalp recorded electrical activity in the brain as participants learned. Researchers tested some of the pairs immediately and the rest after the two-hour nap or wake period. Using different pairs for each test helped prevent the first test from giving participants extra practice. They remembered more of the associations after sleeping, although the study did not show whether that advantage lasted beyond two hours.
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What Theta Activity Revealed During Learning
The signal identified during learning was theta, a brain rhythm that repeats between three and eight times per second. During the first second or so after each pair appeared, theta activity was stronger for information later remembered after sleep.
The researchers describe this theta response as a possible “tag,” a brief signal that may mark certain information for the brain to return to during sleep.
During deep sleep, slow waves set a steady rhythm, while sleep spindles appear as short bursts of electrical activity. Participants with stronger theta activity during learning had more instances in which these two sleep rhythms aligned. Those who had more of these moments remembered more associations.
Theta activity itself did not predict each participant’s total memory score, and the experiment cannot show cause and effect. The researchers use “tag” as a proposed explanation for the relationship between learning and sleep, rather than as a marker they directly observed.
Why the Brain Does Not Keep Every Detail
Forgetting may prevent passing details from competing with information that will be useful later.
“If we remembered everything that we experienced, our brains would quickly reach system overload,” the authors added in the press release.
The experiment cannot say why a real-life conversation, face, or event might receive that priority. Participants learned neutral word-object pairs, so the study did not test how emotion, personal meaning, or future importance affects the process.
Although the experiment did not include people with depression or test any treatment, future research could examine whether a useful system for choosing what to remember sometimes begins giving negative experiences too much weight.
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Article Sources
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- This article references information from a study published in PLOS Biology: Theta oscillations tag episodic memories for sleep-dependent consolidation
