We do not remember everything we go through in a day, and that is a good thing. The brain chooses what to keep for longer, and a recent study shows that it does so at the moment of learning: it tags important information so that it can be strengthened during sleep. Read how researchers caught that moment in brain-wave recordings, and at the end take a short test to see whether you study in line with your memory.
Memories are not an exact copy of everything that happened. They are more of a curated reel of the most salient experiences. If the brain kept everything as it came, it would risk overload: it would be unable to tell critical information from the noise of everyday impressions.
Scientists have long suspected that a tagging process stands behind this filtering. Some memories get an “important” label at the very moment of learning and are prioritised when the brain works offline, for example during sleep.
Animal studies have provided compelling evidence that novelty, repetition and reward influence which memories are processed during sleep. In humans, direct evidence has remained elusive.
A study published in the journal ‘PLOS Biology’, carried out thanks to two EU-funded projects, MemOpt and SLEEPAWAY, fills this gap. The authors write: “How the brain decides what is important to remember and what can be forgotten is still a mystery.” The new findings offer the first clear neural evidence of that choice in the human brain.
The study involved 31 healthy young adults. They learned object-word pairs while electroencephalography (EEG) recorded their brain activity. Each participant was tested twice: immediately after learning and again after a two-hour break.
The whole procedure was repeated across two separate visits. On one, participants were allowed to nap, and on the other they watched nature documentaries during the break. This let the researchers compare what stays in memory after sleep and after time spent awake.
The team analysed the EEG recordings and matched the wave patterns recorded during learning to what participants remembered after the break. Pairs that were accompanied by theta oscillations in the 3-8 Hz range during learning were more likely to survive a period of sleep. After a break without sleep, this link was absent.
Theta activity also predicted something else: stronger coupling between slow oscillations and sleep spindles, which are bursts of neural activity in the sleeping brain. And this coupling was associated with better memory after a nap.
Fun fact: Sleep spindles get their name from how they look on an EEG recording: a short burst of fast waves builds up and then gradually fades. On the trace, it resembles a spindle once used to wind yarn.
The main conclusions of the study in a few points:
The study has its limits. Memory was tested only after two hours, not after several days or weeks. Even so, the findings point to theta oscillations as the cue by which the brain flags experiences for overnight consolidation.
For anyone studying for exams, the lesson is simple: sleep is not a break from learning but a part of it. That is when the brain returns to what it earlier judged important.
The authors see more in their results than a curiosity about memory. “Our new findings shed new light on the brain processes that dictate what is ultimately remembered, and may open new avenues for understanding and treating common mental health problems such as depression,” they state.
The brain does not record everything; it chooses. That choice begins during learning: to information accompanied by theta waves, the brain gives an “important” label and then strengthens it during sleep. This is the first such clear evidence of this mechanism in humans, and possibly a lead for research on depression.
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