How the brain decides what we remember

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The brain subconsciously decides what we remember and what we don’t. According to a new study, experiences end up in long-term memory when specific signals in the brain “mark” the information. Experiments with mice suggest that rewards play an important role in this process.

Whether learning for a test, finding your way in a new environment or getting to know previously unknown people, After people experience something, the nerve cells in their hippocampus generate several different signals.

These include the pointed signal waves known as “sharp wave ripples,” which are generated when the electrostatic charge in the brain cells changes. “Every experience and every event is assigned its waveform,” explains neuroscientist György Buzsaki from the New York University Grossman School of Medicine to science.ORF.at.

Remember or forget?
The fact that the “sharp wave ripples” influence the transfer of information to long-term memory is already known from previous studies. “What we didn’t know until now is how this works in detail and, above all, how the brain decides whether we should remember an experience for a long time or whether we can forget it again soon,” explains Buzsaki.

With a research team led by doctoral student Wannan Yang from the New York University Grossman School of Medicine, the neuroscientist searched for the mechanisms underlying long-term memory formation. The researchers are currently presenting the resulting study in the journal “Science”.

The brain needs breaks
According to the neuroscientist, it is particularly important for memory that the brain does not absorb information continuously. “There are different phases – those in which impressions and information are collected, and then there are always short rest phases.” In these periods, which usually only last a few milliseconds, the brain pauses to process what it has experienced. During these periods, the neurons also produce sharp signal waves.

The fact that the brain depends on several rest periods is also evident in everyday life. For example, they are why our thoughts keep wandering when reading complicated texts. If you want to learn something and remember it for a long time, it is important to take frequent breaks to give the brain cells time to process the new information.

Signal volume is crucial
As part of the study, the researchers showed that the amount of sharp signal waves is the main factor that determines which information is transferred to long-term memory and which is not. “The more of them are produced by the nerve cells immediately after the experience, the more likely you will remember it later,” says Buzsaki.

This is mainly because the signal waves produced during the day are later subconsciously repeated thousands of times during sleep. If more signal waves are present from the outset after an experience, this automatically leads to more repetitions during sleep and, thus, ultimately, to more effective memorisation in the long-term memory.

The team came to this conclusion with the help of mice: the animals had to run through mazes while the researchers recorded the signals in the mice’s brains. If at least five to twenty sharp signal waves were generated in the animals’ hippocampus after an experiment, they could better memorise the various branches to the destination. In contrast, the mice quickly forgot the information, after which fewer or no signal waves were measured.

Reward strengthens memories
It can also influence how many sharp signalling waves are generated after an experience. The experiment with the mice showed that their neurons produced more “sharp wave ripples” when the animals were rewarded with sugar after the maze. “The reward, therefore, plays an important role in memory,” says Buzsaki. According to the neuroscientist, this is probably because the mouse also wants to receive a reward in the future and subconsciously memorises the experience in order to get sugar again later.

This is similar in humans. However, what the reward looks like is usually very different for each individual and depends on the situation. The prospect of doing well in a test the next day can also be such a reward for some people. “Different things are important to different people,” says Buzsaki.

Strengthen memory, erase memories
Knowing the mechanisms underlying long-term memory formation is relevant for several reasons. On the one hand, according to Buzsaki, it is a further step towards a generally better understanding of the human brain and, therefore, also of the causes of diseases such as Alzheimer’s.

After further research in this area, it may also be possible to use the “sharp wave ripples” to improve people’s memory artificially or, conversely, to erase particularly traumatic memories from their memory precisely.