Why do people forget things

Question to the brain

Magdalena Sauvage, neuroscientist at the Leibniz Institute for Neurobiology in Magdeburg

First of all, you have to know that there are different types of memory. We have episodic memory, in which we store memories of unique events, such as the last birthday. There is also semantic memory. This includes, for example, knowing that Paris is the capital of France. We don't remember when and how we learned this, but we know it is. We speak of a "knowledge". Basically, we don't lose knowledge as easily as episodic memories.

Ultimately, it is not known exactly how we forget. But there are different theories about it. One is that we forget so we can remember new things. So it could be a question of our brain's storage capacity. But it has not been proven that our brain has an upper limit.

Another theory is that as time goes on, we tend to forget the details of what happened in the past. The memories are fading. However, these details could be important so that our memory can be put together and thus accessed. For example, I have a photo on the wall with a white dress on. The dress reminds me that my mother gave it to me last year. But when the white dress is erased from memory, I don't remember what my mother gave me last year.

The fading of memories comes from the fact that our memory is distributed over networks in the brain. Long-term memories are mainly stored in the cerebral cortex. Fresh memories are first stored in the hippocampus and then overwritten in a consolidated form in the cortex. But the hippocampus is likely still important for remembering past events. For example, we were able to show that there are four different areas with different functions in the hippocampus. Three of them are named CA1, CA2 and CA3. CA3 is especially important to complete the memory based on details like the white dress. We speak of pattern completion. However, the CA3's cognitive performance is very time sensitive. The details in the area fade over time, so that CA3 can no longer put the memories together properly.

How well we remember, however, also depends on how strong the memory trail is. If an event was very emotional, we remember it vividly because it is important to us. The connections between the neurons are then stronger. So we remember the day our cat died, but not what we read in the newspaper three months ago. The amygdala, the seat of our feelings, puts a positive or negative stamp on memories if they were associated with certain emotions for us. The birthday party is a positive memory, the death of the cat is sad and negative.

However, as we know from war returnees, the hippocampus shrinks under chronic stress. They have a smaller hippocampus. And that also affects memory. In chronic stress, we retain events much worse than in a rested state. This is because the stress hormones, the corticosteroids, ensure that the dendrites between the nerve cells wither and are ultimately much shorter. Sometimes cell death also occurs. Stressed people don't remember as much and forget more quickly.

You might think that if you often tell memories or write them in a diary that they are better remembered. But that is by no means the case. The more we try to remember, the less accurate the memory becomes. Because the memory is overwritten at the moment of our attempt to remember. It's also a kind of forgetting. That is why you should only question witnesses once and as soon as possible after an act.

Recorded by Susanne Donner