Far from being a flaw, the ability to forget may play a crucial role in the brain’s memory system.


We talk to Professor Oliver Hardt, assistant professor in the Department of Psychology at Canada's McGill University.

Your work focuses on forgetting…

The majority of the public, and also many of my colleagues, think of forgetting from the perspective of memory failure. Forgetting is thus considered to be a glitch of memory. It is a relatively recent view in my field that forgetting could be a fundamental function of the memory system, something that it needs to function properly.

On one hand there is a system that constantly makes sure that structures that have been formed during memory formation are maintained, and, at the same time, there’s a process that tries to eliminate these structures.

We now refer to this process as an ‘active forgetting mechanism’, a process that brains use to remove memories with the goal to keep memory optimally functional.

So forgetting is a good thing?

I think there is indeed a bliss in forgetting. Most of what you experience you can really forget. It improves the adaptive aspect of the memory system in the sense that the brain really only retains what is considered useful for the major tasks at hand, to improve chances of survival.

If, for example, you experience a situation that allows for 20 possible reactions based on a comprehensive and unselective catalogue of past events, but actually only two were effective, you have a high chance of selecting a wrong course of action.

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But if the memory system is designed to make sure you only retain the best responses, then perhaps you only have to select from four choices, and the chance of you doing the right thing is much higher.

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How does the brain decide which memories to remember and which to forget?

That is a big question now for our field. If there is this built-in process of forgetting, why do we retain some memories? The thing is, organised forgetting is necessary because we tend to remember too much. That’s the basic problem.

We proposed that the brain is a promiscuous coding device, that it just encodes whatever it can when things happen, because the brain usually has a hard time figuring out what is important and what is not when things unfold.

It is really hard to make that call because things happen too fast, often they only happen once, and you cannot readily make the decision, “let’s keep that for later because I might need to remember it in the future”.

The best approach may be to try to remember as much as possible when things happen and sort it out later. We therefore believe certain forms of forgetting occur predominantly during sleep, counteracting the effects of unselective, ‘promiscuous’, memory formation.

But my view is that there are many other active forgetting processes. They operate on different timescales along the lifetime of a memory.

Is there an inbuilt hierarchy of remembering and forgetting?

There are ways to decide what might be important in the future and thus should be kept instead of being erased.

For example, something that is new, that we have not seen before in this way, has a high chance of being retained. This is because novelty triggers systems that release substances like dopamine, which increase the lifetime of memories formed during this time.

Each surprise is a novelty because based on our existing knowledge and memories we did not predict it. And these kind of emotional reactions like surprise, fear, happiness and so forth, trigger the release of substances and certain other processes that somehow strengthen the connections between neurons, resulting in longer-lasting memories.

In other words, these memories last longer because they are more resistant to active forgetting processes.

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Another way how memories have a higher chance of survival is repetition. I think that this is especially a feature of humans. We talk a lot, and we live in groups. For 30,000 years now we have been sitting around the proverbial fire at night, talking to our kin or loved ones about what happened during the day when we get back home from hunting and foraging, or the office nowadays.

What we experienced we explain to other people, and they ask us probing questions during this reporting, things that interest them particularly.

This kind of guided repetition is another way of filtering out what is important and what can be forgotten.

We usually only talk with people about what we find important and interesting, but when others start to ask us targeted questions to tickle out more information about a certain event, it is a way of making sure that the knowledge of everybody is used to extract the most relevant aspects from your experience and commit this to the memory of many.

This repetition and sharing strengthens the memories of the narrator and distributes it across the brains of the listeners, making the acquired knowledge resistant against the brain’s forces of forgetting by way of strengthening and distribution.

But the question of how, on the biological level, repetition, surprise or other emotional reactions protect against forgetting has not been fully answered. The way my group is looking at this currently is trying to understand how these ways of making memories stronger affect the systems that convey the message ‘forget’ in the brain.

Is it possible that the forgetting mechanism goes haywire in somebody who suffers from a disease such as dementia?

That is a hypothesis I have been proposing for a couple of years now. If there is a hardwired forgetting process in the brain, then, like any other processes of the brain, it can get dysregulated, and this dysregulation could promote diseases of forgetting.

I have suggested that Alzheimer’s disease might not start as a problem of making memories, but as excessive forgetting. Thus, to approach this disease, it may be helpful to try to tone down an overactive forgetting process.

One group has done just that. They used the approach that we used to stop normal forgetting in a healthy rat, and used it in a transgenic [genetically altered] rat that expresses some symptoms of Alzheimer’s disease, such as the famous plaques [fragments of protein that clump together and interfere with signals from neurons].

They found that blocking the forgetting process reduced the plaques and normalised memory retention in this transgenic rat. Thus, I think it is at least a plausible alternative hypothesis that excessive memory loss in certain neuro-degenerative conditions can result from the overactive and dysregulated forgetting process we have discovered.


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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.