Forgetting part I: Remember to forget - how our brain deletes memories

The human brain is a system designed to store and recall huge amounts of information throughout a lifetime. At the heart of this process are our brain cells, called neurons, the brain’s fundamental units.

When we acquire new information, specialized memory neurons that store the specific memory change. They form a network known as memory engrams. So an engram is a network of neurons together storing one particular experience.

The number of neurons involved in forming one memory engram is unknown, but it is thought to be vast, reflecting the complexity and richness of human memory.

Managing countless memories
The human brain continuously forms new memories, accumulating an enormous number of engrams over a lifetime. Managing this vast library of experience requires an efficient memory organization system.

While some researchers believe that the brain simplifies memory engrams by grouping related memories, others suggest that neurons forming an engram become interconnected, allowing memories to be retrieved as a unit.

Some theories suggest that our brain simplifies memory storage by using key neurons serving as indexes, much like a librarian cataloging books for easier retrieval.

Equally important is the ability to remove or weaken memories that are no longer needed. This process, known as active forgetting, is crucial in maintaining cognitive flexibility and preventing memory overload.

The importance of forgetting
While extensive research has explored how memories are formed and stored, recent findings suggest that forgetting is an equally vital process.

If the brain retained every detail encountered, it would become overwhelmed, making it impossible to prioritize essential information. Forgetting, therefore, is an active and necessary function that helps maintain cognitive efficiency.

Forgetting happens through multiple mechanisms. Some memories fade gradually, while others are rapidly erased to prevent interference with new learning.

Research also suggests that forgetting happens in stages: short-term memories may vanish quickly, while long-term memories can endure for decades.

The mechanisms of forgetting
Forgetting is not simply a lapse in memory; it is an active biological process. Active forgetting does not occur simply because a memory is difficult to retrieve. It results from biological processes that weaken or erase memory traces.

Scientists believe that some forgetting happens when specific neurons in an engram detach and lose their connection to the larger memory network, making memory retrieval difficult.

In other cases, chemical changes in the memory neuron gradually erode stored information. That would reduce memory accessibility or even entirely remove outdated or unnecessary memory traces.

Additionally, it is thought that neurogenesis, the formation of new neurons, can erase older memories by replacing neurons that were once part of a memory engram. This challenges the idea that all memories exist indefinitely, merely hidden from conscious access.

However, external factors such as stress, sleep deprivation, and competing memories can also influence forgetting. Some memories may remain intact but become temporarily inaccessible, as seen in the "tip-of-the-tongue" phenomenon.

Forgetting is influenced by emotional and cognitive factors. Motivated forgetting occurs when memories, particularly those linked to negative emotions, are suppressed to protect mental well-being.

Retrieval-induced forgetting happens when recalling certain details weakens access to related but unretrieved details from the same memory. Studies using brain imaging reveal that retrieving some memories can actively diminish others, showing that forgetting is not a passive lapse but an ongoing neural process.

Forgetting dysfunction
Disruptions in the brain’s forgetting processes can lead to significant cognitive and emotional challenges. Conversely, excessive forgetting leads to a progressive decline in memory and cognitive function. You can read about diseases affected by too much or too little forgetting in Part II.

Current research is investigating ways to manipulate forgetting pathways to address these challenges. Potential therapeutic approaches include enhancing memory retention in aging individuals or developing methods to weaken intrusive memories in mental disease.

By understanding the balance between memory retention and forgetting, scientists may find ways to optimize our mental health and improve human capability and resilience.

About the scientific papers:

First author: Jacob A. Berry, Canada
Published in: Nature Molecular Psychiatry, September 2024
Link to paper: https://www.nature.com/articles/s41380-024-02521-9

First author: Ronald L. Davis, USA
Published in: Neuron, August 2017
Link to paper: https://www.cell.com/neuron/fulltext/S0896-6273(17)30498-1?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0896627317304981%3Fshowall%3Dtrue