Why can't you remember things? Science shows: forgetting is also a kind of learning!

"Forget, forget, forget all, the troubles of last year..."

Have you been brainwashed by this divine song and forgotten what to do next?

In our cognition, forgetting is always bad. For example, forgetting what you have reviewed during an exam, forgetting the tasks assigned by your boss at work, etc. These will affect your mood and the outcome of things.

But in fact, scientists believe that "forgetting" may not be a bad thing. It may represent another form of learning.

Recently, Associate Professor Tomás J. Ryan from the Institute of Neuroscience at Trinity College Dublin and Assistant Professor Paul W. Frankland from the University of Toronto explained how memories are stored and how forgetting occurs by observing and manipulating the ability of rodents to form specific memory engram cells.

The findings suggest that changes in the ability to acquire specific memories are based on environmental feedback and are predictable, and that "forgetting" may not necessarily be a bad thing, but rather a functional feature of the brain and a fundamental form of neuroplasticity that represents a higher-order form of learning.

(Source: Nature Reviews Neuroscience)

The related research paper, titled "Forgetting as a form of adaptive engram cell plasticity", has been published in the scientific journal Nature Reviews Neuroscience.

The love-hate relationship between memory and forgetfulness

Speaking of forgetting, the first thing that comes to our mind is certainly memory, because without memory there would be no forgetting, so how does memory come about?

As animals explore the world, they learn to encode information and form memories, which enable them to adapt their behavior and promote development. Just like ancient humans, when they discovered that fire could drive away wild beasts, they learned to use torches to drive away wild beasts when they encountered wild beasts. This allowed humans to survive and civilization to develop.

Research suggests that the act of learning involves a form of plasticity that leaves lasting changes in brain structure that underlie the encoding and future recall of information, also known as a memory engram.

So, what is the relationship between forgetting and memory?

Dr. Ryan said: Memories are stored in groups of neurons called "engram cells," and successful recall of these memories involves the reactivation of these neuronal groups. Therefore, it logically follows that forgetting occurs when memory cells cannot be reactivated.

In addition, forgetfulness is divided into pathological forgetfulness and non-pathological forgetfulness, and the degree of forgetfulness is also different.

At one extreme, there are severe amnesia states, such as in late-stage Alzheimer's disease or severe traumatic brain injury, where the memory engram itself is lost and the probability of recovering the memory is essentially zero.

The other extreme is excessive recoverability, or super memory, where forgetting does not occur. This may be caused by post-traumatic stress disorder in addition to the characteristics of the individual.

Figure｜The scope of forgetting (Source: This paper)

Between these two extremes, there are varying degrees of forgetting, including our usual mode of forgetting: natural forgetting.

But whether from life experience or scientific experiments, we can find that natural forgetting is ubiquitous.

A key question, then, is why does it happen? Is it just a brain bug, or does it have a biological purpose?

The mechanism of natural forgetting

The study found that the prevalence of forgetting in the healthy brain suggests that it may represent an important feature of normal memory function rather than a bug.

Furthermore, when forgetting does occur, not everything is lost. Today, there is growing evidence that the forgotten core memory information still exists in the brain, but cannot be easily accessed through natural recall. It is as if the memory is stored in a safe, but you don't remember the combination to open it.

This provided Ryan and Frankland with a new research idea, helping them to reposition human understanding of forgetting from the perspectives of cognitive function and neurobiological mechanisms.

In traditional research on the mechanism of forgetting, most experiments on the mechanism of forgetting use interventions that are usually pan-neuronal and do not specifically examine their effects on memory engram cells.

The researchers explored the relationship between the accessibility of memory engram cells (i.e., the ability to recover forgotten things) and pathological forgetting in a mouse model of amnesia, as well as the mechanisms associated with forgetting. The study found that natural forgetting is related to the following four mechanisms:

1) The importance of changes in synaptic weight for memory accessibility;
2) Intracellular signaling mechanisms trigger forgetting and act upstream or downstream of synaptic changes;
3) Hippocampal circuit remodeling may change the weight of existing synapses on memory engram cells;
4) Microglia shape brain circuits by eliminating (pruning) weaker synaptic connections marked by complement proteins.

While these mechanisms are likely just a small subset of all the ways our brains forget, they all share one common feature: changes in synaptic strength.

Therefore, the researchers believe that forgetting is due to changes in synaptic weights, which reduces the accessibility of memory engram cells. In other words, forgetting is due to circuit reconstruction, switching memory cells from an accessible state to an inaccessible state.

After understanding the mechanism of forgetting, the question that remains is, what determines which memories are forgotten and why they are forgotten?

The researchers believe that some forgetting is considered a form of learning caused by environmental conditions that change the accessibility of memory engram cells.

Figure | Forgetting is a form of adaptive neuroplasticity. a. According to the model outlined in this paper, animals (in this case mice) form memory imprints as a result of their experiences and perception of the world; b. When the memory is recalled, the prediction is confirmed, and a positive prediction error signal occurs, so the memory is maintained, or if there is significant (novel) relevant information as a result of new learning and plasticity, it is updated. (Source: This paper)

The impact of environment on forgetting

The researchers suggest that changes in the biological environment provide perceptual feedback that regulates the rate of forgetting. In a static environment, forgetting may occur less frequently because information remains useful, whereas in a changing environment, learned information becomes less relevant over time.

And forgetting is not uniform. In some cases, the rate of forgetting can be modulated by environmental experiences that have no obvious relationship to the morphology or function of the forgotten memory.

For example, the temperature conditions in which wood frog tadpoles live modulate their amnesia for the odor of the carnivorous tiger salamander.

So, what specific environmental factors affect the forgetting rate?

Research shows that:

1) Physical exercise modulates memory function and persistence, while stress changes the accessibility of memory, biasing memory retrieval toward more negative memories;

2) Aspects of environmental experience that correspond to the morphology or function of memory also modulate the rate of forgetting. For example, in Drosophila, hunger promotes the retrieval of olfactory memories associated with sugar, while thirst promotes the retrieval of olfactory memories associated with water.

3) Social isolation leads to enhanced RAC1 signaling and forgetting of social memory in rodents, and this change in behavioral state can be reversed by resocialization;

4) Predictive forgetting: When the learned perceptual environment is encountered again, the brain will "delete" the details that do not match the prediction, thereby forgetting the irrelevant details. The brain's tendency to build memories is related to the environment, and memories formed by the most unpredictable environmental features are often forgotten.

Forgetting is a form of learning

In summary, forgetting is an active process of neuroplasticity that does not necessarily lead to memory loss. Forgetting may be reversible, have adaptive functions, be regulated by environmental flux, and be triggered by a mismatch between expectations and the environment.

A core tenet of all theories of adaptive forgetting is that degrading information stored in memory allows organisms to behave more flexibly and promotes better memory-guided decision making.

Forgetting also allows for cognitive flexibility in response to changing circumstances. Because animals can encounter many different environments, forgetting outdated information may be a good strategy for adapting to new ones.

Therefore, in a constantly changing world, such as the one we and many other organisms live in, it may be beneficial to forget some memories, which can make us more flexible in responding to change. If the memories were acquired in a context that is not entirely relevant to the current environment, forgetting them can be a positive change that improves our well-being.

If you were given a choice, would you choose natural amnesia or have hyperthymesia?

References:

https://www.nature.com/articles/s41583-021-00548-3

https://www.sciencedaily.com/releases/2022/01/220113111421.htm

https://www.tcd.ie/news_events/articles/why-do-we-forget-new

theory-proposes-forgetting-is-actually-a-form-of-learning/

Written by: Hao Jing

Editor: Kou Jianchao

Layout: Li Xuewei

Source: Academic Headlines