Source: Academic Headlines
Author: Hao Jing
Editor: Kou Jianchao
Typography: Li Xuewei
"Forgetting and forgetting, last year's troubles..."
Brainwashed by this divine comedy, have you forgotten what you are going to do next?
In our cognition, forgetting is always bad, such as forgetting the content of revision during the exam, forgetting the tasks given by the boss when working, etc., which will affect our mood and the results of things.
But in reality, 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 at the University of Toronto explained how memories are stored and how forgetting occurs by observing and manipulating rodents' ability to form specific memory imprint cells.
The findings suggest that changes in the ability to acquire specific memories are based on environmental feedback and predictability, and that "forgetting" may not necessarily be a bad thing, but rather a functional feature of the brain, a fundamental form of neuroplasticity that represents a higher-order form of learning.
(Source: Nature Reviews Neuroscience)
The research paper, titled "Forgetting as a form of adaptive engram cell plasticity," has been published in the scientific journal Nature Reviews Neuroscience.
Memories and oblivion love each other
Speaking of forgetting, the first thing we think of is definitely memory, because without memory there is no forgetting, so how is memory generated?
In the process of exploring the world, animals learn to encode information, form memories, enable them to adapt to behavior and promote development. Just like humans in ancient times, when they found that fire could drive away wild beasts, when they encountered wild beasts, they learned to use torches to drive away wild beasts, so humans could survive and civilization could develop.
Studies have shown that learning this behavior involves plastic forms that leave lasting changes in brain structure that are the basis for information encoding and future recall, also known as memory imprints.
So, what does forgetting have to do with memory?
Dr. Ryan said that memories are stored in groups of neurons called "memory imprint cells", and successful recall of these memories involves the reactivation of these neuronal groups. So, logically, forgetting occurs when memory cells cannot be reactivated.
In addition, amnesia is also divided into pathological amnesia and non-pathological amnesia, in which the degree of forgetting is also different.
In one extreme case, there is a serious state of forgetting, such as advanced Alzheimer's disease (commonly known as Alzheimer's disease) or severe traumatic brain injury. These severe forgettings are due to the loss of the memory imprint itself, and the probability of recovering the memory is basically zero.
At the other extreme, there is excessive recoverability, that is, super memory, which does not occur forgetting. The cause of this condition, in addition to its own characteristics, may also be caused by post-traumatic stress disorder.
Figure | Scope of Forgetting (Source: The Paper)
Between these two extremes, there are varying degrees of forgetting, including our usual pattern of forgetting: natural forgetting.
But from both life experience and scientific experiments, we can find that natural forgetting is universal.
So, a key question is why is it happening? Is this 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 healthy brains suggests that it may represent an important feature of normal memory function, rather than a bug.
Also, when forgetting really happens, not everything is lost. Today, there is growing evidence that forgotten core memory information is still present in the brain, but is not easily accessible through natural recall. It's as if the memory is stored in a safe, but you don't remember the password to open it.
This provided Ryan and Frankland with a new way of thinking that helped them reorient human understanding of forgetting in terms of both cognitive function and neurobiological mechanisms.
In traditional studies of the mechanism of forgetting, most experiments on the mechanism of amnesia have adopted interventions that are usually pan-neuronal and do not specifically examine the role in memory-imprinted cells.
In a mouse model of amnesia, the researchers explored the relationship between the accessibility of memory-imprinted cells (i.e., what is forgotten can be restored) and pathological forgetting, as well as the mechanisms associated with forgetting. The study found that natural forgetting is associated with the following four mechanisms:
1) The importance of synaptic weight changes in memory accessibility;
2) Intracellular signaling mechanisms trigger amnesia and act upstream or downstream of synaptic changes;
3) Hippocampal circuit reconstruction may alter the weight of existing synapses on memory-imprinted cells;
4) Microglia shape brain circuits by eliminating (pruning) weak synaptic connections labeled by complement proteins.
While these mechanisms may be only a small fraction of all the ways our brains forget, they share a common feature: changes in synaptic intensity.
Therefore, researchers believe that forgetting is due to a change in synaptic weights, resulting in reduced accessibility of memory-imprinted cells. That is: forgetting is due to circuit reconstruction, which switches memory cells from an accessible state to an inaccessible state.
After understanding the mechanism of forgetting, the question remains, what determines which memories are forgotten and why?
The researchers believe that some forgetting is thought to be a form of learning that alters the accessibility of memory imprint cells by environmental conditions.
Figure | forgetting is a form of adaptive neuroplasticity. According to the model outlined herein, animals (in this trial, mice) form memory imprints due to their experiences and perception of the world; When memories are recalled and predictions are confirmed, a positive prediction error signal occurs, so that memories are maintained, or if there is significant (novel) relevant information being updated as a result of new learning and plasticity. (Source: The paper)
The impact of the environment on forgetting
The researchers say changes in the biological environment provide perceptual feedback that regulates the rate of forgetting. In a static environment, forgetting may occur less frequently because the information is still useful, while in a changing environment, the information learned becomes less and less over time.
And forgetting is not equivalent. In some cases, the rate of forgetting can be regulated by environmental experience, and these aspects are not significantly related to the morphology or function of the forgotten memory.
For example, the temperature conditions in which forest frog tadpoles live regulate their oblivion of the smell of carnivorous tiger salamanders.
So, what are the specific environmental factors that affect the forgetting rate?
Studies have shown that:
1) Physical exercise will regulate memory function and persistence, while stress will change the availability of memory, so that the extraction of memory is biased towards those more negative memories;
2) The environmental empirical aspects corresponding to the morphology or function of memory also regulate the rate of forgetting, for example, in fruit flies, the starvation state promotes the extraction of olfactory memories related to sugar, while the thirst state promotes the extraction of olfactory memories related to water;
3) Social isolation leads to enhanced RAC1 signaling and forgetting of social memory in rodents, and this change in behavioral states can be reversed by resocialization;
4) Predictive forgetting: When the learned perceptual environment is re-encountered, the brain will "delete" details that do not match the prediction, thus forgetting irrelevant details, the brain's tendency to build memory is related to the environment, and the memory formed by the most unpredictable environmental features is often forgotten.
Forgetting is a form of learning
In summary, forgetting is a positive process of neuroplasticity and does not necessarily lead to memory loss. Forgetting may be reversible, adaptive, regulated by environmental fluxes, and triggered by mismatches between expectations and environments.
The core tenet of all adaptive forgetting theories is that degrading information stored in memory can make organisms behave more flexibly and facilitate better memory-oriented decision-making.
Forgetting also creates cognitive flexibility in the changing environment. Because animals can encounter many different environments, forgetting outdated information can be a good strategy in order to adapt to new ones.
So, in an ever-changing world, such as where we and many other creatures live, forgetting some memories can be beneficial, which can make us more flexible in responding to change. If memories are acquired in an environment that is not completely related to the current environment, forgetting them can be a positive change that improves our sense of well-being.
If you had to choose, would you choose natural forgetting, or would you prefer to have hyperamnesia?
Resources:
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/