Memory formation and consolidation

How are our memories formed? This is a simple question which I cannot answer properly. I will share some of the things I know, and you may probably want to learn more.

Instinct vs learning

Memories are extremely heterogenic. One way to differentiate memories: separate inborn vs acquired. Some memories were acquired by previous generations and then passed on via epigenetic processes. Those memories are instinctive. Other memories are learned, or acquired.

Usually, we associate instincts with animals and learning with humans. This is not necessarily so. Animals, at least mammals, can learn just like we do. Dolphins learn new hunting strategies in their social groups. Apes learn to apply tools like twigs in small family groups. As people, we can try to suppress our instincts with higher success than other animals. Yet, animals also can overcome their instinctive behavior, especially with a good trainer.

The mechanisms that allow altering DNA and passing acquired knowledge to new generations are only partially known. For example, mice that were taught to dislike certain neutral smells were capable of passing this dislike to their offsprings.

While instinctive knowledge is cool, we usually focus on learning.

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Learning vs unlearning

It is usually easier to learn a new behavior than to learn to suppress an existing behavior. This has something to do with different subtypes of dopamine receptors (D1-like vs D2-like) used for these processes. When we learn something new we feel joy, but when we unlearn there is no such joy…

There are chemical agents for every step of learning and memory formation. Some studies show that increasing the generation of these neurotransmitters is beneficial. Food supplements like zink and magnesium may facilitate the relevant chemical processes.

In any case, relearning is probably the most difficult process. We learn first one behavior, then we learn to suppress it and use another behavior for the same trigger. And then we occasionally confuse and choose the wrong behavior, which only aggregates our frustration.

Mnemonic consolidation

There are several theories dealing with what acquired memory actually means. Sensory information is processed in working memory. Then there is some initial consolidation in short-term memory and further reconsolidation in long-term memory. Different theories use different consolidation models.

In so-called strength theory, multiple presentations of the same information reinforce the strength of connections between nerves. Alternatively, multiple trace theory posits that each time some information is presented to a person, it is neurally encoded in a unique memory trace composed of a combination of its attributes. Further support for this theory came in the 1960s from empirical findings that people could remember specific attributes about an object without remembering the object itself. Then the more some information is presented, the more instances of its trace we have, and the more likely we are to discover the relevant trace fast. This theory works well explaining certain recognitions and cued recall results.

 

In any case, space repetition enables good memorization.

Neurotransmitters

Protein synthesis plays an important role in the formation of new memories. Studies have shown that protein synthesis inhibitors administered after learning, weaken memory, suggesting that protein synthesis is required for memory consolidation.

However, there are additional mechanisms involved in memory consolidation, capable to withstand strong medical influence. Quite possibly not all relevant subtypes of the receptors have been mapped, and some are not as sensitive to certain medical agents as others. We see something similar with serotonin, where some subtypes of serotonin receptors respond to morphine,  others to LSD, and yet more to dibenzyline.

Many people use all sorts of nootropics with complex and uncharted effects on memory formation. As I will show shortly, memory formation has many stages. Improving one of the stages is not enough

Standard model

I will quote:

The standard model of systems consolidation has been summarized by Squire and Alvarez (1995); it states that when novel information is originally encoded and registered, the memory of these new stimuli becomes retained in both the hippocampus and cortical regions. Later the hippocampus’ representations of this information become active in explicit (conscious) recall or implicit (unconscious) recall like in sleep and ‘offline’ processes.

Memory is retained in the hippocampus for up to one week after initial learning, representing the hippocampus-dependent stage. During this stage, the hippocampus is ‘teaching’ the cortex more and more about the information and when the information is recalled it strengthens the cortico-cortical connection thus making the memory hippocampus-independent.

Therefore, from one week and beyond the initial training experience, the memory is slowly transferred to the neo-cortex where it becomes permanently stored. In this view the hippocampus can perform the task of storing memories temporarily because the synapses are able to change quickly whereas the neocortical synapses change over time.

Consolidation is thus the process whereby the hippocampus activates the neocortex continually leading to strong connections between the two. Since the hippocampus can only support memories temporarily the remaining activation will be seen only in the neocortex which is able to support memory indefinitely.

The most important thing here: the memory has separate areas for short-term and long-term storage, and are rewritten from one area to another (probably when we sleep).

Semantic vs. episodic memory

Semantic memory is focused on general knowledge about the world and includes facts, concepts, and ideas. Episodic memory, on the other hand, involves the recollection of particular life experiences. For example, autobiographic memory is a part of episodic memory. These types of memory provably rely on different systems.

Some experts think that autobiographical memories, no matter in which age, involve the hippocampus. Thus certain episodic memories might be stored in the hippocampus while semantic memories in the neocortex.

Amnesic patients with hippocampal damage show traces of memories and this has been used as support for the standard model because it suggests that memories are retained apart from the hippocampal system.

Procedural memory

Declarative memory, semantic or episodic, can be consciously recalled.

Procedural memory describes our implicit knowledge of tasks that usually do not require conscious recall to perform them. One example would be riding a bike: it is hard to recall how to manage the task, but unconsciously it can be easily performed.

Procedural knowledge however has been said to function separately from the mediotemporal lobe and the hippocampal systems as it relies primarily on motor areas of the brain: stratium, basal ganglia, cerebellum…

The working memory is also divided into two systems: one for the procedural and another for the declarative memory.

There are several models for procedural memory practice. All models involve repetitive action and refinement cycles.

The power law of practice states that the logarithm of the reaction time for a particular task decreases linearly with the logarithm of the number of practice trials taken. It is an example of the learning curve effect on performance.

This process is facilitated by dopamine modifying neural plasticity in the memory systems to environmental changes.

Emotional and stressful memories

The amygdala is another area involved in memory formation, especially stressful memories. Let us see the hierarchy: declarative->episodic->autobiographic->flashbulb memory. Stressful memories are often recalled very vividly. Stress hormone epinephrine helps consolidate this sort of memories, even if applied in injection.

It is reasonable to have a different memorization mechanism for dramatic unique life-threatening experiences. These experiences often contribute to PTSD, yet we can rarely forget them… Certain people have more tendency to PTSD than others, maybe due to different activation of amygdala.

Reconsolidation

After being recalled, memories may be modified and then stored again in the long-term memory. We rarely remember the actual event, and more often the processed version of it. This is both a positive and a negative issue. Our memory is not very reliable, but at least we can deal with PTSD. Certain modes of therapy heavily rely on this reconsolidation. At the same time, if we are interrupted while reconsolidating, we may remember something strange.

My kids occasionally love practical jokes, and when I am in rapid eye sleep they sometimes add new narratives to my memories. This was very puzzling until I discovered the source of interference. Sleeping with a working TV may also have a disorienting effect.

So, what did we learn?

There are multiple kinds of memory. Each kind of memory is consolidated in a different area of the brain, yet these processes coexist. Memory consolidation is influenced by environmental factors like sleep and food supplements.

Specific episodic memories are formed pretty fast. Transforming them into semantic realization takes time, as we usually do it in our sleep. Procedural memories are often formed even slower and they rely on different mechanisms.