Neuro Science  








What is Learning ? It is a process of aquiring(gaining) something new in you except those aquired by intaking food. Those new things include new understanding, new knowldge,new skills etc. So gaining weight is not learnng since it is mainly associating with intaking too much or too bad foods :).


What is Memory? Memory is a process (or ability) to store new information in somewhere in your body(mostly in brain) and retrieve(recall) it whenever you want/need.


These are a little bit dry definition. Everybody would have their own definition of learning and memory. Even though you may not be able to express it clearly in words, you may have some intuition about Learning/Memory. Whatever intuition you have, it is likely that you would have correct understanding to some degree mainly because we all go through these processes all the time.


NOTE : If you are reading this just as an hobbiest and at entry level, don't try to read all of them, get overwhelemed and lose interest. Just try with Types of Memory, Big Picture ,Milestonic Research and you can go further only if you are more interested.




Types of Memory


For most of us who has never studied about memory seriously, we may think memory is just memory (storing and recalling). There wouldn't be much classification. But in academia, there are many different types of learning starting from two or three categories and braching out to many different sub categories as illustrated below. You may easily find this kind of diagram just from googling and you may notice a lot of variations of them. I tried to put as many detailed information as possible in a single diagram. It may be a little confusing and daunting at first glance. You don't need to try to memorize all these diagram. Just use this as a cheatsheet whenever you read some articles or watch lectures and come across any vacabulary about memory types that you don't understand.


This diagram is not just drawn to make it look fancy. Each of the group and branching has its own neurological background. It means to completely understand (not just memorize) this diagram would require a lot of study. So don't warry if you don't understand all of these for now and don't try to understand at once. Just try to get familiar with this classification whenever you read / watch related documents/lectures.


If you are completely new to this area, just start from very simple thing like.. there are three types of memory 'Short Term', 'Long Term' and 'Sensory' memory. If you want even simpler one, just remember 'Short Term' and 'Long Term'.




Short-term memory : This refers to the temporary storage of information that is being actively used, such as a phone number that you are dialing or a grocery list that you are trying to remember. It has a limited capacity and information is held for a short duration of time, usually for about 20-30 seconds.

Short-term memory is like your brain's sticky note, it holds information that you're currently using, like a phone number you just looked up or a list of things you need to buy at the store. It's not stored for very long though, usually only for a minute or two


Long-term memory : This is the more permanent storage of information, such as memories of past events or learned knowledge. It has a much larger capacity and can store information for a longer period of time, sometimes for a lifetime

Long-term memory is like your brain's file cabinet, it holds information that you need to keep for a while, like memories of your childhood or facts you've learned in school. This type of memory can last a lifetime



Declarative Memory:  This is the type of memory that we can 'declare'. The word 'declare' in this context is to 'say' or 'talk about' or 'verbalize'. In order to say something, it should be 'explicit'. It refers to the memory about the facts or events that we learned or experienced. Most of things that learn from school (e.g, scientic facts or historical events etc) belongs to declaritive memory. There are also a few different variations (subtype) in declarative memory as listed below.

  • Episodic Memory: Type of memory for personal events or experience in your daily life. I think the type of memory that most frequently researched or utilized in memory research is Episodic memory. Some typical examples of epsodic memory are 
    • what you eat in the morning
    • what you did at the school during the break after the math class yesterday
  • Autobiographical Memory : This is a memory about specific events that you experienced personally in the past. It is the history about self.  Some examples of Autobiographical memories are
    • When did you get married ? (Wedding date)
    • When was your first child was born ?
    • Which elementary school/ High School or University you went to ?
  • Semantic Memory : Type of memory for rote facts or general knowledge. This is the memory about meaning and more conceptual. Some example of Semantic Memory are as follows (from How Semantic Memory Works)
    • Recalling that Washington, D.C., is the U.S. capital and Washington is a state.
    • Recalling that April 1564 is the date on which Shakespeare was born.
    • Recalling the type of food people in ancient Egypt used to eat.
    • Knowing that elephants and giraffes are both mammals.


Non-Declarative Memory:  This is nonconcious memory that cannot be verbally described. It is procedural memory like muscle memory.


NOTE :  For most of the readers including me, Declaritive Memory would be the major interest. I think the reason would be because many people think the declarative memory is something that can add competitive value (e.g, maintaining good memory related to your experties or getting better scores in exams etc) and declarative memory is something we can explain verbally. However I think non-declaritive memory would be more vital to maintain our life itself. Probably becaue of this (probably not), I find much more text or video on internet about declarative memory much more easily comparing to non-declarative memory.




How memory works ?


Even though huge amount of researches has been done about the mechanisms of memory, there are a lot of details which are unknown. However we have seen conginous progress on the researches exploring the further and further details of the meachnism. I will try to consolidate a few well known/widely accepted facts about memory mechanism in this section.



Big Picture


The most common explanation about how memory works can be illustrated as follows. You may see a diagram similar to this in almost every lectures and text book about memory mechanism. I remember I first saw this diagram in almost 30 years ago when I first read the books about this topic and I am seeing the same diagram in the text / video posted even now.


Would you try to verbalize this diagram in your own words ? You may add some additional details of your own which are not shown in this diagram when you convert this diagram into your own words. This is my version.

  • there are roughly 3 types of memory : Sensory Register, Short Term Memory, Long Term memory, but in most textbook/lectures Short Term Memory and Long Term Memory would be the major topics to deal with. (NOTE : You may heard of other type of memory called 'Working Memory' which is not mentioned in this diagram. 'Working Memory' is a type of 'Short term memory'. It is the type of short term memory being used while you are doing some operation/manipulation of the memorized items. For example, if you are doing a math operation 3 + 5 + 10 + 7 in your brain, first you need to memorize all the individual numbers in your brain while you are doing the operation. This kind of memory being stored in the operational area (executive area) of the brain for short term is called 'Working Memory).
  • Objects first gets into our brain through various sensory inputs. Once those objects passes through the sensory input, it is tempariliy store in a very short term memory called 'Sensory Register'. The duration for those items to stay in the Sensory Register varies depending on the type of sensory input. For example, Visual inputs are stored in less than a seconds and Auditory input stays a little bit loger (e.g, several seconds).
  • The items in Sensory Register are transferred to Short Term Memory and stay there for some time. Those are staying in the short term memory just around 30 seconds.
  • The maximum number of items that can be stored in the short term memory is 5~9 (7 +/- 2). This number is not the count of the individual items. It is the number of groups of individual items called chunks. For example, suppose that you remember numbers in short term memory.  Try to rememer each of this number : 4-8-3-9-2-3-5-0-1-9-8-7-6-3. It may be difficult to memorize all of these numbers (14 of one digit number) individually because the counts of the item you have to memorize is much greater than 7. But it may be possible to memorize if you group the number like this : 48-39-23-50-19-87-63. In this case, you just need to memorize 7 of two digit numbers.
  • The items in Short Term Memory are transferred to Long Term Memory. How long those items can stay and how many of items cay be stored in the long term memory is theoretically limitless, but it varies depending on situation and person.
  • The iterms does not transfer automatically from one type of memory to another type memory. We need some effort to transfer those iterms to another type of memory. For example, in order to transfer items from Sensorty register to short term memory, we need to pay attention to those items. In order to transfer items from Short Term memory to Long term memory, you need to do a lot of rehearsal (repetition) and make efforst to maintain proper condition for memory consolidation (e.g, having enough sleep etc).
  • NOTE : Consolidation is a process that stablize a memory for long term storage. It can take days, months, years and the stronger memory is developed over time with the repeated use of the memory.




What is Memory Consolidation ?


I think most of terminologies in above diagram (like Encoding, Storage, Retrieval) may sound familiar, but the term 'Consolidation' may not sound that familiar in this context. I think I'd better explain a little bit further on this terminology. It is important to get a good understanding on this concept because this would be one of the most frequent terms that you may come across when you read documents about memory process.


Consolidation is the process by which short-term memories are transformed into long-term memories. It's the process that allows us to remember something for more than just a few seconds. There are two types of consolidation: synaptic consolidation and systems consolidation.

so consolidation is like the bridge that connects short-term memory to long-term memory. It's the process that helps us remember something for more than just a few seconds. There are two types of consolidation: synaptic and systems.


Synaptic consolidation is the process that occurs within minutes to hours after an experience. It's the strengthening of the connections between neurons that store the memory. This happens when the neurons that were active during the experience start communicating with each other more often.

Synaptic consolidation happens pretty quickly, within minutes or hours after an experience. It's like when you first learn something new and it sticks in your head for a little while. It's the process that makes the connections between the brain cells stronger that hold the memory


Systems consolidation is the process that occurs over a period of days to weeks after an experience. It's the transfer of memories from the hippocampus, which is the temporary storage area in the brain, to the neocortex, which is the more permanent storage area. This process is thought to be the reason why memories that were once vivid and easily accessible can become less so over time.

Systems consolidation takes a little longer, like days or weeks after an experience. It's like when you learn something new and it stays with you for a long time. It's the process of moving the memory from a temporary storage area in the brain to a more permanent one. This is why sometimes you might not remember something as clearly as you did when it first happened.


Both these processes are important in order for the information to be stored and retrieved from long-term memory. Sleep is also important for consolidation, as memories are better consolidated when encoded during wakefulness and consolidated during sleep.


NOTE : Difference between Storage and Consolidation

The terms "storage" and "consolidation" are often used interchangeably in the context of memory, but they do refer to slightly different things.

Storage is the process of retaining information in the brain for later retrieval. Information can be stored in the brain in many different forms and in many different areas, depending on the type and duration of the memory.

Consolidation, on the other hand, is the process of strengthening and stabilizing memories so that they can be stored in the long-term memory. It's the process that allows a memory to be retained over time. Consolidation is a process that occurs after the information has been stored and it is mainly focused on the transfer of memories from short-term to long-term memory and it is also responsible for the integration of memories with already existing knowledge.

So, in summary, storage is the initial process of retaining information, while consolidation is the process of strengthening and stabilizing memories so that they can be retained over time.



Location of Memory formation and storage  in brain


The diagram shown above is very simple and widely used, but it shows purely sequential process and does not show anything about the brain part (location) that are involved in the process.

Convert the simple diagram shown above into another type of diagram mapping onto brain structure, it can be illustrated as below.


NOTE : The part labed as Sensory Cortex is over simplified. Actually this part can be viewed on the cortex of the whole brain, but I just simplified as a single imaginary block to simplify the overall diagram.

NOTE : As you know, the part labeled as Hippocampus is supposed to be at the center of the brain, not out side of the brain. But I put it outside of the brain just to make the part visible clearly.



I would suggest you to read (verbalize) this illustration in your own words. Here goes my version.


Step 1 :

Various stimulus comes in through sensory organs (e.g, eyes, ears, nose, tungue, skin etc) and those stimuli are first processed by Sensory Cortex.


Step 2 :

Those information processed by Sensory Cortex is transferred to Hippocampus (if the information gets a certain level of attention).


Step 3 : At Hippocampus, those information is encoded (converted) into short term memory. And some of the short term memory will be transferred to cortex (Prefrontal Cortex).


Step 4 : Prefrontal Cortex distribute the information to various other cortext area for being stored as permanent memory (long term memory). The final destination would vary depending on the nature of the memory contents.



How is Longterm memory  transferred from Hippocampus to Cortex ?


Before talking about how long term memory transfer from Hippocampus to Cortex, are we sure that the memory transfrom the hippocampus to cortex ? There are some researches strongly showing it is the case and following is an example.

In this research, a same memory test was performed for a long period of time (90 days) and the brain activity was investigated over the period.

As shown in the plot (a) and (b), the activity of Hippocampus decreases over time (b) whereas the activity of a spot in freprotal area (ventral medial prefrontal region) increases over the same period of time. This is interpreted as the memory transfers from Hippocampus to Prefrontal area.


Image Source : Declarative memory consolidation in humans: A prospective functional magnetic resonance imaging study



Even though it is relatively well known that the short term to long term memory conversion happens in Hippocampus and eventually will be stored in cortex area, there still be a lot of mysteries on how exactly this conversion and storage at the final destination happens. I think most of the explanation on this process would be based on hypothesis. One of the hypothesis that caught my attention is as follows.


NOTE : This hyponthsis does not have full scope of memory. It is restricted to a specific type of memory, which declaritive memory obtained from visual input.

Image Source : Modified from Memory Consolidation: Tracking Transfer with Functional Connectivity

      FFA(Fusiform Face Area) : Cortex area that response to face stimuli

      PPC (Posterior Parietal Cortex) : Cortex area that is involved in representation of egocentric space


Step [A] : At the first stages of long term memory (memory consolidation), all of the neocrtex are (FFA, PPC, Early Visual Corbext) has connected to Hippocampus, but does not have strong strong connections among themselves. At this stage, Hippocampus should be involved in every consolidation and retrieval process.

Step [B] : During the 24 hours (observed in the paper), the direct connection between cortex area (connection between FFA and PPC/Visual cortex in this case) gets stronger and the connection between Hippocampus and cortex area getting weaker.

Step [C] : After 24 hours (the number 24 is just the number checked in the proposed papers, it would vary depending on situation and nature of the memory), the connection between Hippocampus and cortex are gets very week (or disappear) and the connection among every part of cortex area are formed. At this stage, memory consolidation and retrial may not get Hippocampus involved. ==> This may explain about the reason why the person who got damaged in Hippocampus can still recall the memory that are formed long time ago.




A Biomarker for Episodic Memory and Planning


There is a specific indicator that indirectly shows activiity of mammalian hippocampus, which is called Sharp Wave and Riiples (SWR). As the name implies, SWR is made up of two part : Sharp part and Ripple part. Sharp waves are large, synchronized bursts of electrical activity that typically occur at a frequency of around 100-200 Hz. Ripples, on the other hand, are smaller, more synchronized bursts of activity that occur at a higher frequency of around 200-500 Hz


SWR is A specific type of brain wave (a sharp wave followed by ripples) observed when Hippocampus is performing a specific task

  • SWR is produced by exteremly synchronized activity of neuron in the mammalian hippocompus and neighbouring regions
  • SWR is generated by tens of thousands of neurons firing together within 30~100 ms window
  • SWR is considered as an biomarker (indicator) of memory process
  • Hippocampal spiking during SWRs can represent past or potential future experience
  • SWR-related interventions can alter subsequent memory performance
  • It is suggested that SWRs support both memory consolidation and memory retrieval for processes such as decision-making
  • It is also suggested that different SWR types correspond to different cognitive functions, such as retrieval and consolidation


Image Source : Hippocampal Sharp Wave-Ripple: A Cognitive Biomarker for Episodic Memory and Planning


SWR primarily occur during sleep and are believed to play a role in the consolidation of new memories. The hippocampus replays the memories during the SWR, this replay process is thought to strengthen the memory and to facilitate the transfer of the memories from the hippocampus to the neocortex for long-term storage.


SWR also occurs during wakefulness, but is more frequent and of higher amplitude during sleep. The exact mechanisms behind SWR are still not fully understood, but research suggests that it is important for the formation and consolidation of memories and other cognitive processes.


SWR occur primarily during non-REM (NREM) stage 2 sleep. They are brief, high-frequency oscillations that occur in the hippocampus, a region of the brain important for memory formation.


During NREM stage 2, the brain is in a state of relative quietness with moderate activity, this state is known as sleep spindles, and it's also during this stage that SWR occur. SWR events are often accompanied by sleep spindles, which are bursts of activity in the 12-15 Hz range, and K-complexes, which are high amplitude, short duration waves.


NOTE : Refer to the note on Sleep cycle, if you want to get further details on Sleep cycle.



How memory/learning is hardwired in Brain ?


When learning happens, what exactly happens in the brain ? Roughly speaking, two stages of changes occurs

    In short term : Major changes in Brain cells at the early stages of learning, most of the changes happens with the chemical properties of synapse in such a way that the synaptic connection gets stronger. How the synaptic connection gets stronger ? Typical mechanism goes as follows.

    • Increase the activity of Ion Channels (e.g, Calcium channel) at the presynaptic end
    • Increase the fusion of neurotransmitter veisicle to cell membrane ==> increase the release of neurotransmitter
    • Increase the sensitivity for a neurotransmitter at the post synaptic end.

    NOTE : In most case, these process is triggered by a certain type of protein (enzyme). So if we block the protein synthensis, these process is blocked or greatly reduced.


    In Long term : If the stimulus that caused the short term changes repeats (continues) for longer period, more fundamental (long lasting) changes happens in brain cell.  Typical changes are new growth of exon terminal and those newly generated axon terminal forms more number of synaptic connection with neighbouring cell (neuron).

    NOTE : This kind of structural (anatomical change) is triggered by the expression of a certain gene that eventually lead to the generation of new axon terminal.

This kind of changes in neurons in nerve system is called Neuroplaciticity. That is, you can say the learning (or memory) is engraved in the brain by the mechanism called Neuroplacticity.  Not all neuroplaciticity is related to learning, but all of the learning process is associated with a certain type of neuroplacticity.


NOTE : The concept and mechanism of neuroplaciticity is a huge topic. So I wrote a separate page for neuroplaciticity. If you are interested (motivated enough) for the cellular and molecular mechanism of memory/learning, I would suggest you to go through the note Neuroplaciticity.




Looking into Hippocampus


As mentioned many times above, the key player of memory formation in the brain is Hippocampus. It can never be overemphsized to understand about the details of Hippocampus as much as possible. The more you know about anatomy and functionality of hippocampus, the better you would understand the process of memory formation and recall.

In this section, I would talk about a little bit of details of Hippocampus, but not too details as in academic paper. What I am trying to do is just to give you some keywords and procedure to help you to read documents (e.g, textbook or academic paper).


The first things that I want to suggest you to get familiar with is the basic anatomy of Hippocampus as illustrated below. First you should be able to image the shape and location of Hippocampus in your brain without looking into any reference (e.g, textbook). Then you should be able to identify the location of hippocampus from various cross section images of the brain. And then you should be able to draw (or imagine) cross section of the hippocampus and identify major part of the substructure of the hippocampus. In short, It would be good enough (at least as an hobbiest) if you can imagine the illustration shown below without looking at this note when you read other document or watching a lecture.


NOTE :  as I always says, don't try to memorize it... just read this note or textbook or watch lectures as many as possible and check if you can visualize in your mind without openning the textbook. If you don't get the image in your mind, then look into this note or other textbook.



        DG : Dentate Gyrus

        Sub : Subiculum

        Pre : Presuiculum

        Para : Parahippocampal gyrus

        MEC: medial entorhinal cortex

        LEC: lateral entorhinal cortex

        Hippocampus Proper = CA1 + CA2 + CA3

        Entorinal Cortex = MEC + LEC


Following is the overal circuitary connecting various part of substructure of Hippocampus and cortex of the brain. I know it look confusing. Don't try to memorize it. Just to try to follow one or two specific circuits when you have chance (e.g, reading other textbook or sitting in a lecture).



Image Source : Space in the brain: how the hippocampal formation supports spatial cognition



Followings are the summary of functionalities of each subregions in Hippocampus : (Mostly based on Exploring the Function of the Hippocampus by Anatomy )



Dentate Gyrus : Dendate Gyrus plays various functionas as below

  • is involved in the mnemonic (memory) processing of spatial information, the formation of episodic and spatial memories, and the spontaneous exploration of novel environments
  • is the input region of the hippocampus, and serves as a pre-processing unit
  • some information from the entorhinal cortex directly enters the dentate gyrus, is processed by dentate gyrus, and then the processed information is delivered to the CA3 region of the hippocampus proper
  • perform pattern separation, which is separating relatively similar input patterns into distinct/unique output patterns
  • is required for Working Memory & Spatial Navigation
  • Undergoes LTP and Neurogenesis
  • Required for Working Memory & Spatial Navigation


CA1 : CA1 is the first region of Hippocampus in terms of anatomy and plays various functions as follows.

  • Recieves inputs from CA3 or CA2 and output the information to entorhinal cortex
  • (more specifically) CA1 recieves temporal information from CA2 and spatial information from CA3.
  • one of the functions of the CA1 region may be to produce combined spatial-temporal code that allows an animal to distinguish the memory of 2 events from each other, even if they occur at the same place but in different times.
  • Performs the retrieval of episodic autobiographical memory


CA2 : CA2 is located between CA1 and CA3 anatomically and plays various functions as follows.

  • Neurons in this region carries less spatial information than in CA1 or CA3.
  • receives input from layer 2 of the entorhinal cortex through the perforant pathway
  • encodes time information more strongly than spatial information and then send the temporal information to CA1
  • has a function pertaining to social behavior and may be involved in social memory processing (e.g, differentiating between new and familiar individual around you)


CA3 : CA3 is located farthest away from CA1 and sitting right next to DG(Dentate Gyrus).

  • receives input from both the mossy fibers of the granule cells in the dentate gyrus, and from cells in the entorhinal cortex through the perforant pathway.
  • considered to be the “pacemaker” of the hippocampus, and thereby has a big role in learning
  • operated as an autoassociation memory to store episodic memories including object and place memories
  • dentate granule cells operated as a preprocessing stage for the CA3 region by performing pattern separation so that the mossy fibers could act to set up different representations for each memory to be stored in the CA3 cells.
  • the mossy fiber system to CA3 connections are involved in learning but not in recall
  • involved in the acquisition of context-dependent fear extinction


Subiculum : Subiculum is between CA1 and Parahipocampal gyrus and plays roles as follow.

  • Functions as main output port of Hippocampus
  • has functions pertaining to memory, spatial navigation, mnemonic or symbol processing, and regulating the body’s response to stress through the inhibition of the HPA axis.
  • Responds to an influx of information as well (e.g,to only familiar objects or a landmarks, and to concurrent location and speed)


Entorhinal Cortex : Entorhinal Cortex (MEC + LEC) located in the medial temporal lobe and is next to the hippocampus

  • Functions as both the major input and the major output hub of information to and from the hippocampus
  • Interface between the neocortex and the hippocampus
  • (with the hippocampus) Works for formation on declarative/explicit (autobiographical, episodic, semantic) memory storage, and spatial memories in particular.
  • Collaborates with hippocampus on memory formation, memory consolidation, and memory optimization in sleep.
  • (with the hippocampus) plays key roles for our sense of direction and spatial navigation ability



Is our long term memory capacity infinite ?


I heard a scientists say it would be a sort of infinite. How to prove it ? I don't think it would be possible to prove it clearly in scientific way and the conclusion would also vary depending on how you define the term 'infinite'. But I think we can find some cases showing the infinite-like memory for a certain type. Note that I said 'a certain type'. As far as I have been collecting cases, I see some people showing almost infinite amount of memory for a certain types but I haven't seen any cases who shows the infinite capacity for every type of memory. Followings are some of the examples :

Jill Price : She shows almost inifinite amount of memory capacity for autobiographic memory.

Stephen Wiltshire : He shows the almost inifite capacity with infinite details for the scenes coming through vision system (visual memory ?). Check out @3:33 of this video.




Is unforgettable memory a blessing ?


Many people (including me to be honest) would wish to have such a memory that never disappear. But would this be a blessing ? Hard to say. The only way to get the answer to this would be to interview many people who has this kind of memory, but we don't have enough cases of these that would reach any common conclusion. But in my personal opinion after watching / reading interviews with a few people (e.g, this, this), such a person seems to have a little bit different opinion about such a capability. It seems the opinion varies depending on how much control they have over such a memory.  If we can keep everything in our memory and can recall (retrieve) it ONLY when we want, it would be considered as blessing, but if not it wouldn't be considered as anything desirable. In many cases, those memory pops up spontaneously regardless of whether they like it or not giving them harsh feeling or interfering with other thought processes. At least, Jill explicitely mentions that she does not like it, but some other persons shows a little bit varying opinion(Watch this)




What is forgetting ?  Why fortet ?


What is forgetting ?  Is it some unwanted problem which everybody suffer from ? or Are we MADE to forget ?

Following statement from a Nature article would be the clear and simple answer to this question.

    Long thought to be a glitch of memory, researchers are coming to realize that the ability to forget is crucial to how the brain works.

Really ? If the forgetting is a crucial function, there might be some biological reason why we evolved the feature (forgetting), what is it ?

  • Every species that has memory forgets without exception. ==> If every species evolved this way, there must be some biology reason for it. Whatever the reason is.
  • Person with highly superior autobiographical memory (HSAM) reports various problems and have difficulties to achieve accomplishment they are hoping for (See what I mentioned in Is unforgetable memory a blessing ?)
  • Various psycological problems like anxiety, post-traumatic stress disorder (PTSD) is from the lack of forgetting certain memories.
  • People with SDAM(severy deficient autobiographical memory) are unable to vividly recall specific events in their lives. As a result, they have trouble imagining what might happen in the future. However, they tend to do particularly well in jobs that require abstract thinking. ==> Less worries, more success :). I am not trying to say we all need to have SDAM to be successful, but just trying to say there would be some important reasons why we need to forget.

If forgetting is a certain biological feature that are built into the species, there must be some biological mechanism which are involved in the process of forgetting. What is it ?

  • One example is from the research on AMPA receptor in a rodent brain. In mammalian brain, memories are econded when the strength of connection between neurons increase. The connection strength is determined by presence of a special receptor called AMPA receptor. In normal situation, the AMPA receptors are very unstable and in-and-out of the synapse constantaly leading to a certain rate of forgetting.  When researchers blocked the removal of AMPA, the rats were prevented from forgetting the location of objects. This implies that the rat brain had to proactively destroy connections at the synapse (refer to this Nature article).


Is it that the memory (information stored in the brain) disappeared ? ot Is it that the memory is still there but we just failed to retrieve (recall) it ?


How do we forget ? On other words, how 'forgetting' works ? A few types of forgetting mechanism are proposed (refer to this video)


Passive Oblivescence : This is a type of forgetting process in which we forget things without any active effort. We just forget things as time goes. In other words, it just fades out.  It may happen because the synapses forming a specific memory gets weeken or the triggers to remember/recall the memory gets weeken or some of the neurons related to the memory is rewired to other memory that causes the loss or interference to the existing memory.


Targeted Forgetting : This is a type of forgetting process that our brain perform mostly during sleep. Brain rearrange / reconsoslidate various information stored in the memory and clean up (purge out) those information/memory which are not worth keeping.


Motivated Forgetting : This is a type of forgetting process that we actively perform, like trying to forget some unpleasant memory.




What is Amnesia ?


The term Amnesia came from Greek αμνησία (amnēsía) which mean  “forgetfulness”. In many english dictionary, it is defined as a total or partial loss of memories. But this definition may mislead you in terms of modern definition of Amnesia. When we say 'loss of memory', you would think of it as something related to the past because the term 'memory' tend to give impression of 'some event happened in the past'. However Amnesia is not only about the past but also about the future.  We can define the term Amnesia in two parts as below.

  • Total or partial loss of memory : this refers to the loss of the memory formed at some point in the past. This type of Amnesia is called Retrograde Amnesia.
  • Loss of capability of forming a new memory : this refers to inability to form a new memory, especiall to form a new long term memory. This type of Amnesia is called Antrograde Amnesia.




Can we trust our memory ?


Is our memory reliable ? Are you sure what you remember in your brain is 100% correct ?

Unfortunately there are hundreds of hundreds of research shows that our memory is not 100% accurate and memory changes in many different ways.


What ??? Then how come most of investigation by law enforcers and even legal testimoney relies on the memory of persons ?

Why we still spend days and nights trying to memorizing so many things out of textbook / lecture note preparing for exam if the memory is not accurate.


I think the reliability of memory is still in active debate.

It seems true that it is not 100% reliable. However it would not be as bad as we should ignore all the witness's testimony or give up studying testbook/lecture note for exam.


What I am trying to say in this section is that memory is not like a tape recorder that plays back the exact the same thing whenever you hit the play button. The memory is constructive and reconstructive. It mean that the memory can be modified in various ways and even the complete false memory that never happened to you can be planted in your brain. Even if memory is still ciritical / vital part of our brain activity and of maintaining our normal life, still it would be good to understand about the vulnerability of memory.



< Memory is Unreliable >


There are so many researches and papers/articles about unreliable aspect of memory, but I would to introduce a few famous cases from early / milestone cases. The cases introduced here from How reliable is your memory?- TEDtalks. If you want to get the full transcript (if you prefer reading), you may check out How Reliable Is Anyone’s Memory?


Case 1 : a Victim Report

  • There was a man who raped a female hitchhiker
  • Later another man named Titus was pulled over by a police. He was kind of resembled that rapist
  • So the police took a picture of Titus and put in in a photo lineup
  • Those pictures were showed up to the victim and she pointed to Titus' photo and said 'That one's the closest"
  • The police and the prosecution proceeded with a trial, when Titus was put on trial for rape and the raped person got on the stand, she said "I'm absolutely positive that's the main".
  • Titus was convicted.
  • Later the actual rapist was found and confessed to commit the rape


Case 2 : Car accident report

  • Researcher showed two group of people a simulated accident (let's call them Group [A] and [B] for convenience). Exactly same scene were shown to both groups
  • Asked [A] how fast were the cars going when they hit each other ? ==> they reported the avarage of 34 mph
  • Asked [B] how fast were the cars going when they smashed into each other ? ==> they reported the avarage of 41 mph
  • Asked [A] if they saw broken glass in the accidenty when they hit each other ? ==> 14% of them said YES
  • Asked [B] if they saw  broken glass in the accidenty when they smashed each other ? ==> 32% of them said YES

NOTE : You would notice what people recall (from  memory) differs by some leading keyword : 'hit', 'smash'.



< Memory is Relliable >





What is Schema ?


You may or may not heard of this term 'schema' in terms of neuroscience or psyhcology, but I think this would be the term that you can easily agree with and associate it with your own experience. In this section, I am going to talk about a few important aspect of Schema.



Definition of Schema


It is not easy to define the concept of Schema in short and easy-to-understanding manner. Whenever I come across with any concept which is hard to grasp, I usually survey about many different definitions from many different sources. At the beginning, it may be even more confusing to see too many different ways of expression but as time goes on you will notice (have a kind of feeling in your mind) that you are getting 'some kind of essence' of it.  I am trying to approach the concept of Schema in the same way.


Following is the list of definitions by pioneering researchers in this area quoted from Naïve to expert: Considering the role of previous knowledge in memory ,

  • active organisation of past reactions or experiences (Bartlett (1932))
  • pre-existing network of interconnected neocortical representations that affect the processing of new information.(Van Kesteren et al. (2012))
  • an associative network structure, which is based on multiple similar experiences (Ghosh and Gilboa (2014) )
  • framework of acquired knowledge, skills or attitudes implemented within a network of connected neurons in which the memory traces of associated information have been stored (Fernández and Morris (2018))

NOTE : If the definition listed above does not make clear sense to you, don't be disappointed. It is not that clear to me either. But I quoted this because it is always good to have some technical background.


Probably following definition would sound clearer and practical. At least to me, it sounds clearer.


What I ask the definition of schema to an AI (ChatGPT) , I got following :

    A memory schema is a mental structure or framework that organizes and integrates information about a particular topic or event. It is a way of organizing and storing information in memory that makes it easier to understand and remember. Memory schemas help us to categorize and make sense of new information, and they also allow us to retrieve information from memory more easily.


    Memory schemas can be thought of as "mental shortcuts" that allow us to make connections between new information and what we already know, making it easier to understand and retain the new information. They can also help us to fill in gaps in our knowledge and make inferences about related topics.

NOTE : If the concept is still not clear to you, try to google a few more documents and read them. Everytime you read new definition, you will feel that your brain gets more and more familiar with the concept.



Why Schema is important ?


Schema is important because it affects on us to interpret or process anything around us regardless of whether you realize it or not. When we perceive on any new things, we try to understand it based on some set of previous knowledge or experience that we is already in our mind. In some case we try to understand new things based on previous knowledge structure (this knowledge structure is called 'schema') even when there is no direct relationship between the new thing and the schema.

For example, if you are talking about something to somebody who has always been crazy about baseball and likes to talk about such a tiny details of baseball game or baseball player etc, you would notice that he often (actually very often) likes to make an analogy between the baseball and whatever you are trying to talk about. In this case, he is taking in the things (even a completely new things) within the schema of baseball.

For sure this thing (percieving things based on a schema) would happen to yourself as well. Just think about what is happening in your mind when your friend or teacher talk about something.

Sometimes this kind of perceiving or understanding things based on an existing schema would cause problem like distorting the real meaning of the new thing, but it is almost impossible to prevent any schema from getting involved in your mental process.

But don't worry I think schema facilitate the process of memorizing and learning new things in most cases rather than causing any problem. I personally try to build my own schema as much as possible since I think it is so helpful to facilitate my learning process and extend knowlege.




What is Knowledge ?


What is knowledge ? A simple definition by Oxford diction is as follows :

    Knowlege is facts, information, and skills acquired by a person through experience or education; the theoretical or practical understanding of a subject.

In other words, Knowledge is sets of facts, information and skills aquired by experinece and education. I think you would agree with this generic defintion. But I want to look a little bit further into several different aspects of knowledge.



Difference between Knowledge and Memory


Memory and Knowledge may sound similar (interchangeable) but they are not the samething even if it is not each to clearly explain about the differences between them. Let me start with simple definition of these two concept first.

  • Memory is a mental function or process that stores something in your brain and recall.
  • Knowledge refers to familiarity or understanding about a subject (facts, information or skills about the subject).

These two are related but not the same. I would say memory is a key part of learning process. A set of information organized for a specific topic is called knowledge.  So the overall impression about the relationship between these concept is as follows.

    Memory ----> Learning ----> Knowledge

NOTE : Even though it is hard to clearly distinquish between memory and knowledge, you may find the difference intuitively based on your own experience. In many cases, person who has great knowledge tend to have good memory but it would not always be the case. You might have seen some people (directly or indirectly) who has extrememly good memory, but does not look knowledgeable. Even thought it would not be common case you might have seen some people with huge amount of knowledge does not seem to have good memory.



What is Explit Knowledge ?


Explicit knowledge is the form of knowledge that you can express it easily in verbal, writing etc and transfer (teach) it to other relatively easily. This would be the most of the knowledge you can obtain or provide via textbook, lectures, user-mannual etc. Also most of the knowledge you can get by googling can be considered as a explicit knowlege.



What is Implicit(tacit) Knowledge ?


Implicit (Tacit) knowledge is the form of knowledge that we have but is extremly difficut to express/explain it in words or writing. Usually we obtain this kind of knowledge from our personal experience or a lot of practices and in many cases you don't even realize that you have such a knowledge until you have chance to do something.  

It is not easy to explain exactly what the tacit knowledge it, but you would notice what it is as you experience more in your field and interact with others.  


Suppose that you had chance to ask (or learn) from an engineer with one year of experience and with 10 years of experience in the same area. You may not see much difference between them in terms of the way they explain or teach. Sometimes you may get the impression that the younger engineer does better with explaining / teaching. But as you have further interaction with them you may see many case that cannot be easily done by the young engineer but so smoothly done by the one with longer experience. If you ask him how he did it, he may say 'it is simple. Just did it'. This would be a typical example of tacit knowledge.


NOTE : I am not trying to say that young engineer does not have much tacit knowledg and you will get a lot of tacit knowledge if you just get older in your area. I was trying to talk a kind of general tendancy assuming that everybody has tried their best through the whole career. Personally I like to (trying hard to) obtain tacit knowlede from others. It takes time and effort, and requires long term interaction. You need to learn mostly from their action, not from their words. Sometimes you would get just single word of precious insight from several hours of dialogues with him.



How the tacit Knowledge is converted into explicit knowledge?


This has been my personal question for long time. I know there are two types of knowledge explicit and implicit as mentioned aboved. I personally thought that implicit knowledge is more solid form know knowledge in me and I wanted to know the mechanism by which the tacit knowledge is formed and how to facilitate the process of converting the explicit knowledge into tacit knowledge.


For this question, I asked ChatGPT with the question : "how is explicit knowledge converted to tacit knowledge ?". I personally like the answer from the chatbot... what an amazing technology !!!

    There are several ways that explicit knowledge can be converted to tacit knowledge, which is knowledge that is more difficult to express or communicate formally:


    Experience: One way to convert explicit knowledge to tacit knowledge is to gain practical experience with the subject matter. By actually doing something, rather than just reading about it or being told about it, you can develop a deeper understanding and a greater intuitive sense of how things work.


    Practice: Repetitive practice can also help to convert explicit knowledge to tacit knowledge. By repeatedly performing a task or skill, you can develop a better sense of how to do it effectively and efficiently, and you may start to develop a "feel" for the right way to approach the task.


    Training: Training programs and courses can also be helpful in converting explicit knowledge to tacit knowledge. By actively participating in hands-on exercises, simulations, and other interactive learning activities, you can develop a more intuitive understanding of the subject matter and how to apply it in practice.


    Overall, converting explicit knowledge to tacit knowledge requires a process of learning through experience and practice. It can be a slow process, but it can also be a valuable one, as it can help you to develop a deeper understanding and a greater intuitive sense of the subject matter.

Personally I agree with this answer and I realize this process is frequently happening in myself. When I learn a new thing as an hobby or as a job, I start gaining knowledge in explitict form (explicit knowlege) at the beginning. As I read more and have more hands-on whenever I get chances, I feel myself that the knowledge is getting internalized (aborbed) into myself. But ironically the deeper the knowledge get internalized into myself, the little I can express it verbally or written form even if I am getting more and more proficient with the specific area. However, don't get disappointed because the difficulties in expressing it out is a nature of tacit knowledge. You still have the knowledge in you more deeply imprinted in you.




Reading a book ten times ? or Reading ten books one time ?


Everybody would want to put things into their long term memory and we all know that repetition would help convert the short term memory to long term memory as mentioned before. But when you are studying new things and trying to put them into long term memory, the 'repetition' is not an easy things to do.

Based on my personal experience (i.e, observing myslef) with learning new things, I think reading ten books on the similar topic once for each book is better than reading one book ten times. It may applies only to me and you may have different opinion. It is perfectly OK if you have different opinion.

But I think I can explain why reading ten books one time for each is better than one book ten times at least applying to myself only.

Reading a book does not start directly from short term memory. It starts from Sensory input(visual memory in case of reading a book). So those information should go to short term memory first before it gets transferred to long term memory. The trigger for the sensory input to short term memory is 'Attention'.  I found it very difficult to maintain the high level of attention for every reading when I am reaing the same thing over and over. I soon lose attention and my brain seems to filter out the input as a kind of background noise and didn't even put it into short term memory. So even if my eyes and primary visual cortex process what comes into the retina (i.e, text on the book in this case) and it tend not to be converted into memory by hippocampus.

However if I read the ten books one for each book, for every reading I am getting a little bit of different variants of the same topic comes into the sensory system. Since each of those input has a certain portions of novelty, my brain tend to pay pretty high attention to every input. In addition, because I am reading books with the same topics most of those inputs are similar. So if I read the ten different books, it is like repeating the same things ten times which helps to get converted to long term memory.

The only problem (personally to me) with reading ten books once for each comparing to reading the same book ten times is cost. That is, I need to spend more money to buy 10 books.

Maybe we can find easy solution for this thanks to technology.  Learning from YouTube has been my solution for long time. You don't need to pay any extra money for watching however many YouTube you get.

This would be the reason why you see a long list of YouTube reference in all of my notes.




What is memory decline and how it happens ?


Memory Decline refer to various form of reduced functionalities in the memory process. It is often known as age-related memory loss, is a natural part of aging that can cause difficulties with recalling recent events, names, or details. Memory decline can affect many aspects of daily life, including work, social interactions, and personal relationships.


As we age, changes occur in the brain that can affect memory. The hippocampus shrinks with age and synapses weaken with age as well. These changes in the brain can make it more difficult to form new memories and to recall previously learned information.

NOTE : It's worth noting that memory decline can be caused by many different factors, not just aging, such as stress, depression, lack of sleep, poor nutrition, and certain medical conditions. It's also important to note that memory decline is a normal part of aging, but it doesn't mean that is an inevitable part of getting older


Other factors that can contribute to memory decline include stress, depression, lack of sleep, poor nutrition, and certain medical conditions such as cardiovascular disease and diabetes.


Here goes some examples of typical memory decline you may experience.

  • Difficulty remembering recent events, such as appointments or conversations.
  • Forgetting names of familiar people or objects.
  • Difficulty remembering details of a story or article that was just read.
  • Losing track of the conversation or losing one's train of thought.
  • Difficulty remembering the steps involved in completing a task, such as a recipe or a project at work.
  • Forgetting where you placed your keys, wallet or phone.
  • Difficulty remembering phone numbers or addresses.
  • Difficulty remembering appointments or birthdays.
  • Difficulty remembering the names of new acquaintances or friends.
  • Difficulty remembering what you did or where you went yesterday.




Milestonic Research



Henry Gustav Molaison (February 26, 1926 – December 2, 2008)


He was an American and is widely known as H.M and may be the most famous and well-known patients. If you have been interested in neuroscience and read some of documents (books, megazines or articles in the internet) or watch lectures, I am pretty sure you might have heard of him, at least the name H.M.  Since he has been mentioned in so many documents and lectures, it would be good if you have some background information about him to follow along with those documents and lectures.  

He had been a patient who greatly contributed to nuerosience, especially learning and memory process, but it was not intentional at all. He just got a brain surgery just to reduce the symptom of epilepsy, but a serious memory related problem is observed after the surgery. This unintended problem led to findings where Hipocampus plays key roles in long term memory formation and memory may be localized (i.e, associated with a specific part of brain not associated with the all across the brain). Watch this video for the importance of findings from H.M


Image Source : Wikipedia


He had a  bilateral medial temporal lobectomy in 1953. With this surgery, he got about two thirds of anterior hippocampus  removed. The removed area were parahippocampal cortices, entorhinal cortices, piriform cortices, and amygdalae. The purpose of this surgery was to cure his epilepsy.

More detailed information of the brain lesion (damage) found from the autopsy were as follows : See this if you are interested in further details.

  • the medial temporopolar cortex,
  • piriform cortex,
  • virtually all of the entorhinal cortex,
  • most of the perirhinal cortex and subiculum,
  • the amygdala (except parts of the dorsal-most nuclei—central and medial),
  • anterior half of the hippocampus,
  • and the dentate gyrus (posterior head and body).
  • The posterior parahippocampal gyrus and medial temporal stem were partially damaged.
  • Spared medial temporal lobe tissue included the dorsal-most amygdala,
  • the hippocampal-amygdalotransition- area,
  • ~2 cm of the tail of the hippocampus,
  • a small part of perirhinal cortex,
  • a small portion of medial hippocampal tissue,
  • ~2 cm of posterior parahippocampal gyrus

What happened after this surgery ?

  • In short, he lost the capability of form any new long term memory(this symptom is called 'Anterograde Amnesia). His skills and momery he aquired before seemed pretty intact (strictly speaking he failed to recall any memory for the 3 years of past period. this is called 'retrograde amnesia), but forming a new long term memory was impossible.  
  • So the doctor doing research on him had to reintroduce herself everytime they meet each other since he does not remember that he met her before.
  • However his procedural memory formation (e.g, reading words in mirror(i.e, backward reading)) seems intact (@45:00 this lecture)

I was not able to find YouTube video showing his symptom directly (he passed away before sharing video over internet is so common), but I found some videos explaining about him like this, this




Clive Wearing (May 1938 ~ )


Clive Wearing is a British former musicologist, conductor, tenor and keyboardist. He had similar brain lesion as H.M (i.e, lesion in Hipocampus) but he got this from his illness not by any physical surgery or accidents.  He lost the functionality of large portions of Hippocampus and temporal lobe which plays crtical function in memory process.


Image Source : Alchetron


After the illness, he has been suffering serious Amnesial.

  • He lost most of his his past episodic memory(retrograde amnesia) and is not able to form new long term memory (anterograde amnesia).  
  • He had around 20 years period of retrograde amnesia meaning his memory about past 20 years has gone (@53:40 this lecture).
  • His new memory goes only around 20~30 seconds and then reset. He is like waking up fresh every 20~30 seconds.
  • His non-declaritive memory (e.g, procedural memory like playing piano) was intact.

Personally Clive Wearing is more familiar to me than H.M because I got several YouTubes (watch this , this) showing / describing details of the problem he is suffering, whereas I was not able to find any video about H.M showing the live behavior of H.M.



Jill Price (1965 ~ )


Jill is famous as a person who never forget (more specifically, never forget about her episodic memory). autobiographical memory is a type of memory from her direct or indirect experience. Indirect experience in this context mean something she learned from TV, news paper, books that she watched / read.  


If you give her any specific events (some events memorized in her brain) she recalls the exact date for the event. Or if you give her any specific date, she recalls every details of what she did or learned in that specific date. Watch the video linked below to get a better sense of what this mean.  Her case lead to a lot of fundamental questions in learning / memory but as far as I know it is still in mystery on how such a memory works.


Image Source : YouTube


NOTE :  you can see the similar case with a few other people. check out this video.  This is a documentary showing more details of the symptom and some effort to figure out the mechanism.



Aplysia californica


This (a sea snail) would one of the most famous animal that lead to the opening of research on learning and memory at synaptic level and molecular level. This animal has very simple architecture of nervous system with much less number of neurons comparing to higher animal (this animal has only 20,000 neurons whereas a human has 100,000,000,000 neurons), but still capable of 'learning' and 'remembering'. The size of a nerve cell is huge (in mm scale in diameter) that you can see it with naked eyes. Only small numbers of neurons (only a few hundreds) get involved in a certain behavior of the animal (Watch @11:25 of this lecture if you want further details).

Scientists was able to train this animial with a certain behavior and direct observe the changes of synaptic networks and molecular composition changes before and after the learning.  You can get a pretty clear idea on how the research has been done from this short video.  If you are willing to tackle the far more details on this research, check out this lecture : Memories are Made of This.



Image Souce : Wiley Online Library


Image Source : (@13:27 of this lecture )




How to Learn to Learn ?


While you are burried with a lot of text and lecture videos, you may think almost everything about memory and learning process have already found, but unfortunately it is not the case at all.

Just try to check if what the text book and lectures can explain exactly what is happening in your brain while you are reading/watching such a complicated topics like neuroscience ? Is just understanding all the books/lecture video can give you the best way to study and learn a new topics / skills ? I don't think so.

What we know about the memory and learning is still at too early stages to explain the whole process of memory and learning.

So if you are interested in the efficient memory and learning, I think you may need to take a little bit different approach from just reading/watching neuroscience book/lectures.

My suggestion is to read/watch books and lectures provided by those who has been learning new things for their whole life in systematic ways and have skills/willingness to share their learning method/experience with others.

Personally I don't like some magical ways for learning (at least it seems magic to me)... like listening a specific type of music, practicing meditation etc. I am not saying that these method does not have any use. I am just trying to say I haven't personally practiced these techniques and they don't sound appealing to me either.

I prefer those skills/methods that can be linked to some neuroscience background even though they cannot be clearly explained by neuroscience alone.

I am trying to find those texts and lectures in YouTube and Reference section. Of course, this is completely my personal choice and you may or may not like them.