Episodic and Semantic Memory

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Introduction

When classifying long term memories, we end up with two major groups. One is Declarative or Explicit Memory and Non-Declarative or Implicit Memory. We will be discussing a detail about the declarative memory in this article. This class is further subdivided into Semantic Memory and Episodic Memory. 

The semantic memory focuses much on the factual and conceptual knowledge about the world and the way it expressed in terms of words. So basically, it supports the ability to interact in terms of language. This includes knowledge about the language and conceptual information. Rather general knowledge also counts in the same. 

The episodic memory focuses on one’s life events that the person has experienced throughout the phases of his life. These are the memories which get stores in one’s limbic system. This is going to involve the memory from one’s perspective but will surely not account for evident facts and figures. It too involves the two major components about the event which are when did the event occur and where? 

Examples of Semantic Memory

  • While eating an apple, you recognize Apple as fruit and from your knowledge, can confer its importance. 
  • When listening to the birds chirping near the window, you straight away point out the bird to be the sparrow. 
  • The calculation of the month’s grocery budget through simple additional methods. 
  • You are scheming and planning to eat your favourite cuisine at your favourite Chinese Restaurant and to pay on spot the charges for what you ate.
  • Introducing yourself with the known qualities a good person may possess. 

Examples of Episodic Memory 

  • The memory you had with your squad over the Friend’s Wedding. 
  • The memory of what you ate in breakfast this morning 
  • It can be an unforgettable tragic memory which you had while an accident. 
  • The failed one-to-one session at an interview recently. 

How episodic and semantic memory may work in integration? 

Conjugating the certain episode in terms of the period can equally be explained through this phenomenon. Like, some days you are not sure about the day you are in. But you may confess, that it is your routine to have a check-up on Monday, so it might be Monday this day. Or it can be like you may fail to recognize one thing, but integration in terms of time and space will help you recall. 

Though, episodic memories particularly about the events when and how it happened but it must not involve remembering the experience. Like you remember being born on 15th September in London, but you don’t remember the overall experience. 

3Rs associated with memory-making 

Sometimes it happens, people fail to make memories. This means that they aren’t able to recognize and recall things even which took place a few seconds earlier. It is mostly seen in the rare case of Herpes Encephalitis, a viral infection. The Central nervous system is ravaged and is among the profound cases of amnesia.

Our memories account a lot in our lives to help us connect our past to the present. Hence, the learning process is summed up eventually to bring about a desired interactive effect. Memories are like not all or nothing things. Despite not able to recall recently processed memories, a person suffering from encephalitis may remember how to eat, what to speak and how to sketch.

Few of the memories store differently and gets into you through automated work processes you go through. Memory is defined to be the learning that has become persistent over time, stored and can be recalled. To access your memory, you need to consider 3Rs. These are recall, recognition and relearning. 

The recall is the retrieval of the memory a person must have learnt earlier. Like, recalling Mango as the king of all fruits. The recognition is like identifying all relatable information and eliminating the odd one. Like, from a list of mango, orange, jasmine and banana you’ll surely be excluding jasmine. The relearning is reinforcing information you have been learning all the way. Like learning the mathematics formulae and then revising it. All work through synaptic connections within our brains. 

How we make memories? 

The memory formation is broken down into three main stages: 

  • Sensory Memory encoded into the brain. The immediate things which we want to record are taken up as sensory input and then shuffle it into short term memory. 
  • Short Term Memory form. The incorporated memory stays there just for 30 seconds without rehearsals. Your mind can’t remember beyond 7 bits of information at a time. 
  • Long term memory encoding. This is the durable storage compartment of your brain where the memory tends to stay for long. The way the short-term memory is calculated to be in the long-term memory is through the principle of ‘Working Memory’. It involves all the ways through which profound cognition is achieved through auditory rehearsals and executing visual-spatial information. 

Episodic Vs Semantic Memory

The episodic and semantic memories are information processing systems. In 1996 Gibson explained memory to be the selective retrieval of information from perceptual systems or other cognitive systems, retain that information, transmit the desire information to other place and latter translating it into conscious and behavioural awareness. 

The episodic memory receives and stores information for short episodes and the events show a temporal-spatial relationship. While the semantic memory is necessary for the use of language.  A perpetual event can be stored in the episodic forms, but it depends solely on how the person perceives while the latter accommodates the integrated word bank and overall knowledge the person possesses. 

The episodic memory is a thought dependent process while the semantic memory is independent of episodic streams. It maintains and records memories through logical inputs. 

The cognitive approach to Long-term memories 

Tulving suggested this idea and he came up with the multi-core model of the theory. He proposed to major classifications which are episodic and semantic. The earlier is concerned with remembering experiences while later with remembering facts. 

The episodic memory is more at an autobiographical front that can be explicitly stated. The semantic memory is a derivative of episodic memory to capture facts and figures. There is a transition from episodic to semantic terms. The episodic memories are more related to hippocampus regions while the latter is known to activate frontal and temporal cortexes. 

Tulving further refined his concepts by adding that subjective time allows possible mental time travel from present to past linking the events. Additive recognitions were self-knowing credits of the episodic memories. 

He too brought attention towards the loss of episodic memory is concerned with temporal medial lobe lesions where semantic memory is known to stay intact. While people suffering from semantic dementia lose this type of memory, while the episodic memory is spared. 

Episodic Foresight 

It explains the future is dependent on the memories made in past. It is the way through which one can see himself in future and can come up with the outcomes beforehand. This is an emerging ability in the younger ones. These processes underlie the same cognitive processes. 

It allows one to analyse situational affairs and getting it in terms of future. Exploring research is mainly done on 3-6 years old children and young adults to find the relationship between episodic memory and future-state-planning. It allows the crosstalk between current situations and future motivational states. 

More and more researches are taking place in terms of this emerging theory. The studies mainly focus on the fact that earlier years in school impact the thinking of young ones letting them plan, prepare and shape their future. They even anticipate the hazards associated with it. 

Interdependence of Episodic and Semantic Memory

It's been a long debate in neuropsychology in concern to both dependence on each other. Tulving's made a great effort in distinguishing between semantic and episodic memory in early 1972. The studies showing interdependence reveals that bot memories have a profound effect on each other. The effect is in terms of encoding and retrieval. 

When there’s damage to medial temporal lobe severe episodic impairment can be seen that has a profound effect on both anterograde and retrograde memories. This will cause impairments in recalling the past event and bringing new connections into the future. The semantic memories are a bit different, MTL loss does not affect factual concerns. For this type of memory takes a valid space in the neocortex. 

But there are certain cases when semantic memory is lost. It happens in the case of semantic dementia where progressive neocortical degeneration is seen. The verbal stimuli are non-differential for such patients and changes in other modalities like odour and taste. 

Neuropsychological studies have after all come to the point that both memories are independent. It reveals that there are certain situations when one memory time remains intact while the other is impaired.  These both are doubly dissociable bringing us to the conclusion that both are relatively distinct. 

Additionally, memory theorists have come up with a different narrative. They say that both memories don’t need to operate in isolation. Rather each one of them has a sound impact on the other. 

Thus, there are different narratives on the perspective of interdependencies varying from theorists to theorists. 

Areas of the Brain associated with Episodic and Semantic Memory 

Episodic memory is a past-oriented memory which will allow you to re-experience the same via the process of recalling. It has several distinct interactive components that allow it to do its task purposely. This whole process can be explained easily through a single term named ‘retrieval’. The areas of the brain which are involved in this process are frontal cortices, temporal and parietal region, cerebellum, diencephalon and hippocampus. The process of retrieval can be altered through genetic makeup and regular retaining capacity of an individual. 

Semantic memory is stored by the same systems as in episodic memory. These include hippocampal and temporal lobes to encode memories. We have an entorhinal cortex and perirhinal cortex involved in all such tasks. These two cortices are collectively called the Para hippocampal cortex. 

Role of the Hippocampus in Memory Formation 

The memory formation is a cognitive process. The brain is divided into four lobes; the frontal. The parietal, the temporal and the occipital lobe. Deep in the medial temporal lobe is a set of structures collectively known as the limbic system. Out of which is the hippocampus.  This is a structure found in both cerebral hemispheres in its medial part. 

The hippocampus means sea horse as it takes such shape. The hippocampus has several discrete parts which include Para hippocampal gyrus, entorhinal cortex, subiculum and dentate gyrus. The hippocampus has roles in spatial awareness, recollection, consolidation of declarative memory. Declarative memory is the type of memory you declare like facts and events including both semantic and episodic memory. 

Brain Lesions have cause memory impairments as stated earlier. However, there are different types of memories monitored by different parts of the brain. Like, non-declarative memory which includes procedural and habitual learning is known to be supported by basal ganglia. 

Trisynaptic Circuit and its association to the Hippocampus 

The Trisynaptic circuit is what hippocampus is occupying major sensory input which enters through the entorhinal cortex. It’s a bundle of fibres with input tracks acting as a hub for the widespread network for memory. These fibres project into the granule cells of the dentate gyrus. This is the prefrontal pathway and the first pathway of the trisynaptic circuit. 

From here dentate gyrus ill transmits its signals to CA3 through the mossy fibres which are the second pathway. These fibres have dense reciprocal connections that can generate new electrical activity. A single mossy fibre projects around 30 pyramidal cells of CA3. CA stands for Cornus Ammonis into four subgroups CA1 - CA4. This is the major division of hippocampus. This name comes from the similarity of hippocampus shape to ancient Egyptian who has same elevated head knots. 

Then there comes the third pathway called Schafer Collateral Pathway connects CA3 to CA1 collateral neurons where it has the greatest number of NMDA receptors in the brain. Finally, CA1 neurons project into the Sibiculum which is considered a major output region for the hippocampus. It goes to cortical and subcortical regions. The input to subiculum from all cortices of the hippocampus and send projects into the fornix but get back into the entorhinal cortex to complete the loop. 

All the cells that make up the hippocampus will collectively act as Hippocampal index. It can retrieve and recall the information that is necessary for a particular memory. It will be built stronger connections to different areas of the brain to make the process of recalling easier. This is mainly seen in the term of when individual is learning or studying. Hippocampus must communicate to many cortical distribution sites and to collect information from all the widespread areas.

Hence, it serves to be an elementary subcortical region while processing in and retrieving different memories either explicit or implicit. These include both episodic and semantic memories. 

Summary

Episodic memory and semantic memory are the two types of declarative memory. These are the long-term memories stored in the brain of a human being. 

Episodic memory is associated with the events that take place in the life of an individual. These memories are stored in the limbic system of the brain. 

Semantic memory, on the other hand, is associated with some facts and figures. It is the conceptual memory that is stored in the brain of a person.

Examples of both these memories have been listed in the article. 

In some instances, both these types of memories may work in integration. 

The memory formation and retrieval system are based on the 3 Rs; recall, recognize and relearn. The process of memory formation is often divided into three steps that have already been described. 

The episodic memory is only involved in storing the information about some events while the semantic memory involves storing logical inputs in the brain. 

Episodic foresight is the phenomenon by which a person can see himself in future and can analyse the outcomes for a better strategy. It can impact the future of a person to a considerable extent. 

Different views exist among the neurophysiologists about the interdependence of episodic and semantic memory. 

Both the episodic and the semantic memories are stored in the hippocampus and other regions of the temporal lobe. In addition, frontal and parietal cortex, as well as diencephalon, also play an important role in this process. Hippocampus has a major role in the process of memory formation as a part of the three synaptic pathways. The hippocampal lesions can severely impair the memories stored in the brain. 

References

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