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Neuroscience

An Overview of Memory and Amnesia

Often described as simply the partial or total loss of memory, amnesia is one of those conditions we’ve all heard of, have seen in media and can somewhat grasp a hold of what it is. But aside from just waking up with no recall like in movie scenes, in reality, amnesia works in a much deeper and complex manner.

Introduction

Often described as simply the partial or total loss of memory, amnesia is one of those conditions we’ve all heard of, have seen in media and can somewhat grasp a hold of what it is.  But aside from just waking up with no recall in movie scenes, in reality, amnesia works in a much deeper and complex manner.

What is Amnesia?

There are two main classifications of amnesia: retrograde amnesia and anterograde amnesia. Retrograde amnesia is when the patient cannot process information and memories typically before the date of what has triggered the amnesia such as an accident or operation. Anterograde amnesia is when new information cannot be transferred from the short term memory store into the long term memory store.  In order to understand how amnesia is affected and affects the brain, you first need to understand how the brain should store memories.

How are Memories Stored in the Brain?

The way memories are processed is not clearly known however there have been several models made by psychologists and neuroscientists.  One of the most commonly referred to encompass the whole is the multi store model (MSM).  This model was curated by Richard Atkinson and Richard Shiffrin (1968, 1971) shows how information flows through the system through processing.

 A stimulus from the environment firstly will pass into what’s known as the sensory register.  Our sensory register has temporary stores for each of our five senses that has a high capacity of approximately one hundred million cells in each eye storing data and information, but it only lasts a period of half a second.  A couple of our biggest is our echoic store, which encompasses auditory information coded acoustically, and an iconic store, which encompasses visual information coded visually.  As our daily lives are full of billions of stimuli, the brain can only usually focus on a couple in order to process on to the short term memory store, hence the key to moving the information on is paying attention to particular stimuli.

Short term memory is a limited capacity store.  Many researchers have studied how large exactly the STM is.  One famous value being Miller’s magic number, 7±2; on average being able to store 5-9 pieces of information.  Information stored in the STM is coded acoustically, meaning that we remember it primarily by how we heard it or how it sounded.  In order for the information to eventually go to the long term memory (LTM) store, it needs to be maintenance rehearsed, otherwise it would last only about half a minute in our STM.

Long term memory store, also known as our permastore is for information that has been rehearsed constantly to the point the information can be remembered and recalled for many years and possibly decades.  The capacity for LTM is proposed to be unlimited.  According to the MSM, memories are ‘retrieved’ from the LTM back into the STM in order to recall it; none of the information is directly from the LTM.

This multi-store model is evaluated as somewhat insufficient as cases of patients with amnesia have proven there to be more stores within the STM and LTM than Atkinson and Shiffrin had proposed.

How Memories are Affected by Amnesia

Curated by Tulving (1985) in response to the MSM not elaborating further on the LTM, are a few different types of memory within long term memory.  They can be classified as declarative, broken down into episodic and semantic memories which you have to consciously think about to recall, and non-declarative as procedural memory, by which you can recall without conscious recognition. 

Episodic refers to our ability to recall from events that occur in ‘episodes’, or events in our daily lives.  These memories are associated to us by time stamps for example recalling something that happened to us last weekend, as well as certain people, places, and behaviours can also be associated to that episodic memory.

Semantic memories are our knowledge of the world, including facts and general knowledge.  These memories are clearly not as personal as episodic as they’re not time stamped, but rather just stockpile if rehearsed enough in your brain.

Procedural memory is also known as muscle memory.  It is our memory of how we physically do things like riding a bike.  Even if we do not ride a bike for an extended period of time, if we learned how to ride it as a child,  it should almost be instant picking it back up later on.  Procedural memories can proceed independently of the brain regions required for declarative memory.  According to fMRI studies, procedural memories activates the basal ganglia, the premotor cortex and the supplementary motor areas in the brain; regions that aren’t typically associated with the processing of declarative memories.

Famous cases of Henry Molaison (H.M.) and Clive Wearing prove that there’s multiple stores of LTM.  Both men were patients with amnesia; Clive had a viral infection in his brain, severely damaging the hippocampus, and Henry had a surgery to cure his epilepsy both resulting in amnesia that affected episodic memories.  They could not recall things they did or what happened to them shortly before, but their semantic and procedural memories were very much intact; Clive could play the piano pieces he knew by heart perfectly and they both knew how to tie shoelaces.  These two cases of amnesiacs had proven evidence the presence of different stores of LTM and they were in different regions of the brain, so if one was affected, the others aren’t  necessarily affected either.

Conclusion

Whilst amnesia detrimentally affects individuals and their loved ones roughly, having patches of memories disappear unbeknownst to the individual, amnesiacs have been one of the biggest contributions to neuroscience, psychology, and the general understanding of memory and how amnesia is initiated in individuals.

Nara Ito, Youth Medical Journal 2021

References

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By Nara Ito

Nara Ito is a student from London, England. She is interested in neurology, immunology, and genetics

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