LONG-TERM MEMORY

Introduction

Long-term memory is the storage of information for a long time. Long-term memory is the final stage in the processing of memory. The Information stored in long-term memory lasts longer than those is short-term memory. Long-term memory decays very little with time and it is easier to recall.

Our conscious mind may not be aware of the information stored in long-term memory. But this information can be recalled with ease and accuracy. Examples of long-term memory are the recollection of an important event in distant past or bicycle riding skills someone learned in childhood.

Some things easily become part of long-term memory while others may need continuous practice to be stored for a long time. It also varies from person to person. Some people can remember complex things with little or no difficulty while others may struggle in remembering easier and daily life information.

Long-term memory is usually defined in contrast to short-term memory. Short-term memories last only for about 18-30 seconds while long-term memories may last for months or years, or even decades. The capacity of long-term memory is unlimited in contrast to short-term and working memory. A lot of researches have shown that different types of long-term memories are stored in different parts of the brain.

Types of Long-term Memory

Long-term is divided into many types. We will discuss all types one by one.

Explicit Memory

Explicit memory usually refers to all the memories and information that can be evoked consciously. The encoding of explicit memories is done in the hippocampus but they are stored somewhere in the temporal lobe of the brain. The medial temporal lobe is also involved in this type of memory and damage to MTL is linked to poor explicit memory.

The other name used for explicit memory is declarative memory. Explicit or declarative memory is divided into two types: episodic and semantic memory.

  1. Episodic Memory
    • Episodic memory stores information about events that happen in a person’s life. It refers to knowing the time and place and details of events. Some examples of episodic memory would be the memory of 1st day of your marriage, or memory of tour to another country and all the events that happened there.
  2. Semantic Memory
    • Semantic memory is responsible for the storage of factual information such as the meaning of words or general knowledge of things. An example of semantic memory would be knowing that Jupiter is the biggest planet of the solar system.  Semantic memory involves conscious thought. Very few differences have been seen in the encoding of semantic information in adults and younger people.

Implicit Memory

Implicit memory is the opposite of declarative memory. It refers to the movement of the body in using objects. An example of implicit memory would be how to ride a bicycle. Several brain areas which include basal ganglia, parietal and occipital regions are involved in implicit memory. This type of memory is largely independent of the hippocampus. Writing, riding, driving, and swimming are all examples of implicit memory because they are non-declarative.

  1. Procedural Memory
    • Procedural memory is the memory of motor skills and it is responsible for knowing how to do things. This memory is automatic i.e. it works at an unconscious level. Procedural memories are non-declarative and retrieved automatically for in procedures that involve motor skills. For example, riding a bicycle is a type of procedural memory.
  2. Associative Memory
    • Associative memory usually refers to the storage and retrieval of specific information through association. The acquisition of this type of memory is carried out with two types of conditioning. One is classical conditioning and the other is operant conditioning. Classical conditioning refers to the learning process in which stimuli and behavior are associated. On the other side, operant conditioning is a learning process in which new behaviors develop according to the consequences.
  3. Non-associative
    • Non-associative memory refers to the learning of new behaviors mainly through repeated exposure to a single type of stimuli. The new behavior is classified into habituation and sensitization. Habituation is the decrease in response to repeated stimuli while sensitization is an increased response to repeated stimuli.
  4. Priming
    • Studies have shown that exposure to certain stimuli influences the response of a person to stimuli that are presented later. This effect of previous memory on new information is what we call priming.

Difference between Short and Long-term Memory

It is thought that Long-term memories differ from short-term memories in the aspect of their longer duration. But the difference between these two types depends upon their definition by someone. Defining both types of memories in clear terms in the first step of differentiating between them.

These memories differ in two fundamental aspects. The first is the duration and the second being chunk capacity limits. There is a huge difference between the duration of these types of memories. Long-term memory has a duration of months and years while short-term memories are thought to stay only a few seconds. There is also a difference in capacity. Short-term memory stores only a tiny bit of information. On the other side, the capacity of long-term memory is thought to unlimited.

Physiologically, the process of establishment of long-term memory differs from that of short-term memory. It involves a change in neuronal structure i.e. long-term potentiation. New neural networks are created and strengthen. The neurons communicate with each other through synapses. The release of neurotransmitters in synaptic clefts enhances the communication between the cells. This whole process does not take place during the creation of short-term memories. Unlike short-term memory, the long-term memories are forgotten only in the case superimposition of a new neural network over the older network. 

Short-term memories can be changed into long-term memories through consolidation, a process involving rehearsal and association of information. Short-term memory relies on visual and acoustic encoding while long-term memories are encoded semantically.

Memory Encoding and its Types

Memory encoding refers to the changing of sensory stimuli or information so that it can be stored and retrieved. The information undergoes this process so that it can become a part of long-term storage. The properly encoded information is very easy to be recalled. There are three main types of memory encoding: visual, acoustic, and semantic.

Visual encoding is converting a visual stimulus to store the information in the brain. This information is first stored in the visuospatial sketchpad. Then, it is temporarily stored in working or iconic memory before its storage in long-term memory.

Acoustic encoding refers to the encoding of acoustic information to understand the acoustic aspects of an event. It is the processing of sounds, words, and other auditory information to store that information in long-term memory. An important part of acoustic information is the phonological loop.

Information that has a particular meaning or context is processed in a way that is called semantic encoding. Concepts, Ideas, and terms are some examples of semantic information. The semantically encoded information is relatively easy to be retrieved. There are also some other types of memory encoding which may include tactile encoding, etc.

Capacity and duration of Long-term Memory

So how much information can be stored in the brain in the form of long-term memory? And for how much time? Well, it depends on several factors. Generally speaking, scientists believe that a human brain can store an unlimited amount for a duration that may go beyond decades.

The first factor that influences the duration of long-term memory is the way memory was encoded. Optimally encoded memories last much longer than shallow processed memories. Another factor is the retrieval of memory. The number of times a specific memory is accessed plays an important role in the strengthening of memory. This is probably the reason for better retrieval of information that is repeated and practiced again and again. Giving attention and focus to the information makes it stick to the brain for a relatively long time.

The capacity of long-term memory is thought to have no limits. According to some studies, the upper bound on the size of visual and acoustic long-term memory has not been reached. We may find it difficult to encode the details of many events but under certain conditions, a person succeeds when he focuses and tries to encode the information.

Changes in Long-term Memories

Long-term memories are not permanently stored in their original condition. Memories are susceptible to change, interference, and also misinformation. Memories are transformed every time they are pulled up. In the process of encoding, the neurons first encode memories in the hippocampus and brain cortices. Whenever a memory is retrieved, it is re-encoded by similar neurons, but not identical to previous ones.

Re-encoding of memories have a great impact on their storage. Details of the memory may change due to re-encoding. Certain aspects of long-term memory may strengthen or weakened depending upon the types of neurons activated. These memories are susceptible to inaccuracies because people sometimes miss details of events. The brain then fabricates the details to fill in the missing gaps. In some cases, old memories may affect the formation of new memories. This may lead to the change in memories or encoding of false memories.

Physiological Aspects of Long-term Memory

Previously, it was believed that only the cortex of the brain stores long-term information. Now we know that they are stored in different regions throughout the brain and other parts of the nervous system depending upon their type. Memories are not somewhat localized but stored through circuitry. Some types of memories may be stored throughout the body because receptors for chemicals in the brain are found everywhere.

When neurotransmitters are activated in the brain, a process called chemotaxis communicates the message to every part of the body. This communication is done basically through blood and cerebrospinal fluid. In this way, some memory may also get stored in muscles. People with organ transplants have reported the emotional reactions and feeling to certain events that they never had before.

Long-term Memory loss

Long-term memory loss refers to the difficulty in recalling the information. It can also be a sign of some serious problems such as dementia.

Sign and Symptoms

Here are some signs and symptoms of long-term memory loss.

  • Forgetting early life events
  • Mixing up names of persons and places
  • Excessive irritability and mood changes
  • Forgetting common and easy words
  • Getting lost in previously familiar places
  • Trouble in recalling details of events
  • Taking a longer time to do familiar tasks

Causes of Memory Loss

There are many causes of long-term memory loss. These causes can be classified into reversible and irreversible causes. Reversible causes can be treated. Examples of these causes include:

  • Depression and anxiety
  • Vitamin B-12 deficiency
  • Hydrocephalus
  • Mental health problems

In some cases, loss of long-term memory may be a result of brain injury. the causes of brain damage are:

  • Alcohol
  • Brain infections
  • Brain tumors
  • Stroke
  • Oxygen deficiency
  • Drug abuse

Irreversible causes of long-term memory include Alzheimer’s disease and dementias. Alzheimer’s disease causes memory loss, difficulty in comprehension, reasoning, and judgment. Dementia is also a big problem in developed countries. Its first symptom is short-term memory loss which is then followed by long-term memory loss.

Diagnosis

Taking the history of patients is the first step in diagnosing long-term memory loss. This history should cover medical history, family history, and history of medication.

The second step in diagnosing this condition is a physical exam. The physical exam may include checking for muscle weakness, brain damage, and vitamin deficiencies. Some times complex neuropsychological testing is done to diagnose this condition.

Treatment

There are various treatments for this condition depending upon the underlying cause. If the underlying cause can be removed easily, then it is removed. Otherwise in conditions like Alzheimer’s disease, Cholinesterase inhibitors, and partial N-methyl D-aspartate (NMDA) antagonists are prescribed by the physician. Regular exercise, adequate sleep, and a healthy diet may also help in some cases.

Ways to Improve Long-term Memory

Attention

Attention is an important requirement to improve long-term memory. Actively attend the information being presented to make it a part of long-term memory. Students should stay away from distractions such as television, music, smartphones.

Good Night-Sleep

Quality sleep is known to optimize the neural processes of the brain. Slow-wave sleep has shown an important role in the consolidation of long-term memories. Sleep deprivation impairs the ability of the brain to encode new memories during the daytime. Optimal sleep of 7-8 hours a day is always recommended.

Exercise

Exercise is known to activate the muscles and keep the heart working properly, which has a positive impact on brainpower. Exercise enhances the chemical and neurotransmitters that empowers the brain to grasp concepts and make them part of long-term memory.

Retrieval

Retrieval is known to be one of the best strategies to convert short-term memories into long-term memories. Retrieving information taking tests is an amazing strategy for students to score more in the exam. Retrieving allows the information to be processed at a much deeper level than the processing of short-term memory. Memories that are not retrieved and recalled weaken and are sometimes replaced by other information.

Visualization

Imagination and visualization refer to an association of images with words to improve neuronal connection strength. Students benefit greatly from visualizing the concepts and information. This association leads to a great improvement in the storage and retrieval of long-term memories.

Role of Gene Transcription

Long-term memory formation requires the synthesis of new messenger RNA (Ribonucleic acid). There is an increased expression of some genes during and after the learning process. Transcription factors and signal transduction mechanisms that guide the process of formation of mRNA have been identified. Epigenetic modifications are critical for memory storage because they play a role in the regulation of transcription. Memory formation also requires molecular processes for the regulation of neuronal transcription.

Effects of Certain Drugs on Long-term Memory

Drugs of abuse like cocaine and marijuana damage neurons to a great extent. Sedative drugs and benzodiazepines which are mind relaxers and stimulants also exert bad effects on memory.

Some drugs are used as memory supplements. Phosphatidylserine is used for the treatment of neurological diseases, the diseases which cause brain damage, like Alzheimer’s disease. These drugs improve cognitive and storage abilities of an individual. These are used as powerful boosters to improve cognition.

Effects of Alcohol on Long-term Memory

Alcoholic consumptions are mainly associated with the destruction of the hippocampus and the nerve cells. The nerve cells responsible for memory encoding, storage, and retrieval are destroyed. An excessive amount of alcohol affects the stomach lining which causes ulcers and other gastrointestinal problems.

Alcohol also interferes in many ways with thiamine. Firstly, it impairs proper thiamine intake as alcoholics more often skip meals. Thiamine converts certain carbohydrates to glucose. Our brain only utilizes glucose for its energy requirements. Once thiamine gets deficient, there is an improper conversion of carbohydrates to glucose. This may lead to brain damage.

Summary

Long term memory is the information stored in the brain for a long time that can be recalled with ease.

Long term memories are divided into explicit and implicit ones.

Explicit or declarative memories are those that can be recalled consciously. These include memories related to some events called episodic memories, and memories about some facts called semantic memories.

Implicit memories are related to some skills that a person learns. They cannot be recalled consciously. These include skills like riding, writing, speaking, swimming, etc.

Long term memories last for much longer time as compared to short term memories and have unlimited storage capacity.

Long term memories are encoded in three ways.

  • Visual encoding involves the conversion of visual stimuli or information
  • Acoustic encoding involves audio information
  • Semantic encoding involves concepts and ideas

The capacity of long-term memory depends on the way it is encoded and the number of times it has been assessed or recalled.

Memories are not stored in their original form. They undergo certain changes during encoding and re-encoding of memories.

Long term memories are not stored in one specific region of the brain. Rather, they are stored in the form of circuits throughout the nervous system.

Long term memory loss can be seen in different memory disorders. The patient presents with a number of signs and symptoms. There might be a number of causes for memory loss.

The capacity of long-term memory can be also be improved in several ways.

References

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