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| The Human Brain |
Can you imagine what you would be without your memory? How would you feel and what would
you do if you woke up one morning and realized you could not remember the food you ate and what you did the previous night before sleeping? What if you can't recognize your spouse, or remember how to wear your shirt or eat your favourite meal? What if you can't find your way to that club you love to visit? Would the world still make sense to you?
Our
memory makes us who we are. It gives us our identities, and enables us to know
our family and the environment we live in. If we can’t remember our name, what
we did, when and where we did it, and if we can’t remember our environment,
then we have lost our identity. Also, it will be extremely difficult for us to
learn new things.
We
learn and relive our past experiences, even childhood experiences, because of the
amazing power to memorize things. Our memory allows us to learn new habits and
unlearn old nasty ones, and also develop relationships. These abilities to
learn new things, remember past events and develop relationships are among the
most basic powers of our brain.
What is memory?
Memory
is the ability of the brain to encode, store and retrieve information. It is the preservation of information over a period of time
in order to influence future action. It is involved in
processing vast amounts of information. This information takes many different
forms, e.g. images, sounds or meaning.
What are the stages of memory ? The three main stages of memory are encoding, storage and retrieval
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| Stages of memory |
Encoding:
When a piece of information or a sensory impulse enters our memory system, it has
to be changed into a form that the brain can handle, so that it can be
stored. Think of this process as similar to changing your money into a
different currency when you travel from one country to another. For
example, a word seen in a book may be stored if it is changed (encoded) into a
sound or a meaning (i.e. semantic processing). In addition,
studies have revealed that attention greatly affects how information is encoded
much more than how fast it is recalled. Hence, “distractions or divided
attention during initial learning may severely impair subsequent retrieval
success, whereas distractions during the recall of information may impede the
process a little, but has no effect on its accuracy.”.
There are three main
ways the brain can encode (change) information that it receives:
1.Visual (picture)
2. Acoustic (sound)
3. Semantic (meaning)
How do you remember a
series of numbers you see in a book? If you can see the numbers, then you
are using the visual coding; but if
you are repeating the numbers to yourself, you are using the acoustic coding. Also, research has
shown that it is the acoustic coding that is employed in short-term memory (STM). When a person is
presented with a list of numbers and letters, they will try to hold them in the
STM by rehearsing them (verbally). On the other hand, the semantic coding
(coding by meaning) is the principle encoding system used in long-term memory
(LTM). But information in the LTM can also be coded both visually and
acoustically.
Storage:
This
is another stage of memory which involves the nature of memory stores in the
brain, i.e., where the information is stored, the duration of the memory, how
much can be stored at any time (capacity), and what kind of information is stored.
How a piece of information is stored in the brain determines the way it is
retrieved. Most adults can store between 5 and 9 items in their
short-term memory. However, if we can chunk information together,
then we can store a lot more of it in our short-term memory. In contrast,
the capacity of LTM is thought to be unlimited, as information can last a lifetime
in the LTM, whereas in the STM, it can last for a about 0.3s.
Retrieval: This is yet another stage of memory
that involves getting information out of the brain. Whenever we can’t recall a
thing, then it means we can’t retrieve it from where it is stored in the brain.
And the difference between the short and long term memory becomes evident when
we try to retrieve information.
STM
is stored and retrieved sequentially. For example, if a list of words is
read out to a group of participants, and then they are asked to recall the
fourth word on the list, the participants will go through the list in the order
they heard it in order to retrieve the information.
On
the other hand, LTM is stored and retrieved by
association. This is why you can remember what you went upstairs for if
you go back to the room where you first thought about it.
Organizing information
can aid its retrieval. You can organize information in sequences (such as
alphabetically, by size or by time).
What are the types of memory?
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| Types of human memory |
The
three main types of human memory are the sensory,
short-term and long-term memory. The long-term memory is then divided into explicit (declarative) memory and implicit (procedural) memory
(Types of memory, n.d.).
“The
sensory and short-term memories must be worked through before a long-term
memory can be established. And each of the three main types of memory has its own particular mode of operation, but they
all cooperate in the process of memorization. This model of memory as a
sequence of three stages, from sensory to short-term to long-term memory,
rather than as a unitary process, is known as the modal (multi-store)
or Atkinson-Shiffrin model, after Richard Atkinson and Richard
Shiffrin who developed it in 1968.”
Sensory Memory:
The sensory memory is a
transient element of memory. It is the ability to retain impressions of sensory
information after the original stimuli have ended. It acts as a kind of buffer for stimuli received through
the five senses of sight,
hearing, smell, taste and touch, which are retained accurately, but very
briefly. For example, the ability to look at something and remember what it
looked like with just a second of observation is an example of sensory memory
Short-term Memory:
The short-term memory is also known as the working memory. It acts as a kind of
“scratch-pad” for temporary recall of the information which is being processed
at any point in time. It can be thought of as the ability to remember and
process information at the same time.
Long -term Memory:
The long-term memory is intended for storage of information over a long period
of time. Even though we forget things everyday, it seems that the long-term
memory actually fades very little over time, which is why it can store a
seemingly unlimited amount of information almost indefinitely.
Short-term
memories can become long-term memory through the process of consolidation,
involving rehearsal and meaningful association. But unlike the short-term
memory (which relies mostly on an acoustic, and to a lesser extent, a visual
code for storing information), long-term memory encodes information for storage
semantically (i.e. based on meaning and association).
Explicit Memory:
The explicit memory, also known as the declarative memory is the conscious, intentional recollection of facts,
previous experiences and concepts. The explicit memory is
divided into episodic and semantic memory. Episodic memory consists
of the storage and recollection of observational information attached to
specific life-events. These can be memories of personal experiences or memories
of events that happened around a person. Episodic memory allows a person to
mentally travel back in time – recalling various contextual and situational
details of one's previous experiences. On the other hand, semantic memory
refers to knowledge of facts, ideas,
meaning and concepts that can be articulated, and is independent of personal
experience. This includes world knowledge, object knowledge,
language knowledge, and conceptual priming.
Implicit memory:
the implicit memory or procedural memory refers to unconscious memories such as
skills (e.g. knowing how to get dressed, eat, drive, and ride a bicycle without
having to re-learn the skill each time). The implicit memory allows us perform
a regular task or a habit without thinking about the procedure.
The
other types of memory include autobiographical
memory and the spatial memory. The autobiographical memory is a memory
system that consists of episodes recollected from an individual's life, based
on a combination of episodic (personal experiences and specific objects, people
and events experienced at particular time and place) and semantic (general
knowledge and facts about the world) memory.
The
spatial memory is the part of memory
that records information about one's environment and its spatial orientation. A
person's spatial memory is needed in order to navigate around a familiar city.
For example, you use
your spatial memory to locate your friend's house, find the ATM when you need some cash, and locate a restaurant when you are hungry. Research has shown
that there are specific areas of the brain, such as the hippocampus, that are associated with spatial memory.
What are the brain structures involved in memory?
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| Mid-saggital section of the brain showing the hippocampus and some structures in the basal ganglia and limbic system. |
There are three most
important brain structures involved in memory. They include the hippocampus, lateral prefrontal cortex (PFC) and amygdala.
However, the hippocampus and its surrounding structures are the most important
in declarative memory (Eichenbaum, 2001).
The
other regions of the brain that play a role in memory include regions of the
diencephalon, which are activated when a remote memory is being recalled (Graham
& Levine,
2004). In addition, the occipital lobe, medial temporal lobe, and fusiform
gyrus all play a role in memory formation (Gabrieli & Kao, 2007).
Hippocampus:
The hippocampus is essential for memory function, particularly the transfer
of information from short to long-term memory.
The hippocampus is also responsible for the control of spatial memory and behavior
(Parts of the brain, n.d). It also has a great ability to retain and recall
episodic memories (Eichenbaum, 2001). Moreover, the formation of new
declarative memories relies on both the hippocampus and parahippocampus (Gabrieli & Kao,
2007)
Amygdala
The amygdala
The
amygdala performs a primary role in the processing
and storing of emotional reactions, social and sexual behaviour, as well as
regulating the sense of smell (Parts of the brain, n.d). It is believed
to be involved in the encoding and retrieval of emotionally charged memories,
such as the Biafra War and the cancellation of the results of the 1993
presidential elections. Such emotional memories have been linked to increased activity
in the amygdala (Sharot et al, 2007).
Lateral Prefrontal Cortex: The lateral prefrontal cortex plays an important
part in processing short-term memories and retaining long-term memories. It is
also essential for remembering contextual details of an experience rather than
for memory formation (Parts of the brain, n.d; Gabrieli & Kao, 2007). The
PFC is also more involved with episodic memory than semantic memory, although
it does play a small role in semantics (Graham & Levine,
2004).
The
factors that affect declarative memory include stress and neurochemical factors
such as cortisol. When subjected to
a stressful condition, an individual might come down with decreased declarative
memory performance (Lupien et al, 1997).
Posttraumatic stress disorder
(PTSD) emerges after exposure to a traumatic event
eliciting fear, horror or helplessness that involves bodily injury, the threat
of injury, or death to one's self or another person (Cabeza & LaBar, 2006).
The chronic stress in PTSD contributes to an observed decrease in hippocampal
volume and declarative memory deficits (Baker et
al, 2005).
Cortisol is the primary
glucocorticoid in the human body. In the brain, it modulates the ability of the
hippocampus and prefrontal cortex to process memories (Damoiseaux & Elzinga,
2007).
A
recent study by Dal Maso et al showed that cardiovascular exercise performed
immediately after practicing a visual-motor tracking task has been shown to
improve the long-term retention of this motor skill through an optimization of
the memory consolidation process. After the experiment, the researchers identified
potential neural substrates through which a single bout of acute exercise, when
performed in close temporal proximity to motor practice, strengthens motor
memories. In another study, it was discovered that sleep boosts memory in young
adults (Jones et al, 2018).
Sleep
plays an active role in the consolidation of declarative memory, and the mechanism
by which it does this is by reactivating hippocampal memory representations.
This reactivation transfers information to neocortical networks where it is
integrated into long-term representations (McClelland et al, 1995).
Examples of memory disorders
Memory
disorders occur as a result of damage to neuroanatomical structures, leading to
impairment in the storage, retention and recollection of memories. Memory
disorders can either be progressive, as in the case of Alzheimer’s disease, or
immediate as in the case of a head injury. Memory disorders include:
- Agnosia
- Alzheimer's disease
- Amnesia
- Brain injury
- Dementia
- Hyperthymestic syndrome
- Huntington's disease
- Parkinson's disease
- Stress
- Wernicke- Korsakoff's syndrome
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