Mind Matters in Children Chapter 6: Memory
This series of articles has been adapted from [Mind Matters in Children] by Dr Kenneth Lyen. It delves into both the normal as well as the abnormal functioning of a child’s mind, and gives practical advice on how to manage children’s mental health and problems like ADHD, dyslexia, autism and depression.
“Wah, you have a good memory!” my student recently told me. “Sorry, but what’s your name?” I asked the student, half seriously. I have no problem memorising biological names like dermatophagoides pteronyssinus, but when it comes to people’s names like Sudjiatmi Notomihardjo, I am totally lost. Have you sometimes felt that you know the answer, but it’s stuck at the tip of your tongue?
We will explore the incredulity of memory, as well as its fickleness. Understanding its amazing capabilities is still one of the major challenges facing mankind.
What is memory?
Memory is the ability of the brain to encode and store information with the facility of recalling them later when required.
Without memory, it would be impossible to develop language, personal identity, social relationships, and using past events to plan future events. Hence memory governs our lives.
Visual images, sounds, touch, smells, taste and other sensations first need to be translated into signals that can travel along nerves. This encoding allows us to select a route on a map that can lead us to our memory destinations in the brain. The different sensations may stop by service stations en route inside the brain and undergo further processing before they reach the main memory banks. Once stored, we can periodically withdraw varying amounts of memories from these banks. Retrieval is an important step because the memories can be put to good use. If we merely store memories but never retrieve them, our memories will languish in the bank, like a dormant account.
Types of Memory
Memory can be divided into three major types: sensory, short-term, and long-term.
1. Sensory Memory
The five major senses detect signals from light, sounds, touch, smells and taste. They correspond to these five subdivisions of sensory memory: iconic, echoic, haptic, olfactory and gustatory. Sensory memory only lasts between a fraction of a second to at most only 3 seconds.
Fig 1. Types of Memory
2. Short-Term Memory
Short-term memory refers to the memory formed while you are paying attention to something, like hearing a brief conversation (Fig. 1). This memory lasts between a few seconds to one minute, assuming you did not reinforce it by repeating or rehearsing the stimulus. Take, for example, we ask you to remember a series of numbers or words like: Piano, Cat, Detergent, Quarantine, Cool, Pencil, Kiasu, etc. Then we ask you to write down as many words as you can. Most people can only remember around 7 items, which is the reason why most telephone numbers (minus the area code) only consist of 7 numbers.
Some people regard “working memory” as equivalent to short-term memory. But theorists nowadays regard them as two distinct types of memory. Working memory is also known as operative memory, and it is the transient storage of information while trying to manipulate temporary information.
Examples of working memory tasks include trying to add some numbers in your head, pattern recognition, or following a series of instructions you have just heard. The information needs to be held in your mind for a few seconds while the problem is being addressed. However, one may sometimes draw on long-term memory to resolve the issue. And this is why working memory can differ from short-term memory.
Long-term memory can be divided into “Explicit” and Implicit”.
a) Explicit Memory
Explicit memory is also known as declarative memory. It is defined as memories that you are conscious of. For example, you can remember events in your childhood, like an accident you had many years ago. This is also placed in a subclassification called episodic memory. Another subdivision of explicit memory is known as semantic memory, and this refers to general knowledge facts, like what is the boiling point of water. Both episodic and semantic memories are conscious, which is why they come under the umbrella of explicit memory.
b) Implicit Memory
The other type of long-term memory is implicit (non-declarative or unconscious) memory. These are memories that you store unconsciously. The first subset of implicit memory is known as procedural memory. This usually involves your motor skills like riding a bicycle, driving a car, buttoning a shirt, or typing on a keyboard. These activities are performed automatically and do not require full consciousness.
A second subset of implicit memory is known as priming. For example, when you see a picture, hear a song, or smell something sweet, and suddenly the sensation triggers off another memory, this is linking or priming a separate memory. Examples of this type of pairing are when you see a banana, you think of the colour yellow, or if you see a rat you think of a cat. It is reminiscent of Pavlov’s dogs that salivate in response to a bell.
Use it or Lose It!
When new memories are formed, the brain transmits signals from one neuron to another across a synapse. Experiments on the sea slug discovered the following: frequent usage of these nerve pathways will strengthen the synapse, but if unused, the connections are weakened and then lost; a principle popularly known as “use it or lose it!” In addition to forming stronger synaptic connections, frequent usage of nerves will generate new branches or dendrites, which will in turn join up to form new connections. Both these actions are involved in the formation of long-term memories. The strength of a synapse is measured by the level of excitability of the post-synaptic neuron in response to a stimulus. Over time, high frequency and repeated stimulations can reinforce synaptic connections which will lead to long-term potentiation. As the name suggests, the resulting potentiated synapses can last a long time, and is the basis of long-term memory.
Memory over the Life-Span
Why do we not remember anything below the age of about 3 years? After all, babies can remember faces, and how to manipulate toys, and point to body parts and pictures. Studies have shown that the infant brain is highly malleable, and that new dendrites form connections every day, and unused neurons are removed. This process slows down by the age of 3 years, and so adults cannot remember their childhood experiences.
b) Childhood and Adolescence
Over the next few years, children’s memories steadily improve. They can remember the names of animals, sometimes even the names of the different family members. From the age of 3 or 4 years upwards, many can do simple addition and subtraction, and read and write simple words. By the age of 10 years old, when they reach Primary 5 (5th grade), their long-term memory is quite powerful, and they can remind the adults what they have forgotten. When they enter the teenage years, their autobiographical memory can be quite strong and they can recount in detail the events they have experienced. Their prospective memory increases, so they are better able to remember and carry out an action that they have planned.
c) Later Teens and Adults
Semantic memory, or the memory of facts, tend to stabilise in teenagers and young adults. Episodic memory, the recall of specific events in our lives, tend to slowly decline after the age of 20 years. We forget where we left our keys, whether or not we switched off the fan or lights, and what we were supposed to buy from the supermarket.
d) The Elderly
Slowly but surely, memory fades with age. Past memories may be retained better but recalling new facts and new plans may suffer lapses of memory. The incidence of dementia increases steadily with age. However, this does not mean that old age is a cause of memory loss: correlation does not mean causation.
How to Improve One’s Memory
The internet abounds with advice on how to improve memory, or how to delay dementia. Some of the information is fake news, so one has to be careful what to believe. Proven methods include the following:
a) Physical Exercise
Physical activity, including aerobic exercises, has long-lasting benefits in improving memory. It improves blood flow carrying more oxygen to the brain, and it protects brain cells by removing waste products. Brain-derived neurotrophic factor, a protein that promotes the growth of nerves and connects the new branches, is increased during exercise, and this promotes brain plasticity along important cortical areas of the brain. Moderate to high intensity exercise also reduce beta amyloid plaques and tau-protein causing neurofibrillary tangles, all of which are believed to lead to Alzheimer Disease. Walking, cycling, and many “cardio” sports undertaken around 4 times a week for at least half an hour each day, will improve one’s memory.
Meeting friends, joining social clubs, dance clubs, choirs, going on hikes, playing mahjong, bridge, and other games, having dinner together, can stimulate the mind and keep it active.
c) Exercise the Mind
There are several ways of exercising the mind, including doing crossword puzzles, sudoku, chess, mahjong, learning a new language, reading and writing. All are effective in improving one’s memory.
The relationship between sleep and memory is complex. Sleep consolidates learning and allows information to be stored more efficiently. Lack of sleep impairs one’s ability to focus and remember information as proficiently. Studies have shown that during sleep some of the toxins that affect the brain are removed, and connections between nerves in the brain are enhanced, thus enhancing the building of memories.
There are lots of advice advocating which foods to eat and which to avoid that will enhance memory. Your parents are right when they ask you to eat more vegetables. Fruits are also encouraged, especially apples, oranges, and berries. Omega-3 fatty acids found in salmon, mackeral, tuna, sardines, and flaxseed, walnuts, almonds, hazelnuts, peanuts, and gingko nuts are also recommended. The common denominator of dietary advice is that they all contain a lot of antioxidants that can remove free radicals that can damage cells throughout the body, including the brain. People who exhibit a gene called the APOE e4 gene are at greater risk of developing Alzheimer Disease, but when taking the above recommended diet, their memory decline is slowed down.
f) Coffee and tea
Caffeine found in coffee and tea have a mild stimulant effect, and not only do they enhance memory up to 24 hours after drinking, they have benefits in promoting long-term memory. Caffeine acts on nerve receptors known as adenosine receptors, and this transiently increases heart rate and blood pressure, but more importantly it also promotes neural activity in the memory forming hippocampus.
Imagining the Future
An important reason why memory is important is it can help us predict the future. Memory of the past is important in helping one to construct a vision of the future. Preventing memory decline in old age, sharpening our current memory, are some of the greatest challenges facing mankind. Answers are slowly trickling in, and in the meantime, it is recommended that we engage in regular physical exercises, socialize more, exercise the mind, get adequate quality sleep, change to a healthy diet, and drink some coffee or tea. Don’t forget!
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Written by Dr. Kenneth Lyen
Illustrated by Lim Daphne