Veni, Vidi, Vici
The search for a better understanding of life begins here.
In 1949, Donald Hebb famously introduced his postulate in his book "The Organization of Behavior" - what is now known as Hebb's Postulate - that "neurons that fire together, wire together". This co-activation is what we believe happens when memories are first created. For one, we know that a majority of neurons in the brain can no longer divide or make copies of themselves after embryonic development - except in the dentate gyrus of the hippocampus and cells in the subventricular zone that continually produce adult neural stem cells. Another notable area is in the olfactory epithelium that lines our nasal cavity allowing us to have the sense of smell. Imagine if our neurons divided every time we make a new memory, our heads would most likely run out of space and explode! Therefore, we have to look deeper into structures that is iconic and unique to neurons.
Synapses are little bulbous connections that form communication links between neurons. It is estimated that the human brain is made up of around 86 billion neurons, that would give us about over a quadrillion possible synapses - imagine the potential for storage space and connectivity! To simplify this, let's break it down. Neurons are specialised cells that form part of the brain - a majority of rest are sort of housekeeping cells that ensure that neurons have a comfortable environment to sustain their functions. Each neuron has dendrites (so called input sensors) and an axon (output projectors). Dendrites branch out like little roots and they link up with axons from other neurons in order to receive information. These little roots contain bulbous synapses that act as transmission points or like a mailbox. Now that you are familiar with dendrites, axons and synapses, we can finally talk about how synapses are truly the places where memories are stored due to their immense quantity and complexity. So, neurons that fire together, wire together. In neuroscience, a competing theory of how memories are sustained for long periods of time revolve around long-term potentiation (LTP). This is because neurons communicate via an electrical-chemical-electrical method. Long-term potentiation just means that activated neurons ("fired") sustain a long period of electrical stimulation that signals to its own nucleus (sort of like the HQ of the cell) to kickstart a whole cascade of processes that enables its continual connection with whichever cell initiated the firing/potential in the first place. This continual connection lies in structural and functional changes in the synapse - therefore, LTP is a process that strengthens connectivity between neurons. Remember how I mentioned that association with other events tend to create stronger memories? Memories are not physical objects in the brain that can be stored and retrieved like your hard drive. Memory is the probability that the same circuit/network of neurons activated upon the first exposure to a stimulus is activated again based on another related stimulus. This probability is further tilted in the favour of "remembering", only if you strengthen it with repetition, associated events or emotions. Let's take your name for example. How do you know your name? "My parents gave me that name.". Alright, I think I've set the bar too low. But have you ever wondered how you remembered that? Perhaps, it was your first day of school and your parents were frantically making sure you remember your name and how to spell it by repeating it over and over again. The excitement or anxiety of your first day at school plus the constant barrage from your parents greatly strengthened your image or memory of your name. So think of this, every time someone asked you about your first of school, you seem to remember the flow of things - parents nagging at you on whether your remembered your name and how to spell it, how you thought your teacher called out your name when they didn't, how embarrassing that must have felt and so on... These related events are then associated with the memory of your name and the events that occurred on your first day of school. Neurons that fire together, wire together. Oh how we have come full circle... Okay, but repetition works too. How come? If you activate the same circuits over and over again, they also tend to sustain LTP. However, the caveat is that without persistent rehearsal, you would probably forget your memorisation in a few weeks or months depending on the context. An additional factor is sufficient sleep since most of memory consolidation occur during our periods of shuteye. Therefore, burning the midnight oil might be doing more harm than you think as it also opens the floodgates to a multitude of cardiovascular and metabolic diseases that can also affect neural function! Try to remember all these words in sequence in 10 seconds, Hairdryer Potato Landyard Yarn Geometry Animals Jargon Nut Unless you have photographic memory or employed the use of memory exercises to remember all the words, you probably only remembered the first two or three words and the last. Why is that so? What did you do constantly when you tried to remember the sequence of words? You kept repeating the first word again and again in order to start the sequence. How about the last word? Well, just so happens that the last word is the most recent word you remembered due to its place on the list - give or take another minute and you might forget it all together. There you go, I have proven today that memory is the probability of a network of neuronal firing - otherwise known as an engram - in the brain that occurs when you are relating it another event. This likelihood can be strengthened by emotional content, context and repetition. I did not go deep into the two different phases of LTP as it may only complicate the general understanding of memory consolidation into long-term memory. Stay safe and curious!
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Ever wondered why and how you remember things? Simple things like your own name, your home address, your favourite past time or food?
While I cannot provide you with a definite answer, there are several concepts that bring to light the core of memory formation and recall. Firstly, how is it possible that we remember significant events yet are oblivious to everyday routine? While the answer to this question seems like a no-brainer. "Of course you remember significant events! It's so... significant!", you might be shouting to the screen as you read the previous paragraph. But what is significance? Memories are more strongly consolidated in the presence of emotional arousal and even fear. To understand what is significant, we have to also look at what is insignificant. Daily routine is a good example of insignificance because we tend to just "go with the flow", incorporating "muscle memory" (although that itself is seemingly a part of procedural memory - like how you still remember how to ride a bike after years of not riding) into our everyday lives. Take for example, having a cake after a long day of work. If I had asked you what cake you had a day after, you probably would remember quite clearly since it was so recent. But if I asked you again a month later, you probably would not even remember having cake a month ago. However, if it was your 21st birthday and you had specially ordered a three-tiered strawberry cake with sparkler candles, it is highly likely that you would still remember it months or even years into the future. This is because of associativity - we associated the basic memory of the cake we had with its flavour (if it was a remarkable tasting cake), the people present at our birthday, the venue you celebrated your birthday at, the feeling you had when you cut the cake and when you received gifts. These additional events strengthen memory, especially if they provide some form of sensory arousal - smell, taste, sight, touch, hearing and proprioception (otherwise known as spatial awareness of your body - perhaps someone dunked your head right into the cake). In another simpler example, something insignificant would be how you are able to sleep every night without being bothered by the traffic outside. This form of habituation occurs because you have the same stimulation at the same time of the day/night - therefore, your brain works to ignore it. But you will always remember your first night living in that home and how you initially couldn't sleep! Amazing, isn't it? However, negative emotions can also strengthen memory - consciously and unconsciously. When was the first time you got scared? Perhaps, at a cinema while watching a horror film when you were 5 years old. You may not remember the exact scenes where the jump scares appeared but you do remember that horror movies tend to make you jumpy and afraid of going to the toilet at night. This "trauma" can also strengthen memory - fear memory. It associates objects or experiences that were similar to your initial exposure to that horror film. Perhaps, in the film the jump scare came out as the protagonist was walking through a dark alley. If that scene was extremely impactful or if you happen to chance upon a dark hallway or alley, you might feel a sense of fear - a tingling of anxiety as you question yourself on whether you should continue to walk through less you might get jumped or see something you shouldn't. However, under normal circumstances, we would forget these triggers and go on with our lives as time passes. Unfortunately, for those who have experienced real and impactful trauma, even with time, a simple reference to said traumatic event may cause them to trigger the same circuits in their brains when they were first exposed to the traumatic event. For example, war veterans hearing firecrackers during Chinese New Year may experience heightened anxiety and fear as they unconsciously/consciously associate the same sound as gunfire during their time of service as their brains activated the same connective circuits that were first created during the time a friend was lost in a firefight or when they thought they were really going to die. This is known as post traumatic stress disorder (PTSD) and it is not limited to war veterans, even victims of domestic abuse and/or sexual assault tend to develop PTSD. "Why have fear memory in the first place?", you might ask. Well, we aren't certain but fear memory does have beneficial purposes. Imagine that your ape ancestors accidentally stumbled into a lion's den while hunting. A pride of lions may have chased them out or even killed members of their tribes. Fear memory is advantageous for them to remember the location of the den and how they felt when they were in the face of imminent danger (the same system (sympathetic nervous system) used for fight-or-flight are then pre-activated as it anticipates danger), causing them to run away as far as they can from said lion's den if they were to chance upon it again. As with everything, too much of anything can become a bad thing - developing into stress disorders and depression as a result of overstimulation of the sympathetic nervous system even in the absence of a real threat. This post is just a brief overview of memory - the good and the not so good. However, we should learn to appreciate our brain as it makes so many decisions everyday to protect us from external threats and deciding which memories are worth keeping or throwing out. Stay safe and curious! |
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