Imagine we didn’t have memory. Without it we wouldn’t be able to learn anything. Without the ability to store and retrieve past experiences we wouldn’t be able neither to predict consequences of our actions nor plan our future. Apart from that, memories can influence our emotional state. Recalling different events from the past can make us feel sad or happy. Memories from early childhood can determine our way of thinking and emotional responding for the rest of our lives. I will explain the role of memories in anxiety, depression, PTSD, and borderline personality disorder in other articles. But for now, it would be sufficient just to familiarize ourselves with basic knowledge about memory.
In essence, memory is the ability to store and retrieve information when it is needed. Imagine a large container storage where you keep things when there is no more space in your house. Memory is like such storage. Depending on duration, memories can be divided into short-term that last for less than 20-30 seconds and long-term that remain for the rest of our lives.
Depending on the type of information, there are visual, auditory, motor, and other types of memories. There are also autobiographical memories, which are memories of specific personal experiences, and semantic memories which are memories of general facts or concepts.
Formation of a memory consists of several steps: encoding, retention, consolidation, and retrieval. With each step, memories become stronger and easier to reproduce. The key to transitioning from one step to another is repetition of information. For example, to learn a poem by heart, we must repeat it many times before we are finally able to reproduce it without a mistake.
Interestingly, repetition also happens without our will. Automatic subconscious replay of learned information occurs during wakeful rest after the learning phase and during sleep at the following night. Importantly, wakeful rest is a period when we are not analysing any new information, but only “digesting” what we have learned. It has been scientifically proven that lack of rest and sleep impedes memory consolidation. For example, in one such scientific experiment, researchers put two groups of mice in the same maze. Their task was to find and memorize a path to a reward. After finding the reward, the first group of mice was allowed to do nothing, rest and sleep without interruption on the night following the experiment. Animals from the second group were subjected to new information during their rest and their sleep at night was also interrupted. The next day researchers put the mice in the same maze and measured how quickly they can find the way to the reward. Less time spent on finding the reward meant better memorization of the path. Scientists observed that animals from the first group, who were not distracted during their rest and who slept without interruption, performed significantly better than the animals from the second group.
But how do rest and sleep improve our memory? It turns out that during the rest immediately after learning something new and during night sleep, our brains replay learned experiences, keep relevant and erase irrelevant information. So, next time you want to learn something, make sure you have enough rest and sleep after the learning. It is very important not to consume irrelevant information during the rest, for example, from social media or TV, because it will make it difficult for neurons to separate relevant and irrelevant information.
Now, regarding the pathology of memory, memory impairments can vary from gross to barely noticeable. The most severe symptom is called amnesia. Amnesia means loss of memory of specific facts, events or even entire periods of life. It is usually observed in neurological diseases such as stroke, dementia, or head injury. There are several types of amnesia. If, for example after a head injury, one doesn’t remember events that preceded the injury, it is called retrograde amnesia. If one does not remember events that followed the injury, it is called anterograde amnesia.
Sometimes people fill these gaps in memory with fictitious events that never really happened. That is called confabulation. If they fill the gap with events that happened but at different time in the past, it is called pseudoreminiscence. And if one fills the gap in one’s own memory with events that happened to other people, this is called cryptomnesia.
Again, amnesia is more common in severe neurological disorders. However, people with less severe disorders, such as depression or anxiety, also show memory impairments, although more subtle. Multiple studies have found that people with emotional disorders have weak autobiographical memory. Autobiographical memory, also called episodic memory, is a type of memory that stores information about unique single events in one’s personal life. The main feature of autobiographical memories is that they contain additional details, such as where and when that unique event occurred. For example, imagine that you were once bitten by a dog. Normally, the memory of this negative event would also contain additional contextual details such as where exactly you were bitten and what dog bite you. Remembering this contextual information is important from a biological perspective because it allows you to avoid a specific place and a specific dog. Remembering contextual information also protects against development of phobia as it restrains fear by binding it to a specific location. If you were bitten by a dog, remembering where and what dog bite you will make you afraid of a specific dog that lives in a specific place. However, in people with anxiety and depression, autobiographical memories lack such specificity. That is why they tend to generalize negative events. So, if a person with weak episodic memory gets bitten by a dog, instead of fearing a specific dog that lives in a specific place, they will fear all dogs in general. We will come back to episodic memory impairment and its role in emotional disorders in our future videos when we speak about anxiety and depression in more detail.
In summary, remember that memory is not only retention of information, but also an erasure of irrelevant information. When having a rest after learning something, try not to consume any irrelevant information because it will contaminate important, including contextual information, and it will take longer for neurons to erase everything that is not relevant.