Where Do our Lost Memories Go?

Babies are able to form memories, they are just unable to recall them. They can still carry on some memories throughout their life like emotional memories which will hugely impact their later life. Also, the rise in synapse production in brain areas implicated in memory roughly maps onto the ages at which the recall is improving.

Recently, my partner and I are intensely studying about neurobiology and emotions. Last week, we found a film which is a great reductive illustration of how emotions work and how each of them plays an almost equally important role in our life. In the film, there are five personified emotions — Joy, Sadness, Anger, Fear, and Disgust — try to lead her through life as her parents move from Minnesota to San Francisco, and she has to adjust to her new surroundings. (feel free to go through the trailer below if you have not seen the film yet — which happens to be the clip that we like a lot).

https://www.youtube.com/watch?v=_MC3XuMvsDI

But here I am not going to go too deep into the film or any specific emotions. I simply want to use a piece of story from the film to introduce the main topic that I am going to explain here - ‘memory and recall’.

In the film, Joy and Sadness get lost in Memory Dump and they encounter Bing Bong, Riley’s childhood imaginary friend, who shows up as a ‘thief’ — stealing some of Riley’s childhood happy memories. My partner asked me at the moment:

“Why do you think he is stealing?”

After a moment of thinking, I answered: “maybe he wants to keep all the memories that he had with Riley because they don’t play anymore and he seems to be forgotten.” We both feel it is a good enough answer and it makes a lot sense here. But later last night, this question came upon to me again. But this time I had a little different version of story to tell:

“ if he is stealing the memories from Riley’s long-term memory storage place basically it means Riley cannot get access to them. Ahh, it makes sense now. Those young childhood memories like the memories we had when we were 1 or 2 year-old are long forgotten or buried for us. They disappeared and we literally lost our ability to re-trace them. So this is actually a great metaphor that author is using. He leverages this long-forgotten childhood friend as a medium to explain the lost of the childhood memory.

From here, you probably start to wondering:

Why cannot we get access to our childhood memory anymore? or
What is memory?

For the first question, I will try to explain it in a simple way based on the books we are currently studying from a great neuroendocrinologist Robert Sapolsky.

Here is a brain structure called hippocampus which is in the medial temporal lobe. It is the brain structure that is necessary for the formation of the declarative memories and it is formed by the end of prenatal period. It is still undergoing development until 12–15months of age. Another area of the brain implicated in memory function is the prefrontal cortex. The density of synapses in this areas increase dramatically at 8 months and peaks between 15 to 14 months. Changes will continue to occur after this period, until well into adolescence. In other words, there are dramatic changes in the brain areas implicated in memory.

In general, the time course of improvements in memory with age is consistent with brain development. Especially late in the first year of life, the medial temporal lobe structures are functionally mature, and there are increase in the density of synapses in the prefrontal cortex. This in turn results in the improved recall abilities of infants near the end of the first year of life.

Further improvements in the reliability of ‘recall’ occur throughout the 2nd year of life followed by the continued increase in synapse formation in both hippocampus and prefrontal cortex areas.

So here you probably have noticed a subtle differences between the word ‘memory’ and the word ‘recall’. Most of time we don’t pay a lot attention to distinguish them. Sometimes when we talk about memory, we actually mean recall.

There are 2 basic types of explicit or declarative memory. First, short-term or working memory— it is the focus of current attention. Second, long-term memory which is broken down further into semantic memory(facts) and episodic memory(specific events). When we talk about recall we are more likely talking about long-term memory here.

In short, there are 3 main steps involving in basic memory processing:

  • Encoding: the process of forming new memories;
  • Storage: the process of information maintenance.
  • Retrieval: the process of gain access to stored knowledge.
In other words, recall is the last step of memory processing. It is an active reconstruction process, not a simply play back of a memory of an event, fact or concept. Every time a memory is accessed for recall, the process actually modifies the memory itself: essentially re-encoding the memory. Good news is that recall makes the memory itself more recallable in the future. Because we learned to keep activating the neurons in a certain patten to facilitate the recall or even a ‘long-term potentiation-LTP’ “which is the process by which the first burst of NMDA receptor activation causes a prolonged increase in excitability of the synapse.”

Let’s go back to our childhood memory. We now can say it more accurately way: Babies are able to form memories, they are just unable to recall them. They can still carry on some memories throughout their life like emotional memories which will hugely impact their later life. Also, the rise in synapse production in brain areas implicated in memory roughly maps onto the ages at which the recall is improving.

Before I end the topic here, I would like to briefly mention the other type of memory — Non-declarative Memory or implicit memory which stands in contrast to explicit memory. It does not require conscious thought and it allows you to do things by rote such as procedural memory — which enables us to carry out commonly learned tasks. Implicit memory can also come about from priming. Priming can involve the acquisition of new information. In other words, you are ‘primed’ by your experiences such as if you have been exposed to something very recently or many more times than another thing like library(my personal favorite place), you are primed to fill up li__ as ‘library’ instead of ‘little’ or ‘light’ etc.

I hope this simplified explanation can solve some of your puzzles. Please feel free to share some of your thoughts here!