What is memory? These days, if you ask anyone this question, most of them will refer to the memory cards or hard disks of the computation devices they are very much familiar with. After all, it’s something which you just store and retrieve right? Or is it? Long before we had handheld supercomputers like today, we were using the computational unit which we are all born with. From simple 1+1 math to solve complex queries or to render amazing graphics within, a skill called “imagination”, all was done using our own brain. Now before someone pulls me into a debate of “Brain: A Computer or Not!”, let’s quickly jump into the topic at hand : P
Memories in our brains aren’t as simple as the plug and play computer chips we know. It’s not only the content of the memory which is important but the processes of storing and retrieval are equally crucial if not more. One of the most important way to distinguish memories is Episodic or Semantic. Episodic memories are stored in a series of, well let’s say, episodes. Here the way it’s received is important as it associates both the time and space of that particular event or memory. Say if someone asks you the way to reach your house. You will tell them various directions to take, the landmarks you see and in within what time, in exact order. Here order matters as that's how an episodic memory is encoded and without order, the memory content won’t make sense, both neuroscientifically and practically as no one would be able to reach your house if you give nonsense directions.
The other type of memory is Semantic. This type associates semantic content and meaning with the facts and information stored in the brain. The important thing is it doesn’t link any spatiotemporal information with those facts, unlike episodic memories. Also, semantic memories need not be related to personal events and can be learned even from someone else’s experience or in general in an abstract way. For example, learning that the capital of Japan is Tokyo doesn’t need you to personally visit Japan as its a fact which everyone knows and that’s a semantic memory.
Now the question is which is more important, Episodic or Semantic? Which is more fundamental and helps the other from neuroscience POV? There are shreds of evidence on both sides saying one helps in the formation of others. Generally, I would have said that it’s the balance of both types which stabilizes the memories in our mind. But in my opinion, Episodic memory might be more fundamental then we think it is.
Everything we perceive in fact is continuous “time-varying” signals. From audio to visual or taste we savor from sour to sweet while eating a mango, is actually encoded as “time-varying” neural spikes in our brain. Here I have highlighted the significance of time in both external events perceived and the internal encoding of the memories of the same.
As all of our sensory experiences are spatial and temporal, our brain must handle this information as well, and the fundamental architecture for memories must include this spatiotemporal information. Hence Episodic seems more fundamental then semantic to me. Converting episodic to semantic memories could be like an optimization function where unnecessary spatiotemporal tags are discarded. For example, when you learned the capital of Japan, its a fact and the information of where and when you learned it, from a book or an online blog like the current one you are reading, is not that important albeit depends on the context. Whereas the other way i.e. adding space-time information to the semantic context isn’t very optimizing.
So these are some of my thoughts on the neuroscience behind the memories which I just retrieved from the memory chip in my brain ; )