Content Reviewers:Rishi Desai, MD, MPH, Yifan Xiao, MD
Contributors:Sam Gillespie, BSc, Simone Taylor
In the movie The Bourne Identity, CIA assassin Jason Bourne tells his new acquaintance Marie Kreutz about washing up on shore, unconscious, with two bullets in his back.
He tells her that he can’t remember anything that happened before regaining consciousness, saying: “No, I’m serious. I don’t know who I am, I don’t know where I’m going, none of it.”
Jason Bourne has amnesia which refers to lacking memory.
Now, memory can be divided into two main types.
The first is implicit memory, also known as procedural memory.
Procedural memory refers to replicating the sorts of skills you can perform automatically, without thinking much about it.
For example, walking, riding a bike, or texting—anything that has become a habit.
The second type of memory is explicit memory, also known as declarative memory. Declarative memory refers to retaining and recalling facts—the sort of stuff you need to win a trivia contest.
These memories do take some degree of conscious effort to retrieve; when trying to remember things like, “How many countries start with the letter J?”, most of us need to stop and focus.
You can think of procedural memory as “remembering how,” and declarative memory as “remembering what.”
When we talk about amnesia, we really mean that some part of a person’s declarative memory, the “remembering what” part, has been affected.
Jason Bourne may not remember who he is, but he has no problem remembering how to speak foreign languages or how to fight.
Now, the process of forming declarative memory can be broken down into four stages, each involving specific parts of the brain.
The first stage is encoding, which occurs in the prefrontal cortex.
Encoding begins as soon as you first sense something.
Say you’re tasting a strawberry. Encoding would involve classifying the strawberry according to sweetness, size, color and texture.
Think of encoding as a process of breaking down an experience into manageable parts for the rest of the brain to use.
The second stage is consolidation.
Consolidation occurs in the hippocampus, deep within the temporal lobes.
In consolidation, the encoded bits of information like the sweetness, size, and texture of the strawberry are linked up to existing memories.
Think of consolidation as solidifying the memory. Comparing that information about the strawberry with memories you already have, like eating a raspberry or a blueberry, helps determine where to put that new information in your memory.
When you consolidate a memory you are organizing it in a way that will be easier to recall.
The third stage is storage, and it takes place throughout the cortex.
In storage, the bits of memory that get linked up to existing memories are preserved or retained.
Now in order to retain a memory, it has to be edited, pared down and simplified.
During this process, the less important details (like what you were wearing or what day of the week it was when you tasted the strawberry) get edited out.
That way the memory really stays focused on the sweetness, size, color and texture of the strawberry—the stuff that matters.
The fourth stage is retrieval, and it occurs in the prefrontal cortex, hippocampus, and cortex.
In retrieval, the brain goes through encoding, consolidation, and storage of the memory over and over again.
And with each run-through, retrieval gets slightly easier.
If you’ve ever tried to memorize facts for a test, you’ve gone through the process of retrieval, and you probably know that the more you run through that information, the more likely you’ll be able to remember it on the test.