Remembrance of things Memento — Part 2: Declarative Memories versus Habits

In the previous post of this series, Part 1, we discovered that a statement made by the main character in Memento, Leonard, was not accurate. He asserted that he has a deficit in “short-term memory” whereas his symptoms indicate a “long-term memory” deficit. That misstatement might, in principle, be meaningless trivia indicating nothing more than the fact that the character Leonard (or perhaps even the screenwriter) was misinformed or mistaken about the technical term for his memory deficit. However, whenever a character in a mystery genre makes a claim that is demonstrably false, the film viewer is well advised to also consider another possibility. Perhaps the screenwriter/director is giving us a clue that Leonard can not always be trusted. We will be pursuing that possibility in more detail in later postings.



Here, I will address another claim made by Leonard, namely, that he has found strategies based on repetition and habit that help him cope with his memory problem. This assertion turns out to be somewhat plausible (perhaps not in the detailed way it was employed by Leonard in the film, but at least in general) based on scientific principles. I am going to summarize those scientific principles in this posting.



There are two qualitatively different kinds of long-term memories stored in the brain, referred to by the technical terms declarative memories and procedural memories. Procedural memories are sometimes referred to as habits, and I will use that term here to make a direct tie-in with the assertion made by Leonard in the movie.



Declarative memories are what we are usually referring to when we use the term memories. They reflect explicit facts that we can remember based on our past experiences. Declarative memories come in two types, called semantic and episodic. Semantic memories are facts that we have stored in our brains some time in the past, irrespective of whether or not we can remember when we learned those facts. An example of a semantic memory would be that I have stored in my brain the fact that a dog is a small mammal with fur. Episodic memories are autobiographical memories of the events in our own lives. For example, I remember that when I was five years old I had a dog named spot.



Habits are memories that involve knowing how to do things. An example would be remembering how to ride a bike. Suppose that you had visited your grandfather’s house when you were 5 years old, and during that visit he taught you how to ride a bike. You are now an adult, and you no longer remember having been taught to ride a bike by your grandfather (an episodic memory) or even that you know how to ride a bike (a semantic memory). Nevertheless, if for some reason you find yourself sitting on a bike, you will be able to ride it. This is because the motor skill of knowing how to ride a bike has been stored in your brain as a habit.



Declarative memories and habits have been dissociated in patients with brain damage. We are going to discuss more details about these clinical conditions, including the specific brain regions involved, in a future posting. For purposes of our present discussion, it is sufficient to know that patients with damage to certain regions of the brain have deficits that involve declarative memories, but habits are unaffected in these patients. Thus, it is theoretically plausible that Leonard could have a clinical condition that allows him to form and use new habits even though his condition prevents him from forming new declarative memories. This does not mean that it is really plausible that someone with total lack of ability to form new declarative memories, such as Leonard, could actually function and live on his own based on a strategy that uses only habits. Nevertheless, if we are willing to give the film a certain amount of poetic license, it is possible to conceive how a strategy based on repetition and habit could facilitate normal functioning in a patient who has declarative memory deficits.



Lets spell out this strategy using an analogy with the bicycle riding example we just described. In that example the environmental condition of sitting on a bicycle seat automatically elicited the stored memory (habit) of how to ride a bike. Suppose Leonard, through repetition, established the following habit; Whenever he is in an environmental condition in which he is confused, a motor action kicks in that causes him to examine his arm for the presence of a tattoo. This habit will bring some information into his (still functioning normally) short-term memory. That information could now direct Leonard to do something else such as, get a note from your pocket and read it. By chaining together in his short-term memory the information in the notes, Leonard could, in principle, bootstrap himself towards accomplishing some goal. Especially, since one of the activities Leonard can engage in during this goal-driven activity is to write new notes and store them for use the next time his habit is elicited.


Click here to go to Part 3 in this series of posts about Memento.




Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s