Remembrance of things Memento — Part 1: Short-term versus Long-term Memory

This is Part 1 in a series of postings I am doing about the movie Memento. My introduction to this series is Part 0.



The main character in Memento, Leonard, asserts that he has a “memory problem” that affects his short-term memory. MUCH more to come later about his “memory problem”, but what I want to address in this posting has to do with his assertion that his problem involves deficits in “short-term memory”. Leonard is clearly mistaken (perhaps just out of ignorance or misinformation) when he makes this specific assertion. All of his symptoms revealed in the film are ones that are related to a specific form of deficit in “long-term memory”. In what follows, I will delineate some of the scientific facts regarding short-term and long-term memory (as well as a third type called sensory memory).



Memory refers to our capacity to retain and later retrieve information about our prior experiences. As we go about our daily lives, detailed information about what we see, hear, touch, taste, and smell is being processed continuously by our sense organs. However, we do not store all of these details in our memories. In fact, only a tiny fraction of what we experience in our day to day lives gets stored. Another way of expressing this is to state that memory is highly selective. For every experience that is stored in the form of a memory, thousands of other experiences are forgotten. This selective aspect of memory turns out to be adaptive . Consider the information overload that would occur if we remembered every detail of every experience we have had since infancy.



Scientific theories of memory often make reference to a three-box model of memory systems. The basic idea of these models is that memories are stored in three distinct neural systems (boxes) that operate sequentially on memories having different time spans. The first neural system (the first box) is involved in storage of immediate sensory memories. Information coming into the brain from our sense organs (what we see, hear, touch, taste, and smell) is stored initially in this sensory-memory system. Most of this information is lost within a second or two. Clever experiments performed by psychologists have demonstrated some of the properties of the sensory memory system. In a typical set of experiments, an observer sits in a dark room in front of a screen. A strobe flashes a bunch of numbers onto the screen for just a moment. Then a fraction of a second later, the observer is asked (technical details about how this is done would take too long to describe here) a question such as, What was the number that was flashed in the fifth column of the third row? Observers can answer such questions with an amazing level of accuracy, close to 100%. However, if the experimenter waits just a few fractions of a second longer before posing the question, the performance of the observer drops off dramatically to not much better than expected based on guessing. These kinds of experiments demonstrate that there is a sensory memory system that holds a great deal of information, but only for fractions of seconds.



Not all is lost however. A few items of information that were initially stored in sensory memory are transferred into a second neural system (the second box), short-term memory, where they are held for 30 seconds or so. Scientific experiments studying short-term memory have, once again, used observers sitting in a dark room where an image is flashed momentarily onto a screen. After a delay on the order of 10 to 30 seconds, the observers are asked to report everything they can remember about what they just saw. If the image was a bunch of numbers, as described above for the experiments on sensory memory, observers can typically report accurately about 4 to 10 of the numbers that were present. Similarly, if the flashed image was an ordinary scene of people, places, or events, observers can typically report accurately on the “gist” of what was in the scene, something on the order of 4 to 10 facts about the scene. The factor that determines which of the items in the sensory memory box (the privileged few 4 to 10 items) become chosen for transfer into the short-term memory box is attention, a fascinating topic in its own right, but one that we will not discuss further here.



I stated above that items in short-term memory are retained for only a few seconds. That is not quite accurate. A more precise statement is that short-term memories fade away within about 30 seconds UNLESS they are rehearsed. An example would be when you look up someone’s telephone number and have to remember it long enough to dial the number. If the time from when you look it up until you actually dial the number is more than a few seconds, then the only way to retain this number in your memory is to repeat it to yourself over and over, at least once every 30 seconds or so.



The upshot is that most short-term memories are lost within 30 seconds or so (or more precisely, within 30 seconds or so after we stop rehearsing them internally). However, some short-term memories get transferred into the long-term memory box where they are stored (semi)permanently in your brain. These stored. long-term memories constitute what you remember about your past.


Psychologically, memories give us a feeling of continuity with our past. If we do not have long-term memories, then our only way of maintaining a sense of “self” is to continuously chain together from one set of short-term memories to the next. The neurologist Oliver Sachs in a recent article in the New Yorker (September 24 issue, 2007) describes the case of Clive Wearing, who has no long-term memories. (NOTE: technically, this statement is not completely accurate, but close enough for our discussion here). Clive tries to maintain some sense of self by chaining together thoughts in short-term memory for as long as possible, but eventually he always falls into a psychological abyss: “It was as if every waking moment was the first waking moment. Clive was under the constant impression that he had just emerged from unconsciousness because he had no evidence in his own mind of ever being awake before…” Nevertheless, Clive can maintain a conversation and perform similar activities by using this strategy of chaining together from one short-term memory to the next.



Similarly for the character Leonard in Memento. (NOTE: there are also some important differences between the symptoms of Clive and those of the character Leonard. We will discuss Leonard’s symptoms in more detail in later postings). Leonard exhibits no problems involving his sensory or short-term memory systems. His symptoms relate to the fact that he can not remember anything that happened to him more than 30 seconds or so since he stopped rehearsing it. This is because none of his short-term memories are getting transferred into permanent long-term memories. In summary, the symptoms exhibited by Leonard reveal NO problems with the operation of his sensory or short-term memory systems. Instead they reveal a deficit in his ability to form new long-term memories. Information coming into his senses flows normally into box one (sensory memory) and then into box two (short-term memory). The problem is a bottleneck between box 2 and box 3; Nothing flows into box three so no new long-term memories are formed.

The next post in this series is Part 2




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