Short Term versus Long Term Memory

Beginning in the late 1950s and increasing rapidly thereafter, the research journals in learning and memory began to be flooded with research on a new topic. What was being studied and reported on was not a new phenomenon, but a new dimension of the already well studied area of human memory. The new dimension being investigated was the nature of memory over very short intervals‑seconds or minutes. The name given to this phenomenon was short‑term memory, or simply STM.

Memory theorists long had proposed that there may not be one, but two, mechanisms for memory storage. What they suggested was that one type of storage mechanism is available for events recently experienced. This mechanism is the realm of STM. Another type of storage system seems to exist, however, for traces of experiences developed over longer periods through repetition, habit, and study. This aspect of memory is called long‑term memory, or LTM.

Several differences between STM and LTM were hypothesized. First, it was contended that STM involves "activity" traces in contrast to LTM's "structural" traces. That is, STM is dependent on ongoing electrochemical brain activity; in contrast, LTM is based on relatively permanent changes in brain cell structure. Another, related contention was that STM decays autonomously, whenever attention is diverted from what is to be remembered. LTM, however, is based on irreversible, non-decaying traces. Third, obvious differences in capacity between STM and LTM were noted. Whereas STM has relatively fixed limits, LTM was judged to have apparently unlimited capacity.

These distinctions match well with our own introspective assessment of our memory capabilities. For instance, when we encounter new information, we generally need to continue to pay attention to it and rehearse it in order to "keep it in mind." Remembering a phone number we have just looked up or the names of several new acquaintances, for most of us, requires some attention and repetition. Especially on first encounter, our memory for such information can be exceedingly fragile‑even a brief interruption or distraction may cause us to loose the thought entirely.

Once information has been well learned and committed to memory, however, rehearsal and repetition seem much less critical. We easily can state our uncles' names, recall the names of two large cities on the Red Sea, or give three examples of large hairy animals without having to rehearse any of this information‑despite the fact that we may not have thought of these topics for months or even years!

In more recent models of memory, however, the importance of the STM‑LTM distinction has diminished. Although memory theorists have continued to pay attention to the differences between STM and LTM, most models of memory have shifted from storage to a "processing" emphasis. This processing emphasis is retained in most current models. Rather than being conceived of as a "place" where information is held for brief periods, the concept of STM has been broadened so that it reflects the many different ways in which we deal with information. The STM now more and more reflects the concept of "working memory"‑that part of our cognitive systems we would refer to as our consciousness. For example, J.R. Anderson's ACT model incorporates a "working memory" and a long‑term memory." These two are not emphasized as "separate places," however, but rather as being closely interrelated. The current contents of consciousness set up a pattern of activation in LTM; this activation of LTM, in turn, may "reverberate" back into working memory.