I N 1949, DONALD HEBB ARGUED THAT IT WAS UNLIKELY that any chemical process could occur fast enough to accomodate immediate memory yet remain stable enough to accomodate permanent memory. Hebb was not alone in his thinking, as many psychologists advanced the notion of two memory systems: long-term memory (LTM), and short-term memory (STM).

In general, STM is characterized by a limited capacity of up to seven pieces of independent information, and in the brief duration of these items in STM, usually anywhere from three to 20 seconds. Additionally, decay appears to be the primary mechanism of memory loss in STM (Kalat, 1998).

In cognitive psychology, memory is usually divided into three stores: the sensory, the short-term, and the long-term. The progress of information through these stores is often referred to as The Information Processing Model.

The sensory information store has unlimited capacity, and reacts to both visual and auditory information. However, the duration of information in sensory memory is extremely brief, perhaps only 300 miliseconds, and is subject to rapid decay.

After entering sensory memory, a limited amount of information is transferred into short-term memory. In a classic study in 1956, George

Miller found that the amount of information which can be remembered on one exposure is between five and nine items, depending on the information. Applying a range of +2 or -2, the number 7 became known as Miller's Magic Number, the number of items which can be held in STM at any one time. 

Within STM, there are three basic operations: iconic memory, acoustic memory, and working memory (McMahon, 1982). Iconic memory refers to the ability to hold visual images in STM, while acoustic memory refers to the ability to hold sounds in STM. Of the two, acoustic memory can be held longer than iconic memory. On the other hand, working memory is an active process where the goal is not so much to move the information from STM to LTM, but merely to keep it until it is put to use (think of a phone number you’ll repeat to yourself until you can dial it on the phone.)

The process of transferring information into LTM from STM involves the encoding or consolidation of information. Initially, it was thought that the transfer of information into LTM was a function of time; that is, the longer a memory stayed in STM, the more likely it was to be placed into LTM (Fleming & Levie, 1993). Recent research has moved away from this view, instead focusing on the necesity of the brain to organize complex information in STM before it can be encoded into LTM.

In this process of organization, the meaningfulness or emotional content of an item may play a greater role in its retention into LTM. Also, on a more concrete level, the use of "chunking" has been proven to be a significant aid to STM transfer to LTM. Because STM’s capacity is limited to seven items, regardless of the complexity of those items, chunking allows the brain to automatically group certain items together. For example, we often do not remember phone numbers as seven individual digits, but rather as two units: the first three numbers, then the last four numbers.

For the educational technologist, the affordances and limitations of STM will play a recurring role in instructional design. Two issues in particular stand out. First, the instructional designer must keep the limits of STM (seven item capacity, short duration) in mind in the conception and design of instructional systems. And second and most importantly, the designer must consciously look to find ways to make learning relevant and meaningful enough for the learner to make the important transfer of information to long-term memory. 

Kalat, J. W. (1998). Biological psychology (6th ed.). Pacific Grove, CA: Brooks/Cole.

McMahon, F. B., & McMahon, J. W. (1982). Psychology: The hybrid science (4th ed.). Homewood, IL: The Dorsey Press.

Fleming, M., & Levie, W.H. (1993). Instructional message design: Principles from the behavioral and cognitive sciences (2nd ed.). Englewood Cliffs, NJ: Educational Technology Publications.

Marc Pastor ，Graduate Student, SDSU Department of Educational Technology