According to the Standard Model of Memory (SMM), when you pay attention to information in sensory memory, it gets transferred to the short-term subsystem. As was the case with sensory memory, short-term memory may be described in terms of five characteristics.

Level of Awareness of Short-Term Memories

Since the SMM defines attention as the mental process that transfers information into the short-term subsystem, short-term memory is, by definition, at the conscious level of awareness.

Duration of Short-Term Memories

H. M. is a famous set of initials in the history of memory research because they refer to a man who, on September 1, 1953, had portions of his brain removed by William Beecher Scoville — a neurosurgeon at Hartford Hospital in Hartford, Connecticut — as a treatment for epilepsy. Not until H. M. died on Dec. 4, 2008, did the world learn that his full name was Henry Gustav Molaison (Carey, 2008). Sadly, after the operation, Henry developed profound memory problems similar to those observed in the case of Jimmie G. described in Section 6-2 (see Scoville & Milner, 1957):

From H.M.’s moment in surgery onward, every conversation for him was without predecessors, each face vague and new. Names no longer rose to the surface, neither histories nor endearing moments came any more. Reassurances of welcome had to be sought every moment from each look in every pair of eyes [because he could not remember anyone he had met since the operation].... When Dr. Scoville came home and told his wife of the surgery, she said that he told her in the form of a joke: Guess what, I tried to cut out the epilepsy of a patient, but took his memory instead! (Hilts, 1995, p. 100)

Although Henry still was able to perceive the world, to remember events for brief periods of time, to remember many episodes of his life from before the surgery, and to perform well on a standard intelligence test, he apparently had lost the ability to form new long-term memories.

After his surgery, Henry was tested intensively almost until his death 55 years later. The results of these tests have taught us much about the memory system. Because Henry’s short-term memory subsystem was intact, it was possible to measure directly the duration of short-term memories by determining how long he could maintain a new memory:

In one test, for example, he successfully kept the number 584 in mind for 15 minutes, and when asked how he did this, he replied, “It’s easy. You just remember 8. You see, 5, 8, 4 add to 17. You remember 8; subtract from 17 and it leaves 9. Divide 9 by half and you get 5 and 4, and there you are — 584. Easy.” Yet, a few minutes later, after his attention had shifted to something else, he could not remember the number or the mnemonic scheme[∂] he had used, or even that he had been given a number to remember. (Milner, 1970)

Based on this account, it might seem that short-term memories can be retained for at least 15 minutes. However, Henry was maintaining the memory in the short-term store longer than usual by using maintenance rehearsal, which is a type of rehearsal[∂] in which one repeats over and over again the material to be remembered . You probably do this often during your day whenever you want to remember something for a short period of time, such as a phone number that someone has given you to dial. (Note: You can create your own mnemonic schemes here.)

In order to study the actual duration of short-term memories, researchers prevent participants from using maintenance rehearsal by asking them to perform a mental task, such as counting backward from 100 by 3’s, immediately after giving them material to memorize (such as a word list). After a short period of time (say, 5 seconds), the researchers ask the participants to recall as much of the material as possible. By varying the amount of time that passes before recall, memory researchers can determine how quickly a memory is lost from the short-term store. Such research has demonstrated that we begin forgetting short-term memories within the first few seconds after receiving new information (see Figure 1). By about 15 seconds after receiving the information, any memory of it has virtually disappeared from the short-term store. Although we can increase the duration of short-term memories with maintenance rehearsal (as Henry did in the passage quoted above), the memories will disappear very rapidly when we stop repeating the information. Therefore, if our memory system contained only the short-term store and the mental process of maintenance rehearsal, we would retain virtually no memories beyond what is occurring right now.

Figure 1. The percentage of information maintained in the short-term store over 18 seconds (Fernald, 1997, p. 237)

Capacity of the Short-term Store

The amount of information that can be held in the short-term store at any one time is easily measured. For example, we can present word lists with varying numbers of words and determine at what point people start to have trouble:

• 3 Items:  grand    bear       top
• 4 Items:  kite        hive        core       wean
• 5 Items:  rod         ice          week      gate       pin
• 6 Items:  ear         axe         zoo         lake       joke        vase
• 7 Items:  fine         dime      cake       nice       moat      shell       oar
• 8 Items:  lint         pine       year       urn         bore       zinc        air         mine
• 9 Items:  sea         time       oak         earn       wine      care        jam       like        born
• 10Items: vine       nook      rare        yarn       lawn      bone      hook      wear      grain     door

Encoding of Short-term Memories

Encoding involves various ways of processing (manipulating and transforming) information in order to mentally label it — to form a memory code for the information. How we encode depends on the nature of the material to be remembered and what we are trying to do with this information. When invesitgating how we encode short-term memory, most studies have used rapidly presented verbal material, such as a word list or a list of nonsense syllables, which research participants then immediately recall (see the description of nonsense syllables in Section 5-3). Under these conditions, people tend to use mostly maintenance rehearsal to memorize the items.

Let’s say that we present the following list of words to a group of people:

cat   run   suit   junk   frame   clothes   dress   stone   age   log   boat   watch

We probably will find that, although most people can recall several of these words, they also may remember incorrectly some words that were not presented. These misremembered words can tell us what kind of encoding a person used. For example, someone might incorrectly remember hearing some of the following 12 words:

bat   bun   fruit   bunk   lame   droves   mess   bone   rage   bog   goat   botch

It is very improbable that any one person would incorrectly remember all these words; but an individual might make one or more of these mistakes. Now, in what way are these misremembered words similar to the original list? If you repeat the words out loud, you will find that the misremembered words rhyme with the original list of words. These mistakes suggest that the original list of words was phonemically encoded in short-term memory. In other words, when verbal material is encoded for storage in short-term memory, it tends to be encoded according to the way it sounds. If the words had been semantically encoded, what kinds of mistakes would we have observed when subjects recalled the list? In this case, the mistakes would have contained words such as the following:

feline   jog   tuxedo   trash   border   shirts   skirt   rock   old   wood   ship   clock

If we encoded information semantically in short-term memory, people would misremember words that have a meaning similar to those in the original list. The fact that this rarely happens shows that we do not often use semantic encoding for the initial formation of short-term memories.

Forgetting of Short-Term Memories

In Section 6-6, decay theory was defined as, "the forgetting of a memory ... caused by the disappearance over time of its engram." When an engram disappears, the memory no longer exists: it has "decayed." A memory subsystem that is limited in terms of the duration of its memories contains engrams that decay over a period of time. In the case of short-term memories, engrams decay within 15-20 seconds unless maintenance rehearsal is used. Thus, the neural changes that form the foundation for short-term memories degrade over a very short period of time.

Another reason why short-term memories are quickly forgotten involves the limited capacity of the short-term store. Once this capacity is met, the addition of new information requires that information already in the store be "pushed out" or displaced. Displacement theory states that the forgetting of memories occurs when new information pushes old information out of the memory store. Because the short-term store can hold only about seven items of information (at least, when this information is single-syllabled words or numbers), displacement is an important cause of forgetting from this memory subsystem.

Study Questions for Section 6-7

1. According to the SMM, what does attention do?
2. At what level of awareness are short-term memories?
3. Why was H. M. able to remember a set of numbers for 15 minutes even though he was unable to transfer this memory from the short-term store to the long-term store?
4. How would you define a mnemonic scheme in your own words?
5. What is the duration of unrehearsed short-term memories? What is the duration of short-term memories on which you are using maintenance rehearsal?
6. What is the approximate capacity of the short-term store (for single-syllabled words or numbers)?
7. Of what use is the short-term store if it is so limited in terms of its capacity and duration?
8. How are short-term memories of verbal material usually encoded?
9. If I try to recall a word list that I learned 5 minutes ago, and mistakenly recall that it included the word "pig" when the word actually was "big," what kind of memory code did I form for the word?
10. If I try to recall a word list that I learned 5 days ago, and mistakenly recall that it included the word "pig" when the word actually was "pork," what kind of memory code did I form for the word?
11. When a memory subsystem is limited in terms of its duration, which theory of forgetting best explains why memories are lost from its memory store? Why?
12. When a memory subsystem is limited in terms of its capacity, which theory of forgetting best explains why memories are lost from its memory store? Why?
13. Which theory (or theories) of forgetting best explain(s) why memories are lost from the short-term store?

Go to Quiz 6-7 questions

Go to Readings Section 6-8