What is short-term memory? |

Introduction

A woman needs to make a telephone call and a friend has just told her the number to call; she does not have pencil and paper to write down the number. Two options are immediately available to help her remember the number. She could repeat the number over and over until she makes the call (a technique known as maintenance rehearsal), or she could give the number some kind of meaning that would help her recall it (elaborative rehearsal). The mental process that allows a person to perform those operations is commonly called short-term, or working, memory.

William James, in 1890, used the term “primary memory” to describe the information under conscious awareness (immediate memory) and the term “secondary memory” to describe inactive information (indirect memory). This type of dualism evolved into the terms “short-term memory” and “long-term memory,” a distinction that was based on the idea that each memory type was independent and was the result of different underlying mental processes.

In 1968, Richard Atkinson and Richard Shiffrin further developed this approach by proposing a stage model, or modal model, of memory that included the sensory register, the short-term store, and the long-term store. Subsequently, extensive research programs focused on the short-term store. These experimental findings resulted in the view, postulated by Alan Baddeley in 1986, that emphasizes the mental processes involved in the memory function rather than describing a static (inactive or passive) storage bin where information is saved. With this approach came the label “working memory” and the metaphor of a mental workbench performing a wide range of cognitive operations. As Henry Ellis and Reed Hunt explained in 1991, “Memory is determined by what is done to the information, not by where the information is stored.” This is the view of an active, mental process characterized by specific functions and limitations.

Characteristics

Three basic characteristics define short-term memory: trace life, storage capacity, and nature of the code. With respect to trace life (the amount of time information can be retained in working memory without further processing), Lloyd Peterson and MargaretIntons-Peterson demonstrated in 1959 that current, active information in the working memory bank is subject to rapid forgetting (in about twelve seconds) if the information does not receive further processing. They showed that if people are not allowed to rehearse or elaborate information they have just encountered, that information is lost. For example, they asked people to recall a series of letters, but immediately after they indicated the letters to be recalled, the people in the experiment were required to count backward by threes. The activity of counting backward interfered with remembering the letters. Similarly, if one is trying to recall a telephone number one has just heard but is interrupted on the way to the telephone to call it, the telephone number is usually lost.

In 1956, George Miller wrote a paper entitled “The Magic Number Seven, Plus or Minus Two” that made a strong and influential case regarding the storage capacity of short-term memory. His notion has been tested in a variety of settings, using a variety of information units. Whether people are asked to remember a list of letters, numbers, or words, or even a group of objects, most people remember about seven items. This finding has produced a wide range of applications. Telephone numbers, one may note, are composed of seven numbers.

Further study of the capacity of short-term memory revealed the ability to “chunk” information and, in so doing, remember more information than merely seven individual, independent bits of information. This process involves reorganizing single bits of information (with the assistance of information previously encoded in long-term storage) into larger units of information. For example, one could remember each individual letter of the word “chunk” and recall five letters, c-h-u-n-k (hence, five units). One might also form the letters into one unit and, instead, recall the word “chunk” (one unit). Chunking dramatically increases the amount of information that can be retained in short-term memory.

To account for the nature of the code (the form used to understand and store information), Baddeley designed a model of working memory that includes the phonological loop (a concept describing the coding of speech-based information in working memory) and the visuo-spatial sketchpad. A wide range of experimental evidence indicates that a phonetic (or acoustic or sound) code is used in short-term memory. For example, if a person is asked to retain a list of words or letters and the items sound alike, fewer items are recalled and more errors are found. On the other hand, if the items sound different, recall is better and fewer errors are found. Baddeley referred to this as the phonological similarity effect and explained that this effect occurs because the short-term memory store is based on a phonological code. Accordingly, items that sound similar will have similar codes.

In addition, related to the phonological code is the word-length effect. In essence, this refers to the finding that words with more syllables take more time to read and are less likely to be recalled from short-term memory than are monosyllabic words. On the other hand, if a word takes longer to read and to pronounce (either aloud or to oneself), the opportunity for a strong memory trace is greater.

The visuo-spatial sketchpad in Baddeley’s model refers to the use of an imagery code in short-term memory. For example, one might imagine one’s kitchen and focus on the location of the sink. To do this, one most likely generates a mental image of the kitchen. Another example of the visuo-spatial sketchpad involves recognizing words or patterns when a person is reading. Still another example of this function is the process in which people engage when comparing two shapes. In a test in which people are asked to indicate whether geometric clusters are similar or different when they are presented in different orientations, experimental evidence indicates that most people engage in mental rotation to make their decisions about the figures. In other words, they imagine the particular geometric cluster turned in different directions and compare it to each of the other figures.

Importance to Daily Life

The essential role of short-term memory is usually taken for granted until some event disrupts the memory process. In a hypothetical example of the consequences of not having a properly functioning short-term memory system, a man named Bill wakes up one morning without one. First, he gets out of bed (he can still walk, because he has the benefit of long-term memory) and trips over his cat. He finds himself on the floor but cannot remember how he got there. He walks to the bathroom to brush his teeth, but when he gets there he does not remember why he is there. He wanders into the kitchen to make coffee; he puts water into the coffeemaker and bends down to pet the cat. Then he rises and fills the coffeemaker with water again, because he does not remember the event that happened only a few seconds before.

The telephone rings, and Bill answers it. His friend Jane asks him to meet her in fifteen minutes; he agrees and hangs up. In the meantime, the water from the coffeemaker is spilling over the kitchen counter, and Bill has no idea why that is happening. The doorbell rings, and his next-door neighbor asks to borrow some milk. Being a good neighbor, Bill agrees to get some milk and goes into the kitchen. Once there, Bill realizes that he must turn off the coffeemaker. He steps in the spilled coffee, then goes into the bedroom to change his socks. Meanwhile, the neighbor whom Bill has forgotten leaves. Eventually Jane calls to ask why Bill did not meet her.

One’s very existence and quality of life depend on the functioning of short-term memory. The preceding example may seem preposterous, yet there are a large number of cases of people who have impaired short-term memory. Several types of events can result in memory deficits and disorders, including head injuries, strokes, and disease-related dementia.

Amnesia

The term “amnesia” refers to a class of disorders that involve various types of memory dysfunction. Some types of amnesia are associated with loss of long-term memory functioning. In these cases, a person may be able to learn new information but has difficulty recalling previous information. Other types of amnesia are associated with impaired short-term memory. These people have difficulty learning new information but can recall previously learned information. There are amnesiacs who have memory deficits relating to both short-term and long-term memory.

In general, when short-term memory is impaired, people are unable to process and retain new information effectively. The case of American neurosurgeon William Scoville’s patient H. M., described in the 1950s, provides an interesting example of short-term memory impairment. H. M. suffered from a severe form of epilepsy that could not be controlled by medication. Scoville surgically removed portions of H. M.’s brain (the temporal lobes) in an attempt to remedy the epilepsy problem.

After the surgery, H. M. experienced striking short-term memory impairment, called anterograde amnesia. H. M.’s memory disorder was studied in depth, and many of the characteristics that define short-term memory functions were illuminated. H. M. was able to remember information from his past, but he was unable to remember new information. For example, H. M.’s mother reported that he could still mow the lawn, because he remembered how to do so, but he was unable to find the lawn mower when he left it parked somewhere.

H. M. was unable to remember or recognize anyone he met or any place he visited after the surgical procedure. He engaged in intense conversations with people but subsequently could recall neither the conversation nor the person with whom he had the conversation. Moreover, H. M. was taught procedures for accomplishing tasks, and his performance of a task revealed that he had learned the task; however, H. M. consistently claimed that he had never before performed the task. In other words, he did not remember the event of learning the task, but his performance revealed that he had retained some of the skills associated with the learning event.

Clearly, the short-term memory process, though often taken for granted, is an essential and integral part of mental functioning. This process also plays a major role in the study of psychology.

Early Interest in Memory

With the first humans came the first speculations about mental processes. Inherent in studies of mental activities are studies of memory, since memory is necessary for learning. The role of memory was a central element of philosophies of the mind. This point is exhibited in historical accounts of the mind that referenced memory processes. These accounts reveal the underlying theories of mental processes postulated at the time of the philosophies.

For example, both Aristotle and Plato used the analogy of a wax tablet to describe the memory process. According to this perspective, experience was merely stamped into the brain. These views are consistent with the idea of memory being a static store or receptacle rather than a dynamic process. These static views assume a passive organism rather than an active, dynamic information processor.

This concept of memory continued through the ages, changing very little until the science of psychology arose in the late nineteenth century. As psychology evolved, so did the field of memory. This evolution is reflected in a change from the static, storage view of memory to the idea of memory as an active process. This change parallels the evolution of the image of humans as passive experience-storage units to seeing humans as active information processors; this evolution is often called the cognitive revolution. Whether the change is called evolution or revolution, it happened largely as a result of the foundation provided by innovative research such as that conducted by the German psychologist Hermann Ebbinghaus.

Ebbinghaus’s Contributions

In the early 1880s,
Hermann Ebbinghaus moved the study of memory from the domain of philosophy to the domain of science when he embarked on an intensive investigation of the memory process. Ebbinghaus used himself as an experimental subject and spent two years memorizing nonsense syllables to see whether simple repetition would facilitate the recall process. Keeping copious notes in a strict scientific environment, he found that rote rehearsal improved the memory retrieval process.

Ebbinghaus’s work was particularly important because it showed that mental processes could be simplified and studied using a rigorous, scientific method. In addition, he proved data that are fundamental for understanding memory processes, and that paved the way for the vast program of memory research that is flourishing today.

Studies associated with short-term or working memory focus on dynamic, immediate, cognitive activities. In general, these mental processes are involved in understanding the world; specifically, these investigations advance knowledge about learning, comprehension, problem solving, thought construction, and expression. Trends include mapping regions of the brain and discovering neurotransmitters (brain chemicals) that are affiliated with working memory activities. The working memory process provides a rich domain for investigations of mental activities. Many researchers believe that future investigations of the process will reveal the keys to discovering the essence of mental activity.

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