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Short-term memory

Short-term memory

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Short-term memory is the capacity for holding a small amount of information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...

 in mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

 in an active, readily available state for a short period of time. The duration of short-term memory (when rehearsal or active maintenance is prevented) is believed to be in the order of seconds. A commonly cited capacity is 7 ± 2
The Magical Number Seven, Plus or Minus Two
"The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information" is one of the most highly cited papers in psychology. It was published in 1956 by the cognitive psychologist George A. Miller of Princeton University's Department of Psychology in Psychological...

 elements. In contrast, long-term memory
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...

 indefinitely stores a seemingly unlimited amount of information.

Short-term memory should be distinguished from working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

 which refers to structures and processes used for temporarily storing and manipulating information (see more details below).

Existence of a separate store


The idea of a division of memory into short term and long term dates back to the 19th century. A classical model of memory developed in the 1960s, now known to be flawed, assumed that all memories pass from a short-term to a long-term store after a small period of time. This model is referred to as the "modal model" and has been most famously detailed by Shiffrin
Richard Shiffrin
Richard Shiffrin is the Luther Dana Waterman Professor of cognitive science for the Department of Psychological and Brain Sciences at Indiana University, Bloomington. Shiffrin has contributed a number of theories of attention and memory to the field of psychology. He co-authored the...

. The exact mechanisms by which this transfer takes place, whether all or only some memories are retained permanently, and indeed the existence of a genuine distinction between the two stores, remain controversial topics among experts.

One form of evidence, cited in favor of the separate existence of a short-term store comes from anterograde amnesia
Anterograde amnesia
Anterograde amnesia is a loss of the ability to create new memories after the event that caused the amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact. This is in contrast to retrograde amnesia, where memories...

, the inability to learn new facts and episodes. Patients with this form of amnesia, have intact ability to retain small amounts of information over short time scales (up to 30 seconds) but are dramatically impaired in their ability to form longer-term memories (a famous example is patient HM
HM (patient)
Henry Gustav Molaison , famously known as HM or H.M., was an American memory disorder patient who was widely studied from late 1957 until his death...

). This is interpreted as showing that the short-term store is spared from amnesia and other brain diseases.

Other evidence comes from experimental studies showing that some manipulations (e.g., a distractor task, such as repeatedly subtracting a single-digit number from a larger number following learning; cf Brown-Peterson procedure) impair memory for the 3 to 5 most recently learned words of a list (presumably still held in short-term memory), while leaving recall for words from earlier in the list (presumably stored in long-term memory) unaffected; other manipulations (e.g., semantic similarity of the words) affect only memory for earlier list words, but do not affect memory for the last few words in a list. These results show that different factors affect short term recall (disruption of rehearsal) and long-term recall (semantic similarity). Together, these findings show that long-term memory and short-term memory can vary independently of each other.

Not all researchers agree that short-term and long-term memory are separate systems. Some theorists propose that memory is unitary over all time scales, from milliseconds to years. Support for the unitary memory hypothesis comes from the fact that it has been difficult to demarcate a clear boundary between short-term and long-term memory. For instance, Tarnow shows that the recall probability vs. latency curve is a straight line from 6 to 600 seconds (ten minutes), with the probability of failure to recall only saturating after 600 seconds. If there were really two different memory stores operating in this time frame, one could expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar for recall of a list immediately after learning (presumably from short-term memory) and recall after 24 hours (necessarily from long-term memory).

Further evidence against the existence of a short-term memory store comes from experiments involving continual distractor tasks. In 1974, Robert Bjork and William B. Whitten presented subjects with word pairs to be remembered; however, before and after each word pair, subjects had to do a simple multiplication task for 12 seconds. After the final word-pair, subjects had to do the multiplication distractor task for 20 seconds. In their results, Bjork and Whitten found that the recency effect (the increased probability of recall of the last items studied) and the primacy effect (the increased probability of recall of the first few items) still remained. These results would seem inconsistent with the idea of short-term memory as the distractor items would have taken the place of some of the word-pairs in the buffer, thereby weakening the associated strength of the items in long-term memory. Bjork and Whitten hypothesized that these results could be attributed to the memory processes at work for long-term memory retrieval versus short-term memory retrieval.

Ovid J.L. Tzeng (1973) also found an instance where the recency effect in free recall did not seem to result from the function of a short-term memory store. Subjects were presented with four study-test periods of 10 word lists, with a continual distractor task (20-second period of counting-backward). At the end of each list, participants had to free recall as many words from the list as possible. After free-recall of the fourth list, participants were asked to free recall items from all four lists. Both the initial free recall and the final free recall showed a recency effect. These results went against the predictions of a short-term memory model, where no recency effect would be expected in either initial or final free recall.

Koppenaal and Glanzer (1990) attempted to explain these phenomena as a result of the subjects’ adaptation to the distractor task, which therefore allowed them to preserve at least some of the functions of the short-term memory store. As evidence, they provided the results of their experiment, in which the long-term recency effect disappeared when the distractor after the last item differed from the distractors that preceded and followed all the other items (e.g. arithmetic distractor task and word reading distractor task).
Thapar and Greene challenged this theory. In one of their experiments, participants were given a different distractor task after every item to be studied. According to Koppenaal’s and Glanzer’s theory, there should be no recency effect as subjects would not have had time to adapt to the distractor; yet such a recency effect remained in place in the experiment.

One proposed explanation of the existence of the recency effect in a continual distractor condition, and the disappearance of it in an end-only distractor task is the influence of contextual and distinctive processes. According to this model, recency is a result of the final items’ processing context being similar to the processing context of the other items and the distinctive position of the final items versus items in the middle of the list. In the end distractor task, the processing context of the final items is no longer similar to the processing context of the other list items. At the same time, retrieval cues for these items are no longer as effective as without the distractor. Therefore, the recency effect recedes or vanishes. However, when distractor tasks are placed before and after each item, the recency effect returns, because all the list items once again have similar processing context.

Biological basis


It is proposed by Tarnow that short term memory involves the firing of neurons which depletes the Readily Releasable Pool (RRP) of neurotransmitter vesicles at presynaptic terminals. The pattern of depleted presynaptic terminals represents the long term memory trace and the depletion itself is the short term memory. After the firing has slowed down, endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...

 causes short term memory to decay. If the endocytosis is allowed to finish (the memory is not activated again), the pattern of exhausted postsynaptic terminals becomes invisible and the short term memory disappears. The long term memory remains as the metastable pattern of the neuronal excitations
Metastability in the brain
In the field of computational neuroscience, the theory of metastability refers to the human brain’s ability to integrate several functional parts and to produce neural oscillations in a cooperative and coordinated manner, providing the basis for conscious activity.Metastability, a state in which...

.

Relationship with working memory


The relationship between short-term memory and working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

 is described differently by various theories, but it is generally acknowledged that the two concepts are distinct. Working memory is a theoretical framework that refers to structures and processes used for temporarily storing and manipulating information. As such, working memory might also be referred to as working attention. Short-term memory generally refers, in a theory-neutral manner, to the short-term storage of information, and it does not entail the manipulation or organization of material held in memory. Thus while there are short-term memory components to working memory models, the concept of short-term memory is distinct from these more hypothetical concepts. Within Baddeley
Alan Baddeley
Alan David Baddeley FRS, CBE is a British psychologist. He is professor of psychology at the University of York. He is known for his work on working memory, in particular for his multiple components model.-Education:...

's influential 1986 model of working memory there are two short-term storage mechanisms: the phonological loop
Baddeley's Model of Working Memory
Alan Baddeley and Graham Hitch proposed a model of working memory in 1974, in an attempt to describe a more accurate model of short-term memory....

 and the visuospatial sketchpad
Baddeley's Model of Working Memory
Alan Baddeley and Graham Hitch proposed a model of working memory in 1974, in an attempt to describe a more accurate model of short-term memory....

. Most of the research referred to here involves the phonological loop, because most of the work done on short-term memory has used verbal material. In recent years, however, there has been a surge in research on visual short term memory
Visual short term memory
In the study of vision, visual short-term memory is one of three broad memory systems including iconic memory and long-term memory. VSTM is a type of short-term memory, but one limited to information within the visual domain....

, and also increasing work on spatial short term memory.

Duration of short-term memory


The limited duration of short-term memory quickly suggests that its contents spontaneously decay over time. The decay assumption is part of many theories of short-term memory, most notably Baddeley's model of working memory
Baddeley's Model of Working Memory
Alan Baddeley and Graham Hitch proposed a model of working memory in 1974, in an attempt to describe a more accurate model of short-term memory....

. The decay assumption is usually paired with the idea of rapid covert rehearsal: In order to overcome the limitation of short-term memory, and retain information for longer, information must be periodically repeated, or rehearsed — either by articulating it out loud, or by mentally simulating such articulation. In this way, the information will re-enter the short-term store and be retained for a further period.

Several researchers, however, dispute that spontaneous decay plays any significant role in forgetting over the short term, and the evidence is far from conclusive.

Authors doubting that decay causes forgetting from short-term memory often offer as an alternative some form of interference: When several elements (such as digits, words, or pictures) are held in short term memory simultaneously, their representations compete with each other for recall, or degrade each other. Thereby, new content gradually pushes out older content, unless the older content is actively protected against interference by rehearsal or by directing attention to it.

Capacity of short-term memory


Whatever the cause or causes of forgetting over the short term may be, there is consensus that it severely limits the amount of new information that we can retain over brief periods of time. This limit is referred to as the finite capacity of short-term memory. The capacity of short-term memory is often called memory span
Memory span
In psychology and neuroscience, memory span is the longest list of items that a person can repeat back in correct order immediately after presentation on 50% of all trials. Items may include words, numbers, or letters. The task is known as digit span when numbers are used. Memory span is a common...

, in reference to a common procedure of measuring it. In a memory span test, the experimenter presents lists of items (e.g. digits or words) of increasing length. An individual's span is determined as the longest list length that he or she can recall correctly in the given order on at least half of all trials.

In an early and highly influential article, The Magical Number Seven, Plus or Minus Two
The Magical Number Seven, Plus or Minus Two
"The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information" is one of the most highly cited papers in psychology. It was published in 1956 by the cognitive psychologist George A. Miller of Princeton University's Department of Psychology in Psychological...

, the psychologist George Miller suggested that human short-term memory has a forward memory span of approximately seven items plus or minus two and that that was well known at the time (it seems to go back to the 19th century researcher Wundt). More recent research has shown that this "magical number seven" is roughly accurate for college students recalling lists of digits, but memory span varies widely with populations tested and with material used. For example, the ability to recall words in order depends on a number of characteristics of these words: fewer words can be recalled when the words have longer spoken duration; this is known as the word-length effect, or when their speech sounds are similar to each other; this is called the phonological similarity effect. More words can be recalled when the words are highly familiar or occur frequently in the language. Recall performance is also better when all of the words in a list are taken from a single semantic category (such as sports) than when the words are taken from different categories. According to the available evidence, the best overall estimate of short-term memory is about four pieces or "chunks" of information. In free recall it has been shown, to the contrary, that there is no such "quantized" limit, rather it is a function of memory decaying with time.

Chunking


Chunking
Chunking (psychology)
Chunking, in psychology, is a phenomenon whereby individuals group responses when performing a memory task. Tests where individuals can illustrate "chunking" commonly include serial and free recall, as these both require the individual to reproduce items that he or she had previously been...

 is the process by which we can expand our ability to remember things in the short term. Chunking is also a process by which a person organizes material into meaningful groups. Although the average person may only retain about four different units in short-term memory, chunking can greatly increase a person's recall capacity. For example, in recalling a phone number, the person could chunk the digits into three groups: first, the area code (such as 215), then a three-digit chunk (123) and lastly a four-digit chunk (4567). This method of remembering phone numbers is far more effective than attempting to remember a string of 10 digits.

Practice and the usage of existing information in long-term memory can lead to additional improvements in one's ability to use chunking. In one testing session, an American cross-country runner was able to recall a string of 79 digits after hearing them only once by chunking them into different running times (e.g. the first four numbers were 1518, a three-mile time.)

Factors affecting short term memory


It is very difficult to demonstrate the exact capacity of STM because it will vary depending on the nature of the material to be recalled. There is currently no way of defining the basic unit of information to be stored in the STM store. It is also possible that STM is not the store described by Atkinson and Shiffrin. In that case, the task of defining the task of STM becomes even more difficult.

However, capacity of STM can be affected by the following:
Influence of long-term memory, Reading aloud, Pronunciation time and Individual differences.

See also

  • Related to the concept of working memory
    Working memory
    Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

  • Contrast long-term memory
    Long-term memory
    Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...

  • Iconic memory
    Iconic memory
    Iconic memory is the visual sensory memory register pertaining to the visual domain. It is a component of the visual memory system which also includes visual short term memory and long term memory . Iconic memory is described as a very brief . A small decrease in visual persistence occurs with age...

  • Visual short-term memory
  • Attention versus memory in prefrontal cortex
    Attention versus memory in prefrontal cortex
    A widely accepted theory regarding the function of the brain's prefrontal cortex is that it serves as a store of short-term memory. This idea was first formulated by Jacobsen, who reported in 1935 that damage to the primate prefrontal cortex caused short-term memory deficits...

  • Memento, a film
  • 50 First Dates
    50 First Dates
    50 First Dates is a 2004 American romantic comedy film directed by Peter Segal and written by George Wing. The film stars Adam Sandler as a woman-chasing veterinarian and Drew Barrymore as an amnesiac, along with Rob Schneider, Sean Astin, Lusia Strus, Blake Clark, and Dan Aykroyd.Most of the film...

    , a film
  • Ghajini, a film
  • Patient HM
  • The Last Hippie in An Anthropologist on Mars
    An Anthropologist on Mars
    An Anthropologist on Mars: Seven Paradoxical Tales is a 1995 book by neurologist Oliver Sacks consisting of seven medical case histories of individuals with neurological conditions such as autism and Tourette syndrome...

    , a medical case history
  • Clive Wearing
    Clive Wearing
    Clive Wearing is a British musicologist, conductor, and keyboardist suffering from an acute and long-lasting case of anterograde and retrograde amnesia, meaning that he lacks the ability to form new memories.-Musical career:...


External links