Short-term memory acts as a kind of “scratch-pad” for temporary recall of the information which is being processed at any point in time, and has been referred to as “the brain’s Post-it note”. It can be thought of as the ability to remember and process information at the same time. It holds a small amount of information (typically around 7 items or even less) in mind in an active, readily-available state for a short period of time (typically from 10 to 15 seconds, or sometimes up to a minute).
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| A 2010 University of Stirling study has suggested a possible link between poor short-term or working memory and depression.|
The 10 to 15% with the poorest working memory in the study tended to mull things over and brood too much, leading to a risk of depression.
People with a good working memory, on the other hand, are more likely to be optimistic and self-assured, and more likely to lead a happy and successful life.
For example, in order to understand this sentence, the beginning of the sentence needs to be held in mind while the rest is read, a task which is carried out by the short-term memory. Other common examples of short-term memory in action are the holding on to a piece of information temporarily in order to complete a task (e.g. “carrying over” a number in a subtraction sum, or remembering a persuasive argument until another person finishes talking), and simultaneous translation (where the interpreter must store information in one language while orally translating it into another). What is actually held in short-term memory, though, is not complete concepts, but rather links or pointers (such as words, for example) which the brain can flesh out from it’s other accumulated knowledge.
However, this information will quickly disappear forever unless we make a conscious effort to retain it, and short-term memory is a necessary step toward the next stage of retention, long-term memory. The transfer of information to long-term memory for more permanent storage can be facilitated or improved by mental repetition of the information or, even more effectively, by giving it a meaning and associating it with other previously acquired knowledge. Motivation is also a consideration, in that information relating to a subject of strong interest to a person, is more likely to be retained in long-term memory.
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A recent study at the University of Michigan suggests that attention and short-term memory processing are directly affected by a person’s surroundings and environment.|
Two groups of individuals were tested on their attention and working memory performance, one group after a relaxed walk in a quiet park and the other group after navigating busy city streets.
Those who had been walking the city streets scored far lower on the tests.
The term working memory is often used interchangeably with short-term memory, although technically working memory refers more to the whole theoretical framework of structures and processes used for the temporary storage and manipulation of information, of which short-term memory is just one component.
The central executive part of the prefrontal cortex at the front of the brain appears to play a fundamental role in short-term and working memory. It both serves as a temporary store for short-term memory, where information is kept available while it is needed for current reasoning processes, but it also “calls up” information from elsewhere in the brain. The central executive controls two neural loops, one for visual data (which activates areas near the visual cortex of the brain and acts as a visual scratch pad), and one for language (the “phonological loop”, which uses Broca’s area as a kind of “inner voice” that repeats word sounds to keep them in mind). These two scratch pads temporarily hold data until it is erased by the next job.
Although the prefrontal cortex is not the only part of the brain involved – it must also cooperate with other parts of the cortex from which it extracts information for brief periods – it is the most important, and Carlyle Jacobsen reported, as early as 1935, that damage to the prefrontal cortex in primates caused short-term memory deficits.
The short-term memory has a limited capacity, which can be readily illustrated by the simple expedient of trying to remember a list of random items (without allowing repetition or reinforcement) and seeing when errors begin to creep in. The often-cited experiments by George Miller in 1956 suggest that the number of objects an average human can hold in working memory (known as memory span) is between 5 and 9 (7 ± 2, which Miller described as the “magical number”, and which is sometimes referred to as Miller’s Law). However, although this may be approximately true for a population of college students, for example, memory span varies widely with populations tested, and modern estimates are typically lower, of the order of just 4 or 5 items.
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Short-term working memory appears to operate phonologically.|
For instance, whereas English speakers can typically hold seven digits in short-term memory, Chinese speakers can typically remember ten digits.
This is because Chinese number words are all single syllables, whereas English are not.
The type or characteristics of the information also affects the number of items which can be retained in short-term memory. For instance, more words can be recalled if they are shorter or more commonly used words, or if they are phonologically similar in sound, or if they are taken from a single semantic category (such as sports, for example) rather than from different categories, etc. There is also some evidence that short-term memory capacity and duration is increased if the words or digits are articulated aloud instead of being read sub-vocally (in the head).
The relatively small capacity of the short-term memory, compared to the huge capacity of long-term memory , has been attributed by some to the evolutionary survival advantage in paying attention to a relatively small number of important things (e.g. the approach of a dangerous predator, the proximity of a nearby safe haven, etc) and not to a plethora of other peripheral details which would only interfere with rapid decision-making.
“Chunking” of information can lead to an increase in the short-term memory capacity. Chunking is the organization of material into shorter meaningful groups to make them more manageable. For example, a hyphenated phone number, split into groups of 3 or 4 digits, tends to be easier to remember than a single long number. Experiments by Herbert Simon have shown that the ideal size for chunking of letters and numbers, whether meaningful or not, is three. However, meaningful groups may be longer (such as four numbers that make up a date within a long list of numbers, for example). With chunking, each chunk represents just one of the 5 – 9 items that can be stored in short-term memory, thus extending the total number of items that can be held.
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| The use of mnemonic devices can significantly increase memory, particularly the recall of long lists of names, numbers, etc.|
One case, known as “S.F.”, was able to increase his digit span (the longest list of number that a person can repeat back in correct order) from 7 to 79 with the use of mnemonic strategies.
Akira Haraguchi and Lu Chao’s record-breaking recitations of the digits of the number Pi (100,000 and 67,890 digits respectively) also make use of mnemonic systems.
It is usually assumed that the short-term memory spontaneously decays over time, typically in the region of 10 – 15 seconds, but items may be retained for up to a minute, depending on the content. However, it can be extended by repetition or rehearsal (either by reading items out loud, or by mental simulation), so that the information re-enters the short-term store and is retained for a further period. When several elements (such as digits, words or pictures) are held in short-term memory simultaneously, they effectively compete with each other for recall. New content, therefore, gradually pushes out older content (known as displacement), unless the older content is actively protected against interference by rehearsal or by directing attention to it. Any outside interference tends to cause disturbances in short-term memory retention, and for this reason people often feel a distinct desire to complete the tasks held in short-term memory as soon as possible.
The forgetting of short-term memories involves a different process to the forgetting of long-term memory. When something in short-term memory is forgotten, it means that a nerve impulse has merely ceased being transmitted through a particular neural network. In general, unless an impulse is reactivated, it stops flowing through a network after just a few seconds.
Typically, information is transferred from the short-term or working memory to the long-term memory within just a few seconds, although the exact mechanisms by which this transfer takes place, and whether all or only some memories are retained permanently, remain controversial topics among experts. Richard Schiffrin, in particular, is well known for his work in the 1960s suggesting that ALL memories automatically pass from a short-term to a long-term store after a short time (known as the modal or multi-store or Atkinson-Schiffrin model).
However, this is disputed, and it now seems increasingly likely that some kind of vetting or editing procedure takes place. Some researchers (e.g. Eugen Tarnow) have proposed that there is no real distinction between short-term and long-term memory at all, and certainly it is difficult to demarcate a clear boundary between them. However, the evidence of patients with some kinds of anterograde amnesia, and experiments on the way distraction affect the short-term recall of lists, suggest that there are in fact two more or less separate systems.