Working memory is part of the cognitive domain called the executive function. The executive function is the mental processes that enable us to plan, focus attention, remember instructions, and juggle multiple tasks successfully. Scientists more or less agree that the triad of executive function is: working memory, flexibility, and inhibitory control.
Some important properties
The concepts of short-term memory (STM), or short-term store (STS), working memory (WM), etc. are fundamental to the field of cognitive science. The ideas date back to the time when George Miller, Brown, and Peterson & Peterson did various studies on selective attention during the ‘60s. For example, one has to remember a set of 3 alphabets. These sets are forgotten easily after 15 seconds when the person’s attention is diverted to another distracting stimulus. Hence, the concept of STM is closely related to selective attention and cognition in general.
The concepts of short-term memory (STM), or short-term store (STS), working memory (WM), etc. are fundamental to the field of cognitive science. The ideas date back to the time when George Miller, Brown, and Peterson & Peterson did various studies on selective attention during the ‘60s. For example, one has to remember a set of 3 alphabets. These sets are forgotten easily after 15 seconds when the person’s attention is diverted to another distracting stimulus. Hence, the concept of STM is closely related to selective attention and cognition in general.
In general, they are different from the so-called memory or the long-term memory in which the content has to be consciously recalled. William James mentioned that long-term memory has to be “brought back”, whereas STM is currently available. It is also related to conscious awareness. As a more special term, WM bears a connotation that it is a mental workbench where mental effort is applied. A popular example of WM is when you try to solve an arithmetic problem (4 + 30/2 = ?) or when you want to understand a foreign language.
If STM is a type of memory, what is the capacity? It turns out that STM has a limited capacity. Miller originally proposed it has a magical container that can take up to 7 ± 2 items at once. Later research shows that a lesser capacity of around 4 ± 1 items at once is more acceptable (e.g. Cowan, 2010). Hence, STM is the bottleneck in our information processing system. Such capacity can be overcome by chunking or recoding strategy.
If STM is a type of memory, what is the capacity? It turns out that STM has a limited capacity. Miller originally proposed it has a magical container that can take up to 7 ± 2 items at once. Later research shows that a lesser capacity of around 4 ± 1 items at once is more acceptable (e.g. Cowan, 2010). Hence, STM is the bottleneck in our information processing system. Such capacity can be overcome by chunking or recoding strategy.
Related to the STM capacity is the term “forgetting” or losing the strength in the consciousness. In studies by Brown (1958) and a husband-wife Peterson & Peterson (1959), subjects were presented with a 3-letter (trigram) that did not make any linguistic sense (to prevent chunking), e.g. XMH. Afterward, subjects were given a number, say, 450. Then they had to count backward by three/four for a given interval, after which they have to recall what trigram they just saw. The performance is no better than chance after 3-4 seconds counting. A decay curve of “forgetting” can be fitted to predict the recall accuracy as a function of distractor interval. It seems that Brown-Peterson task points to the idea that WM storage decays as a function of time. However, Waugh and Norman (1965) proposed that it is the interference between the counting and the trigram that causes the forgetting, not time-dependent property per se. Indeed the number of interfering items in between the presentation of an item and the recall period contributes to the severity of forgetting. This temporal decay versus interference theory is still unresolved.
Once placed in the STM, how can the items be retrieved? Serial position curves reveal two properties of STM: the recency and primacy effect. In other words, an item held last or most recent in the queue and the earliest (deliberate attempt to rehearse, or put in the long-term storage) can be retrieved very accurately than the items in between (e.g. Glanzer et al.). A more developed model of STM retrieval comes from S. Sternberg (1969) which includes the way we search a desired item in the STM container, brings an item of interest to consciousness, and do an appropriate response. He was interested in whether people scan items in the short-term memory one at a time (serial processing) or all at once (parallel processing).
In one study, Sternberg presented a display of numbers from 1 to 6 different digits to subjects. These items are so-called the memory set. Then he showed the subjects a test digit and they had to decide whether the test digit had been included in the display just shown. If parallel processing occurred, the length of the presentation should not influence the reaction time, but if serial processing occurred, the reaction time should increase as a function of presentation time. An interesting finding was that the reaction time was about the same whether the response was a "yes" or a "no." In other words, participants did not stop responding when they found a match but continued searching the entire display in their memory. This process is called an exhaustive search. Thus, Sternberg concluded that people perform an exhaustive search when retrieving information from STM.
Distinct Components of WM
In the 60's, Atkinson and Shiffrin talked about STM model. The more elaborate model, originally proposed by Baddeley and Hitch (1974), was based on Shallice & Warrington's and other clinical cases. According to Baddeley's model, WM by no means has a central controller and two different components or "slaves" that work independently.
1) Phonological loop: maintains, manipulates acoustical, verbal information, e.g. rehearsing words.
2) Visuospatial sketchpad: maintains, manipulates visual and spatial information, e.g. playing chess.
3) Central executive: acts as a manager that controls and oversees the use of different WMs.
4) A more recent development is a component called episodic buffer.
Phonological loop is related to observed phenomena e.g. phonological similarity, articulatory suppression, and clinical studies of aphasic patients with dyspraxia. The primary goals of the central executive controller is binding information from a number of sources into coherent episodes, coordination of the 3 components, shifting between tasks/retrieval strategies, and selective attention & inhibition. The prefrontal cortex is important for such purposes.
The visuospatial sketchpad has also been expanded quite recently. For e.g., Logie expanded the Baddeley's visuospatial sketchpad by dividing the system into optical (visual) and spatial (mental imagery, movement information). Smyth et al. (1988) shows how subjects performed a recall of a series of movement sequence and this is thought to involve the visuospatial sketchpad. Other studies have shown that there is less interference between visual and spatial tasks than between two visual tasks or two spatial tasks. This suggests that the two entities may be more independent than initially thought.
There are tons of neuropsychological test batteries to investigate WM in humans and patients. Digit span test, where a person is presented with a series of digits one after the other and has to repeat the digits in the same order, is arguably the most common test to access verbal working memory. A variant of this, alphabets, may be used. Corsi Block Test is the spatial equivalent of the test that taps into the spatial working memory. Mental rotation test is another visuospatial test that may also involved mental imagery. Because of the online nature and the link between WM, cognition, and problem solving, WM is sometimes thought to describe some kind of intelligence.
Neural Substrates of WM
The dorsolateral prefrontal cortex (DLPFC), equivalent to BA 9 and BA 46, (or Area 9/46, Petrides & Pandya in primates), is thought to be the most important part for WM both in humans and monkeys. Much of the earlier works in non-human primates were done by Goldman-Rakic in the '80-'90s using a paradigm called delayed-response task. She found that lesion to DLPFC impaired the task performance. A series of neuroimaging studies have corroborated this finding, confirming the involvement of DLPFC in working memory tasks. According to d'Esposito and colleagues, there is a lateralization of functions, in the sense, verbal working memory is more left-lateralized versus visuospatial which is right-lateralized.
Distinct Components of WM
In the 60's, Atkinson and Shiffrin talked about STM model. The more elaborate model, originally proposed by Baddeley and Hitch (1974), was based on Shallice & Warrington's and other clinical cases. According to Baddeley's model, WM by no means has a central controller and two different components or "slaves" that work independently.
1) Phonological loop: maintains, manipulates acoustical, verbal information, e.g. rehearsing words.
2) Visuospatial sketchpad: maintains, manipulates visual and spatial information, e.g. playing chess.
3) Central executive: acts as a manager that controls and oversees the use of different WMs.
4) A more recent development is a component called episodic buffer.
Phonological loop is related to observed phenomena e.g. phonological similarity, articulatory suppression, and clinical studies of aphasic patients with dyspraxia. The primary goals of the central executive controller is binding information from a number of sources into coherent episodes, coordination of the 3 components, shifting between tasks/retrieval strategies, and selective attention & inhibition. The prefrontal cortex is important for such purposes.
The visuospatial sketchpad has also been expanded quite recently. For e.g., Logie expanded the Baddeley's visuospatial sketchpad by dividing the system into optical (visual) and spatial (mental imagery, movement information). Smyth et al. (1988) shows how subjects performed a recall of a series of movement sequence and this is thought to involve the visuospatial sketchpad. Other studies have shown that there is less interference between visual and spatial tasks than between two visual tasks or two spatial tasks. This suggests that the two entities may be more independent than initially thought.
There are tons of neuropsychological test batteries to investigate WM in humans and patients. Digit span test, where a person is presented with a series of digits one after the other and has to repeat the digits in the same order, is arguably the most common test to access verbal working memory. A variant of this, alphabets, may be used. Corsi Block Test is the spatial equivalent of the test that taps into the spatial working memory. Mental rotation test is another visuospatial test that may also involved mental imagery. Because of the online nature and the link between WM, cognition, and problem solving, WM is sometimes thought to describe some kind of intelligence.
Neural Substrates of WM
The dorsolateral prefrontal cortex (DLPFC), equivalent to BA 9 and BA 46, (or Area 9/46, Petrides & Pandya in primates), is thought to be the most important part for WM both in humans and monkeys. Much of the earlier works in non-human primates were done by Goldman-Rakic in the '80-'90s using a paradigm called delayed-response task. She found that lesion to DLPFC impaired the task performance. A series of neuroimaging studies have corroborated this finding, confirming the involvement of DLPFC in working memory tasks. According to d'Esposito and colleagues, there is a lateralization of functions, in the sense, verbal working memory is more left-lateralized versus visuospatial which is right-lateralized.
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