Working memory is a limited capacity part of the human memory system that combines the temporary storage and manipulation of information in the service of cognition. Short-term memory refers to information-storage without manipulation and is therefore a component of working memory. Working memory differs from long-term memory, a separate part of the memory system with a vast storage capacity that holds information in a relatively more stable form. According to the multi-component model, working memory includes an executive controller that interacts with separate short-term stores for auditory-verbal and visuo-spatial information. The concept of working memory has proved useful in many areas of application including individual differences in cognition, neuropsychology, normal and abnormal child development and neuroimaging.
The term working memory is used most frequently to refer to a limited capacity system that is capable of briefly storing and manipulating information involved in the performance of complex cognitive tasks such as reasoning, comprehension and certain types of learning. Working memory differs from short-term memory (STM) in that it assumes both the storage and manipulation of information, and in the emphasis on its functional role in complex cognition. A range of different approaches to the study of working memory have developed with differences reflecting the interests of the researcher, whether neuropsychological (Vallar, 2006), neurobiological (O'Reilly et al., 1999), psychometric (Engle et al., 1999) or oriented towards providing practical guidance on human factors (Kieras et al., 1999). Despite very different theoretical methods and styles, there is general agreement on a need to assume a role for some form of executive controller, probably of limited attentional capacity, aided by temporary storage systems, with visual and verbal storage probably operating separately (Miyake & Shah, 1999). Such a structure had in fact been proposed by Baddeley and Hitch (1974). While accepting that this is now one of range of models, the Baddeley and Hitch multicomponent model provides a convenient structure for summarising research on working memory over the 30 years since it was first proposed.
Before moving on it is important to note that the term working memory was developed independently in the study of animal learning where it refers to the type of learning or memory thought to underpin tasks such as the radial arm maze, in which an animal has to remember which of several arms have already been visited on that day, a task which we would regard as depending on long-term memory (Olton, Becker & Handelmann, 1979).
The multicomponent model of working memory
In the 1960s there was a short period of consensus among researchers that human memory consisted of a system that could be divided into two principal components. One was a short-term store capable of holding small amounts of information for a few seconds. This fed into a separate long-term store holding vast amounts of information over longer time intervals. This so-called modal model could account for a range of experimental data and was able to account for selective effects of different types of brain damage on short- and long-term recall.
Baddeley and Hitch (1974) set out to test the hypothesis that the short-term store also functioned as a working memory. They did so by requiring participants to perform reasoning, comprehension or learning tasks at the same time as they were holding in STM between 0 and 8 digits for immediate recall. If STM does function as a working memory, then loading it to capacity should lead to massive disruption of cognitive processing. It did indeed cause some disruption, with time to perform a reasoning task increasing with load, but the effect was not huge, and there was no influence on error rate. Baddeley and Hitch (1974) therefore abandoned the modal model, according to which STM is a unitary store, proposing instead a multicomponent model assumes an attentional controller, the central executive aided by two subsystems, the visuo-spatial sketchpad concerned with visual storage and processing, and its acoustic/verbal equivalent, the phonological loop.
The phonological loop
This is the subsystem that is assumed to hold digit sequences for immediate recall. The fact that reasoning was slowed as number of digits increased suggests that it does play a role in reasoning, but the unchanged error rate indicates that it is not essential. It is assumed to have two basic components, a temporary speech-related/acoustic store and a subvocal articulatory rehearsal process.
The phonological store is indicated by the presence of the phonological similarity effect, whereby people are much less accurate in repeating back sequences of similar-sounding words such as MAN CAP CAT MAT CAN, than dissimilar words such as PIT DAY COW PEN TOP. Similarity of meaning (HUGE LARGE BIG WIDE TALL) has little effect on immediate recall. On the other hand if several trials are given to learn a longer list of say 10 words, meaning becomes all-important and sound loses it power, consistent with different systems for short-term and long-term storage (Baddeley, 1966a; 1966b).
Evidence for the importance of rehearsal comes from the word length effect, whereby immediate recall of long words (e.g. REFRIGERATOR UNIVERSITY TUBERCULOSIS OPPORTUNITY HIPPOPOTAMUS) is much more error-prone than for short words (Baddeley, Thomson & Buchanan, 1975).
Baddeley and Hitch suggested that the memory trace of items in the short-term store would rapidly fade, but could be maintained by saying them to oneself. Long words take longer to say, allowing more fading and hence more forgetting to occur. Consistent with this interpretation, preventing subjects from saying words to themselves by requiring the continuous utterance of an item such as the word 'the', removes the word length effect. Since the initial demonstration of the word length effect (Baddeley, Thomson and Buchanan, 1975) other interpretations have been proposed, differing principally in the implications of the effect for whether items in the short-term store are forgotten as a result of spontaneous decay of the memory trace, or by disruption from later material (See Baddeley, 2007 Chapter 3 for a discussion).
The concept of the phonological loop has influenced a number of attempts to simulate human performance in verbal STM tasks using more detailed computational models. The first tranche of such models focused on specifying mechanisms for handling information about the serial order of items, an aspect that was left unspecified in the original account of the loop. These models tend to agree that serial ordering involves 'competitive queueing' (Grossberg, 1987), a process whereby items are simultaneously active and compete for serial selection. The models differ principally with respect to the nature of the ordering cues that determine these activation levels (Burgess & Hitch, 1992; Page & Norris, 1998; Brown, Preece & Hulme, 2000). Recent attempts at computational modelling have gone further by specifying how the short-term phonological storage system interacts with long-term memory (Burgess & Hitch, 2006; see also Botvinick & Plaut, 2006), an essential step to understanding the role of the loop in long-term learning.
Function of the phonological loop
Given that one accepts the evidence for a temporary verbal or phonological memory system, the question of its evolutionary significance arises. One possibility is that the phonological loop supports the acquisition of language, providing a temporary means of storing new words, while they are consolidated in phonological LTM (Baddeley, Gathercole & Papagno, 1998). Evidence for this comes from the study of a patient with a very pure phonological STM deficit, who found it extremely hard to learn to link new foreign words to their meaning, while performing normally when learning to link pairs of words in her native language (Baddeley, Papagno & Vallar, 1988).
A series of studies followed up this hypothesis. One study tested eight-year-old children with specific language impairment (SLI), who had normal general intelligence but the language of six-year-olds. They found it very difficult to repeat back nonwords such as SKITICULT. As they showed no sign of impaired hearing or speech production , their deficit was attributed to impaired phonological STM (Gathercole & Baddeley, 1990). Twin studies have shown that the deficit in nonword repetition in SLI is inheritable, but that other deficits also contribute to the disorder (Pennington & Bishop, 2009). Performance on nonword repetition also correlates highly with the level of vocabulary development in young normal children although as children get older, other factors such as intelligence and exposure to language become increasingly important (Baddeley, Gathercole & Papagno, 1998). More recently, increasing attention has been paid to the role of the phonological loop in controlling behavior through self-instruction (Emerson & Miyake, 2003) a function initially emphasised by the Russian psychologists, Luria (1959) and Vygotsky (1962).
The visuo-spatial sketchpad
The study of visuo-spatial STM has developed enormously in recent years and is very well described in the entry by Luck, who suggests that its principal function is to create and maintain a visuo-spatial representation that persists across the irregular pattern of eye movements that characterise our scanning of the visual world.
Another function of the sketchpad is to create and maintain visual images of the type that we might for example, use in attempting to answer questions such as whether the ears of a collie dog are floppy or pricked, in describing the route from the station to home, or that an architect might use to imagine a building he is designing. It has been shown that spatial tasks can interfere with spatial skills such as driving a car, while a more purely visual activity such as seeing a sequence of pictures or colour patches may interfere with capacity to remember objects or shapes (Logie, 1986, Klauer & Zhao, 2004). Such results, together with the occurrence of brain-damaged patients who show one deficit and not the other (Della Sala & Logie, 2002), suggests that information about space, and about objects and their visual characteristics, may be stored separately (See Luck's entry for further detail). It seems likely that the sketchpad may also be involved in the storage of movement sequences, suggesting a capacity to store kinaesthetic information as well as visuo-spatial (Smyth & Pendleton, 1990, Smyth & Scholey, 1992). The presence of similarities between storage of serial order in visual and verbal memory suggests an analogous process, though not necessarily within a single system (Smyth et al. 2005).
The central executive
This is assumed to be an attentional control system of limited processing capacity that has the role of controlling action. Baddeley (1986) adopted a model proposed originally by Norman and Shallice (1986) which suggested that actions are controlled in two ways. Behavior that is routine and habitual is controlled automatically by a range of schemas, well-learned processes that allow us to respond appropriately to the environment. An experienced driver on a routine trip would be a good example of this, sometimes arriving at the destination with no memory of the journey. When such procedures are no longer adequate, for example finding the normal route blocked by an accident, a second system, the Supervisory Attentional System (SAS) comes into operation. This is capable of using long-term knowledge in order to set up possible solutions, and reflect on them before choosing the best. In the case of our interrupted journey, this might involve the central executive of working memory, probably in connection with LTM, the visuo-spatial sketchpad, and possibly the phonological loop. In its original version, the central executive was regarded as a general system capable of both processing and storage. In the interests of parsimony Baddeley and Logie (1999) proposed that it had a purely attentional capacity. Subsequent research has however, suggested the need to supplement the executive with a separate storage system, the episodic buffer (Baddeley, 2000).
Although the term 'central executive' might suggest a single monolithic controller, it seems more likely that it comprises an integrated alliance of executive control processes, probably including the capacity to focus attention, to divide attention between two or more tasks, and to control access to long-term memory (Baddeley, 2007; Baddeley et al., 1991; Logie et al, 2004), possibly based on one or more types of inhibition (Engle et al, 1999; Miyake et al., 2000).
Executive functioning has been extensively investigated by Shallice (2002), particularly in connection with its disruption following damage to the frontal lobes of the brain, a deficit referred to as the dysexecutive syndrome. This may result in major problems of attentional control, including sometimes repeatedly perseverating on a single action, while at others failing to maintain a goal against distraction. In the case of memory, this may result in confabulation where, in attempting to retrieve a memory, recall is captured by inappropriate associations, sometimes resulting in totally false memories (Baddeley & Wilson, 1986).
The episodic buffer
The initial three-component model of working memory ran into problems in accounting for the way in which the various subsystems could work together and in particular how they could interface with long-term memory. To tackle this problem, Baddeley (2000) proposed a fourth component, the episodic buffer (See Figure 1). This was assumed to be a temporary store of limited capacity that was capable of combining a range of different storage dimensions, allowing it to collate information from perception, from the visuo-spatial and verbal subsystems and LTM. It was assumed to do so by representing them as multidimensional chunks or episodes, which were assumed to be available to conscious awareness. The capacity to bind a range of separate sensory channels into the perception of integrated objects is often regarded as an important function of consciousness (e.g. Baars, 2002). Our investigation of this binding function in recent years (Allen et al., 2006; Baddeley et al., 2009) has caused us to modify the Baddeley (2000) model which predicts that disrupting the central executive will interfere with binding. This is not the case, suggesting that the episodic buffer should be regarded as a passive store, and that the processes of binding do not depend crucially on executive control. In this respect, the current model differs from the proposal by Baars (1997) that consciousness operates like a stage on which actors perform, replacing it with a concept more closely resembling that of a screen on which the results of binding processes operating elsewhere can be projected and utilised by the central executive. For a similar concept see Potter's (1993) idea of conceptual short-term memory.
An alternative view of working memory is provided by Cowan who, postulates an attentional system with a capacity of about four chunks as the central feature of working memory (Cowan, 1988;1995;1999;2001). Outside this central focus, short-term storage is assumed to depend on activated long-term memory. Cowan's model could be seen as one way of specifying the interaction between the central executive and the episodic buffer. Cowan's emphasis on working memory as activated long-term memory might seem to provide a clear contrast with the multicomponent model. The difference is however more apparent than real. Both assume that interaction with LTM plays an important role, with the multicomponent model assuming that such links operate at a number of different levels in ways that the simple concept of "activation" fails to capture (Baddeley, 2009).
Cowan's work has however, raised some important, and as yet unresolved issues, including:
- Modularity: could the apparent separation of visuospatial and verbal working memory be accounted for on the basis of a more general principle of similarity-based interference in activated long-term memory?
- Capacity: Do storage and processing draw on a single limited capacity, as proposed in the initial Baddeley and Hitch model, or are they separate, as in the episodic buffer version?
- Decay or Interference: Is information lost through temporal decay of the memory trace, or is it displaced or over-written by other material?
These are not new questions, but have become issues of greatly renewed activity, largely as a result of Cowan's ideas and his extensive experimental program.
Individual differences in working memory
Daneman and Carpenter (1980) were interested in the role of working memory in comprehension. They developed a task that involved simultaneously processing sentences and remembering the last word of each, which they called working memory span. They found a remarkably high correlation between performance on this task and measures of reading comprehension in their college student participants. This predictive capacity has been replicated many many times, and shown to generalise to a wide variety of tasks that combine temporary storage with processing. Some of these tasks are quite complex, for example remembering words that are sandwiched between arithmetical calculations (Turner & Engle, 1989), but even tasks involving quite simple operations may correlate with measures such as scholastic achievement, provided they involve combining memory and rapid processing (Lépine et al, 2005). There is furthermore a very high correlation between tasks of this type and performance on conventional intelligence tests based on reasoning capacity (Kyllonen & Christal, 1990). This has led a number of groups to search for the crucial capacity that allows these apparently simple tasks to predict such a wide range of cognitive skills. A summary of the current state of play in this area is given in a book edited by Conway et al. (2008).
Probably the most sustained attempt to solve the problem of why working memory span predicts so many cognitive tasks has come from Engle and his co-workers who have shown that the capacity to predict cognitive function is not limited to memory tasks, but can also be found in attentional control paradigms such as that involved in the antisaccade-task (See Kane et al. 2008). This task involves participants moving their eyes from a fixation point to a target as rapidly as possible. Performance can be facilitated by a peripheral warning light occurring at the point where the target will appear, as there is a strong tendency for the eye to move automatically to a new stimulus. In a second condition however, rather than directly indicating the location of the target, the warning light signals participants to move their eyes to the opposite side. This information still proves helpful to high span but not low working memory span participants. Based on this and a range of other studies Engle and colleagues argue that the crucial feature of working memory is the capacity to maintain attention against distraction, whether this is perceptual or comes from other sources such as earlier memories.
However, while Engle makes a strong case for an association between working capacity and the ability to inhibit distracting stimuli, it is not clear that this is the only feature that characterises working memory capacity; it may indeed be just one example of a number of functions of a more general, multi-faceted attentional control system. Furthermore the concept of inhibition itself is open to a range of interpretations at both a psychological and physiological level.
The fact that we still do not fully understand how these complex working memory span tasks work does not, of course, mean that we can not use them profitably. Susan Gathercole and colleagues used the multicomponent working memory model to develop a working memory battery suitable for children of school age, using complex working memory span tasks as a measure of central executive capacity and other tasks to assess phonological and visuo-spatial subsystems. Factor analysis supported the multi-component model and showed that the structure of the working memory system remains remarkably stable as children develop (Gathercole, Pickering, Ambridge & Wearing, 2004). Although its capacity increases with age (Case, Kurland & Goldberg, 1982), there are nevertheless marked developmental changes in the way working memory is utilised. For example the developmental progression whereby the child masters increasingly complex intellectual operations has been linked to the growth of central executive capacity (Halford, 1993). There are also significant developmental changes in the subsystems of working memory, the best known being the expansion in the range of operation of the phonological loop, ranging from the development of the capacity for inner speech and rehearsal strategies in children (Hitch, 2006) to the involvement of a wider range of aspects of executive control in adults (Saeki & Saito, 2004).
Gathercole's test battery is able to identify children who are at risk of encountering academic difficulties, with different patterns of working memory deficit associated with problems in different subject areas. Careful observation of children at school has showed that children with poor working memory skills tend to struggle because of a difficulty in following the sometimes surprisingly complex instructions provided by teachers. They also have trouble in coping with many of the techniques and strategies that are designed to help children cope, since these often require additional working memory capacity. Such children resemble those characterised as suffering from the attention deficit component of the ADHD syndrome. A programme that helps teachers identify such children and optimise teaching methods has been developed (Gathercole & Alloway, 2008).
Neural substrates of working memory
A good deal of research has been carried out on this topic, initially through the study of patients with localised lesions and subsequently using neuroimaging methods. Broadly speaking, the results fit the three-component model, with the phonological loop being represented in the left hemisphere where storage is associated with a region in the temporoparietal junction (Brodmann area 40), and rehearsal with the more anterior Brodmann area (44) that is known to be associated with speech production (Paulesu, Frith & Frackowiak, 1993). The visuo-spatial sketchpad appears to involve a number of predominantly right hemisphere areas, one visual, presumably reflecting the processing and retention of objects and their visual features, a second area is more parietal, presumably involving spatial aspects, while two frontal areas of activation have been associated with control functions (Henson, 2001). There is general acceptance that the frontal lobes play an important role in executive control, although opinions differ as to the extent to which different executive functions may be separately localisable (Duncan & Owen, 2000; Shallice, 2002). There is as yet little evidence as to the localisation of the episodic buffer, which seems likely to reflect a broadly distributed system which may possibly not give rise to activation in any one specific area.
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