Discussion: Short-Term Memory Loss.
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Have you ever entered a room to retrieve an object, only to forget why you went into the room in the first place? Your short-term memory betrayed you. Discussion: Short-Term Memory Loss.
Have you ever been introduced to several people in quick succession, instantly forgetting their names? Again, your short-term memory was to blame.
Short-term memory is limited in duration: Information retained for a period of seconds soon vanishes if not repeated or rehearsed. It is also limited in capacity: as new information enters your working memory buffer, previously stored items are displaced and lost, as can happen during introductions among groups of individuals. Finally short-term memory is susceptible to distraction. Unexpected stimuli, new scenes, or interjections can disrupt your train of thought.
For this Discussion, consider types of short-term memory. Think about strategies to improve short-term memory.
With these thoughts in mind:
Post an example of short-term memory loss that you have experienced. Then explain your short-term memory loss within the context of the working memory model. Finally, explain strategies you might use to improve your short-term memory function.
Be sure to support your postings and responses with specific references to the Learning Resources.
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The theoretical concept of working memory assumes that a limited capacity system, which temporarily maintains and stores information, supports human thought processes by providing an interface between perception, long-term memory and action1–3. There are many approaches to the study of working mem- ory, using a range of empirical and theoretical tech- niques. However, most theories agree on the need for a system of limited attentional capacity, supple- mented by more peripherally-based storage systems4. The account that follows emphasizes this fractiona- tion, stressing the importance of executive control, but concentrating initially on the more tractable peripheral storage systems. Within experimental cog- nitive psychology there are several different but com- plementary approaches to working memory: some emphasize the role of attentional control in memory5, whereas others attempt to explain working memory data in terms of models that were originally devel- oped for long-term memory (LTM)6,7. An example of this approach is provided by Nairne8, whose account seems to criticize the multi-component model of working memory. However, most of Nairne’s criticisms apply only if one attempts to explain all the phenomena of working memory in terms of the phonological loop — one component of a complex system. As this review shows, the phonological loop has received more attention than other components, but this reflects its relative tractability, compared with the visuospatial and executive subsystems. A third,
influential approach relies on correlational techniques that capitalize on individual differences across the various components of working memory (BOX 1). Rather than attempting to give an account of each of these approaches, I describe a single multi-component model of working memory. In general, deviations from the other models represent difference of emphasis and scope, rather than direct conflict. Points of clear disagreement will, however, be identified and discussed.
The term ‘working memory’ seems to have been invented by Miller, Galanter and Pribram9, and was adopted by Baddeley and Hitch10 to emphasize the differences between their three-component model and earlier unitary models of short-term memory (STM). These differences include its multi-component character, its emphasis on combined processing and storage, and the stress on its functional importance as a system that facilitates a range of cognitive activities, such as reasoning, learning and comprehension. This approach resulted in the development of a set of experimental tasks that could be used to analyse different activities and subject populations1. Because the available empirical evidence provided few con- straints, the model was initially loosely specified: for example, it had no mechanism for storing serial order. However, it was simple and robust, and had the potential to develop and become much richer and more clearly specified. The Baddeley and Hitch model continues to flourish, and will be used as a basis for this review. The article will, however, extend beyond
WORKING MEMORY: LOOKING BACK AND LOOKING FORWARD Alan Baddeley
The concept of working memory proposes that a dedicated system maintains and stores information in the short term, and that this system underlies human thought processes. Current views of working memory involve a central executive and two storage systems: the phonological loop and the visuospatial sketchpad. Although this basic model was first proposed 30 years ago, it has continued to develop and to stimulate research and debate. The model and the most recent results are reviewed in this article.
Department of Psychology, University of York, York YO10 5DD, UK. e-mail: firstname.lastname@example.org doi:10.1038/nrn1201
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memory, then patients with STS impairment should show little capacity for long-term learning or for every- day cognitive activities. Such patients were identified18, but had few cognitive problems beyond grossly impaired STM19.
Baddeley and Hitch10 used secondary tasks to deplete the availability of STM in subjects performing tasks, such as reasoning or learning, that were assumed to rely on working memory. They found clear but far from catastrophic impairment, and proposed a three- component model of working memory in place of the unitary system. The three components (FIG. 1) comprised a control system of limited attentional capacity, termed the central executive, which is assisted by two subsidiary storage systems: the phonological loop, which is based on sound and language, and the visuospatial sketchpad.
In this review, I focus on each of these components in turn, emphasizing in particular those aspects that have developed or changed in recent years.
The phonological loop Structure of the loop. The model of the phonological loop comprises a phonological store, which can hold memory traces for a few seconds before they fade, and an articulatory rehearsal process that is analogous to subvocal speech (FIG. 2). Memory traces can be refreshed by being retrieved and re-articulated. Immediate mem- ory has a limited span because articulation takes place in real time — as the number of items rehearsed increases, it reaches a point at which the first item will have faded before it can be rehearsed. Much of the work in this area has used immediate serial recall, typically using a small set of digits, letters or unrelated words, with the characteristics of the material remembered being used to give an indication of the nature of the code on which the recall is based. For unrelated letters, the code is acoustic or phonological; sequences of simi- lar sounding letters such as V, B, G, T, P, C are recalled less well than a dissimilar set, such as W, X, K, R, Y, Q20,21. Similarity of sound is also crucial for unrelated words, whereas meaning is relatively unimportant22. When the model is switched from STM to LTM using several presentations of longer lists, sound becomes irrelevant and meaning crucial23.
Evidence for the role of articulation comes from the word-length effect: immediate memory span declines as word length increases from one to five syllables24. The suggestion that this reflects the slower rehearsal of
the range of the model to speculations as to the proba- ble future development of a more general theory of working memory.
The multi-component model of working memory In 1949, Hebb11 proposed a distinction between STM, which is based on temporary electrical activation, and LTM, which is based on neuronal growth. A decade later, support for this distinction came from studies showing that small amounts of information were rapidly forgotten unless actively rehearsed12,13. A counter attack14 argued that results such as these could be explained in terms of a unitary LTM system, but in the mid-1960s many studies argued for a separation between LTM and STM15. By the late 1960s, many new models had emerged around the concept of STM. The most influential model16 proposed that information from the environment flows through a series of tempo- rary sensory registers into a limited capacity short-term store (STS), which feeds information into and out of LTM. This system was also assumed to act as a working memory, supporting complex cognitive activities, although this aspect was not further explored. Discussion: Short-Term Memory Loss.
This model encountered problems both in terms of its assumptions regarding learning17, and from data concerned with the impact of neuropsychological dam- age to the STS. If the STS served as a unitary working
Box 1 | Individual differences in working memory
In 1980, Daneman and Carpenter164 developed a task in which subjects were required to combine storage and processing, first reading a series of unrelated sentences, and then recalling the final word of each. Working memory span was defined as the maximum number of sentences for which this task could be performed perfectly. They found a high correlation between working memory span and reading comprehension, a result that has been replicated many times165. Similar results occur when sentence processing is replaced by other tasks, such as arithmetic calculation166 or colour–word association167. Prediction is not limited to comprehension but extends from spelling168 to acquisition of logic169, and from note taking170 to following directions171, with correlations typically being in the region of 0.5. Kyllonen and Christal172 compared a cluster of working memory span measures with more conventional measures of intelligence based on reasoning. They found a high correlation, with the main difference being a slightly higher reliance of working memory span on processing speed, and of reasoning on previous knowledge. Discussion: Short-Term Memory Loss.
Although much of the work in this area has simply assessed the correlation between working memory span and performance on cognitive tasks, there is a growing interest in understanding the underlying processes that contribute to the relatively complex working memory span task. Studies using factor analysis and latent variable analysis have supported the broad concept of separate phonological173 and visuospatial storage systems174, together with a main executive system. Attention is now focusing on the processes that underlie executive control173,175,176.
Theories of intelligence have typically been developed as statistical models of data from large batteries of tests. These methods have been used to address controversies such as whether intelligence is best regarded as a single general capacity (G), as proposed by Spearman177, or as a range of separate capacities, as proposed by Thurstone178. A meta- analysis by Carroll179 of more than 400 studies has identified three components, one resembling Spearman’s G, one visuospatial and one verbal-arithmetic. The analogy with the three-component working memory model is clear. Although extremely successful practically, the psychometric study of intelligence seems, in recent years, to have generated more political controversy than theoretical progress180,181. Applying the sophisticated statistical methods that have been developed within psychometrics to the experimentally and neuropsychologically grounded multi-component working memory model might be a fruitful way forward. Discussion: Short-Term Memory Loss.
Figure 1 | The three-component model of working memory. This model, proposed by Baddeley and Hitch10, comprises a control system, the central executive, and two storage systems, the visuospatial sketchpad and the phonological loop.
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interpretations assume that each item forms a cue or stimulus for the following item, with the result that once the initial item is activated, the sequence runs off rela- tively automatically31,32. Such models have difficulty in dealing with sequences in which the same item recurs (7, 1, 9, 3, 1, 5, 8), which leads to a slight impairment in performance33. More problematic for chaining models is the pattern of data that is observed when phonologi- cally similar and dissimilar items are interleaved (C, X, P, W, D, Y). According to chaining models, similarity should cause confusion among similar stimuli, leading to the cueing of the wrong responses. Errors should therefore follow similar letters and involve the dissimilar items. In fact, performance on dissimilar items is unaffected by the interleaving of similar items, which continue to show a higher error rate30,34.
A range of computationally explicit models of ver- bal STM have been proposed6,7,31, with several based on the phonological loop concept35–38. Burgess and Hitch36 assume that order is carried by associating successive items with an ongoing contextual cue, the exact nature of which is unspecified. The phonologi- cal similarity effect occurs because the items that are specified by each cue are encoded phonologically, with similar items having fewer distinguishing cues. The similarity effect is assumed to occur at retrieval rather than at the cueing stage.
The Page and Norris model38 assumes that recall of order is based on positional associations between the first and subsequent items. The associations become progressively weaker as more items are added, providing a simple explanation for limited memory span. Retrieval involves competitive queueing: the strongest association is retrieved first and the associated item is emitted and then inhibited, allowing the next strongest to be retrieved and so on to the end of the list, or to the point at which the associations become too weak and the process breaks down. A related model that also uses serial position as its context is Henson’s start–end model37, in which both the first and last items act as cues.
All of the contextual models give good accounts of the distribution of intrusion errors, whereby the most common error is for two items to transpose (present 1, 2, 3, 4, 5, 6; recall 1, 2, 3, 5, 4, 6). Henson’s model, however, gives a better account of what he calls protrusions, a tendency for an omitted item to be replaced by an item in the same serial position within the previous list (present 1, 2, 3, 4, 5, recall 1, 2, 3, 4, 5; then present 6, 7, 8, 9, 0, recall 6, 7, 3, 9, 0). It seems possible, however, that this reflects a separate, more long-term component, as discussed later.
Despite differences in the nature of the contextual cues, computationally explicit models are possible, and they typically separate the mechanism for storing order from the mechanism by which the items are registered. Bearing this in mind, we should return to the basic phenomena that prompted the model.
The phonological similarity effect. The phonological similarity effect is highly robust39, and is often used as a marker of the phonological loop. It tends to disappear
longer words is supported by the abolition of the word- length effect when subvocal rehearsal is prevented by requiring subjects to repeat an irrelevant sound24,25.
The two-component model of the loop is also supported by neuropsychological evidence (see REF. 19 for a recent review). Patients with phonological STM deficits in the absence of more general language impairment typically have lesions in the left temporoparietal area (BRODMANN AREA (BA) 40). When words are presented visually, these patients show neither a phonological similarity effect nor a word-length effect, a pattern of behaviour that is consistent with their avoiding the use of the defective phonological store, despite having a nor- mal capacity to articulate26. Subvocal rehearsal does not seem to depend on the capacity for peripheral control of articulation, as it can be preserved in ANARTHRIC patients who have lost the capacity for overt articulation27. On the other hand, DYSPRAXIC patients who have lost the capacity to set up internal speech–motor programmes do show the predicted reduction in memory span28.
Although the two-component model of the loop gave a good account of a range of evidence, it was a lim- ited model that left three crucial questions unanswered. How is serial order maintained? How does the loop interact with LTM? And what is the biological function of the phonological loop?
The problem of serial order Explanations of how serial order is remembered, learned and retrieved fall into two broad categories: chaining models and contextual models29,30. Chaining
(BA). Korbinian Brodmann (1868–1918) was an anatomist who divided the cerebral cortex into numbered subdivisions on the basis of cell arrangements, types and staining properties (for example, the dorsolateral prefrontal cortex contains subdivisions, including BA 46, BA 9 and others). Modern derivatives of his maps are commonly used as the reference system for discussion of brain- imaging findings.
Unable to speak because of defective articulation.
Having an impairment of the ability to perform certain voluntary movements, often including speech.
Phonological output buffer
Broca’s area–premotor cortex
Inferior parietal lobe
Visual analysis and STS
Orthographic to phonological
Figure 2 | A functional model of the phonological loop. a | Phonological analysis. b | Short-term storage (STS). c | The programming of speech output. d | Visual encoding. e | Grapheme-to-phoneme conversion. Auditory information gains direct access to a phonological store, after which it passes to an output buffer for recall or recycling through rehearsal. Visual material can be recoded verbally and then gain access to the phonological store through rehearsal. Modified, with permission, from REF. 19 (2002) John Wiley & Sons Ltd.
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items to be ignored60–62. This prompted alternative inter- pretations of the effect7,41,55,63. Jones and colleagues55,63
have found that the effect is not limited to speech or music, but can also be produced by variable tones. The crucial requirement seems to be a fluctuation in the state of the irrelevant stimulus stream. They suggest that this sets up a competing representation that disrupts the storage of serial order.
The proposal of separate item and order mechanisms in the phonological loop allows a range of explanations of the irrelevant sound effect40. A detailed account within the primacy model of the phonological loop is given by Page and Norris64, but this remains an area of lively controversy41,65. Indeed, Jones and colleagues deny the need to assume separate visual and verbal storage systems65, although the evidence they cite has not proved readily replicable, and their model fails to give a good account of data from neuropsychological and neuroimaging studies40.
Function of the phonological loop. We have proposed that the phonological loop evolved to facilitate the acquisition of language66. Evidence for this view came initially from the failure of a patient with a pure phono- logical loop deficit to acquire the vocabulary of a new language, despite otherwise normal verbal LTM66. It was supported by the demonstration that factors that disrupt the phonological loop — such as articulatory suppression, phonological similarity and word length — also disrupt the acquisition of foreign vocabulary, but not of learning to associate pairs of unrelated native language words; such learning is typically based on semantic coding67,68. Phonological loop capacity is a good predictor of the ability of children69 and adults70 to learn a second language.
Acquisition of native vocabulary in children is well predicted by non-word repetition, the capacity to hear and repeat back an unfamiliar pseudo-word, which is assumed to depend on the phonological loop71,72. Children with a specific language disability, coupled with normal non-verbal intelligence, also perform badly on non-word repetition, but have no obvious hearing or articulatory problems73. This and further evidence led to our proposal that the phonological loop evolved to facilitate language acquisition66.
Most studies of verbal STM rely on the retention of sequences of items such as digits and letters that are already familiar. If we are correct, however, this system has evolved to master new words, comprising unfamiliar sequences of phonemes. Studies within the field are therefore beginning to focus strongly on the sublexical level of analysis, and on the more detailed structure of the phonological loop system74,75. There are two main questions: how temporary storage can enhance the learning of new words, and conversely, how language influences STM.
Immediate recall of non-words is better when they are similar in phonotactic structure to the native language of the person remembering76. This indicates that long-term implicit knowledge can be used to aid immedi- ate recall. However, performance on phonotactically
when error rates increase beyond around 50%, indicating that subjects are abandoning the loop and opting for alternative strategies such as sematic or visual coding 40,41. Given the importance of strategy, there have been surprisingly few attempts to control it by instruction, with the notable exception of Hanley and Bakopoulou42; we are likely to see more such studies, preferably backed up by neuroimaging measures.
The word-length effect. Although almost as robust as the phonological similarity effect39, the word-length effect is open to a wider range of interpretations43. One approach has been to reject time-based decay, arguing that long words are more difficult to recall because they contain more components and are therefore more frag- ile44,45. When long and short words are mixed, however, long words are no more difficult to remember than their shorter neighbours, with recall depending on overall list duration, as predicted by the loop hypothesis46. Finally, the abolition of the word-length effect for either auditory or visual presentation, when rehearsal is suppressed24,47, fits more naturally into the phonological loop model than into its competitors.
Less radical modifications of the model have also been proposed. It has been suggested that the word- length effect stems largely43,46, or even entirely48, from delay during output, rather than from rehearsal. Recent experiments controlling output time, however, implicate both, as the loop model would predict47.
The assumption that forgetting in the phonological store reflects trace decay has also been challenged. Evidence for time-based decay came initially from a study comparing recall of disyllabic words that were spoken quickly (bishop, tipple) or slowly (harpoon, Friday)24. As predicted, longer duration words were less well recalled. However, there were failures to replicate this using other sets of simuli28,49. It is possible that, in these studies, spoken duration was not adequately measured, and the two sets were not equated for phono- logical similarity50 (but see REF. 51), but other studies using different sets of study words further questioned the importance of duration relative to other factors such as linguistic complexity52. However, the most method- ologically rigorous study so far, which considered the whole range of materials that have been used previously, concluded that when duration and similarity are measured carefully, all sets of material give results that can be accounted for in terms of spoken duration and phonological similarity53. It seems, therefore, that the simple trace decay assumption, though controversial, is still adequate, obviating the need for more complex interference models.
Irrelevant sound effects. Immediate recall is impaired by the concurrent or subsequent presentation of irrelevant spoken material7,54–57. This was initially interpreted in terms of mnemonic masking within STM57,58. However, irrelevant speech had no greater effect on phonologi- cally similar items than on dissimilar items59, and per- formance was unaffected by phonological similarity between the material to be remembered and the spoken
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which a random 50% are filled (FIG. 3b). The matrix is then removed, and the subject attempts to recall which cells were filled. Testing begins with a 2 × 2 matrix, with the matrix size increasing until performance breaks down. Della Sala et al.85 have shown a double dissocia- tion between visual and spatial span. For normal sub- jects, the Corsi task is disrupted more by spatial than visual interference, whereas the reverse is true for pat- tern span (FIG. 3c). Neuropsychological cases have also
unfamiliar items was much better at predicting subse- quent vocabulary acquisition than performance on phonotactically familiar items. It is possible that both types of material depend on the phonological store, which is responsible for long-term learning, but that the articulatory output process can also benefit from previous language habits. Support for this view came from a study in which immediate memory for words and non-words was measured using either recall or recognition. Sequences of words show a clear advantage over non-words for spoken recall, but there is virtually no difference for serial-order recognition74,77. This indi- cates that language habits influence performance through the articulatory output component of the phonological loop, whereas the storage component is comparatively language independent. This could be advantageous for a system that has evolved to acquire new words, without being excessively influenced by existing knowledge. So, the phonological loop should facilitate language acquisition in two ways: the store should provide relatively unconstrained temporary representation for new phoneme sequences, and the articulatory system should facilitate learning through rehearsal, provided that the new sounds can be repre- sented using existing output processes. This faciliation is likely to occur immediately for phonotactically regular sequences, but might require further exposure when the sequences are irregular and unfamiliar.
The simple phonological loop model has proved to be robust and productive. Future developments seem likely to link it more directly to theories of language perception and production.
The visuospatial sketchpad Like its verbal equivalent, visual working memory is limited in capacity, typically to about three or four objects. This results in the phenomenon of change blindness, whereby objects in scenes can change colour, move or disappear without people noticing78,79. The visual world typically persists over time, and itself pro- vides a continuing memory record, making detailed visual retention largely redundant80. Objects comprise features such as colour, location and shape, with features within a given dimension competing for storage capac- ity, whereas features from different dimensions do not81,82. Wheeler and Triesman83 propose a model whereby feature values are stored in parallel dimension- specific registers or caches, with competition within but not between such registers. Retention of objects is dependent on the binding together of constituent features, a process that demands attention.
The visual–spatial distinction. Neuropsychological studies have indicated the need to distinguish between visual and spatial memory. The Corsi block task measures spatial span. It comprises an array of nine blocks; the experimenter initially taps two of them, and the subject attempts to imitate the sequence, with sequence length increasing until performance breaks down84 (FIG. 3a). The visual, non-spatial counterpart of this task is pattern span, in which the subject is shown matrices of cells of
Figure 3 | Visual and spatial short-term memory tasks. a | Spatial short-term memory is measured using the Corsi block tapping test. The tester taps a sequence of blocks, which the subject attempts to imitate. Sequence length increases until performance breaks down. Testing is facilitated by the numbers on the tester’s side of the blocks. Digit span is typically about two more than Corsi span. b | Spatial span involves presenting the subject with a series of matrices in which half the cells are filled. The pattern is removed and the subject is required to mark the filled cells on a response sheet. The size of the matrices is steadily increased until performance breaks down. c | Effects of interference from visual or spatial stimuli on Corsi and pattern span. Corsi span is disrupted by spatial activity and pattern span by visual activity. Panels b and c modified, with permission, from REF. 85 (1999) Elsevier Science.
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whereas in the second the crucial adjectives are non-spa- tial and rely on rote verbal memory. Typically subjects can remember eight spatial sentences, but only six non- spatial ones91. The requirement to perform a concurrent tracking task, such as steering a car, disrupts the spatial code, but not the verbal condition. Although this task is predominantly spatial in character, other imagery mnemonics seem to depend much more on visual or pattern memory. Subjects who are required to learn to associate pairs of items (elephant/umbrella) can enhance performance by imagining the two interacting, so that when they are given the word elephant, the umbrella springs to mind. This kind of imagery can be disrupted by simultaneously presenting pictures of objects, patches of colour91 or even flickering patterns of visual noise92.
Function of the visuospatial sketchpad. The capacity to hold and manipulate visuospatial representations pro- vides a measure of non-verbal intelligence that predicts success in fields such as architecture and engineering93,94. There are many examples of the importance of visual or spatial imagery in scientific discovery, including Einstein’s development of his general theory of relativ- ity95. Attempts to study the design phase of engineering architecture have also identified the role of visuospatial imagery and mental synthesis93,94. In recent years, attempts to study the process of mental synthesis experi- mentally have increased. A typical study might involve presenting the subject with an array of verbally described shapes, for example, the capital letters J and D, and asking the subject to combine them into an object, with size being flexible. A suitable answer might be an umbrella96,97. The memory load can be increased by adding further shapes, and by requiring the subject to remember and manipulate them mentally as opposed to allowing the shapes to be drawn98,99. A number of studies have applied the working memory framework to such tasks, and have found that subjects will use coding in the phonological loop to store items while manipulating them visuospatially100, although this might have the disadvantage of losing visuospatial information in the process101. By analogy with the role of the phonological loop in language acquisition, it seems plausible to assume that the sketchpad might have a role in acquiring semantic knowledge about the appearance of objects and how to use them, and for understanding complex systems such as machinery, as well as for spatial orienta- tion and geographical knowledge. So far, there seems to have been little work on this potentially important topic.
Fractionation. Logie102 has proposed a fractionation of the sketchpad that is analogous to that of the loop. He distinguishes between a visual storage component, the visual cache, and a more dynamic retrieval and rehearsal process which he terms the inner scribe. He argues that the sketchpad is not a perceptually-based store, but occurs after visual information has been processed in LTM. Support for this view comes from two types of patient, both showing visuospatial neglect after right hemisphere damage86. One type of patient cannot report the contents of their visual left hemifield, but
been identified, showing either disruption of visual but not spatial STM, or the opposite pattern86.
Although the most common distinction is between visual and spatial coding, Smith et al.87 prefer a distinction between the spatial and object codes, linking the two to the separate visual processing paths that are proposed for the encoding of ‘what’ and ‘where’ information88. This idea is supported by neuroimaging studies, which broadly fit the predicted pattern, although other interpretations are possible86. Another characterization is in terms of a dynamic (spatial) and static (pattern) distinction89. To complicate matters further, a kinaes- thetic or motor dimension of coding has also been proposed90. This is an area that is likely to benefit from careful studies that aim to separate out these potential contributions to visuospatial STM, preferably accompa- nied by careful and systematic neuroimaging.
Visual imagery. We can, of course, set up visuospatial representations from verbal instructions, such as when attempting to describe a familiar room or reading a visually descriptive passage from a novel. A number of techniques have been developed to investigate visuo- spatial imagery. One is illustrated in FIG. 4. The subject’s task is to remember and repeat back a sequence of sentences while viewing a matrix in which one cell is denoted the starting square. Under one condition, the sentences can be recoded as a path through the matrix,
In the starting square put a 1.
In the next square to the right put a 2.
In the next square up put a 3.
In the next square to the right put a 4.
In the next square down put a 5.
In the next square down put a 6.
In the next square to the left put a 7.
In the next square down put an 8.
In the starting square put a 1.
In the next square to the quick put a 2.
In the next square to the good put a 3.
In the next square to the quick put a 4.
In the next square to the bad put a 5.
In the next square to the bad put a 6.
In the next square to the slow put a 7.
In the next square to the bad put an 8.
1 2 5
Memory with tracking
Figure 4 | The task devised by L. H. Brooks to study the role of visual imagery in verbal recall. (see REF. 182). a | Subjects attempt to recall the sequence of sentences, typically succeeding on around eight sentences when spatially codable material is used, and six with non- spatial material. b | Requiring the subject to perform the two memory tasks at the same time as they are tracking a moving light stimulus disrupts the use of visuospatial imagery, but has no effect on remembering the sequence of verbally coded sentences.
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perseveration on some occasions, and excessive distractability on others107–109.
The distinction between automatic, habitual control and attentional, supervisory control is an important one, which can also be linked to extensive evidence from social psychology. Bargh and col- leagues110,111 showed that behaviour can be deter- mined by habits and prior attitudes, often without the awareness of the subject. Examples include uncon- scious imitation of body posture in conversation112, influence of action, such as speed of walking by implicitly activated stereotypes111, and environmental determination of strategy choice113.
The concept of an SAS fits neatly into research by Baumeister and colleagues on self-control, which they argue can be measured by a simple questionnaire that predicts a range of behaviours from resistance to petty temptations, through academic performance, to social and emotional coping capacity114. The capacity for self-control is assumed to mediate these effects, based on a system that is limited in capacity and subject to fatigue-like decline115,116. The link between working memory and ‘CONATIVE PSYCHOLOGY’117 presents an important challenge.
Fractionating the executive. It could be argued that the SAS is little more than a homunculus, the little man taking all the important decisions. I would agree, but regard the homunculus as offering a useful scientific strategy, provided we accept that the homunculus defines the problem area, but is not the solution. This requires that we attempt first to specify the processes attributed to the homunculus, and then to explain them118,119.
I began by postulating the capacities that are needed by any attentional controller, namely to focus, to divide and to switch attention, together with the need to connect working memory with LTM119. We made some progress in isolating the capacity to divide attention. We chose two tasks that demand very different peripheral processing, namely auditory digit span and visuomotor tracking, and we titrated the level of difficulty for each subject to a standard level of performance. We then tested simultaneous performance. We found that patients with Alzheimer’s disease were clearly impaired in contrast to normal elderly people, whose capacity for dividing attention was not reliably poorer than for young subjects120,121.
Our three-part model for working memory encoun- tered problems when trying to address the interaction with LTM. These problems stemmed from our simplify- ing assumption that the executive was a purely atten- tional system. This assumption was challenged by a densely amnesic but highly intelligent patient who, despite impaired LTM, showed normal immediate memory for passages of prose comprising some 25 idea units, and extending well beyond the capacity of the loop or sketchpad122. Our model also lacked a system whereby ‘chunking’ could occur, allowing information in LTM to supplement immediate serial recall. Chunking results in an immediate memory span for
show no such occlusion when describing a familiar view from memory, while other patients show the opposite: normal visual attention coupled with neglect of the left hemifield when describing a scene from memory, presumably reflecting impaired scanning of a post- perceptual memory store.
In conclusion, visuospatial working memory is an active but poorly integrated area of research. It would benefit greatly from more interaction among research communities at both a methodological and theoretical level, and from linkage with the extensive literature on visual attention, including work using single-unit recording in non-human primates103,104.
The central executive The central executive is the most important but least understood component of working memory. In the original model, it was simply treated as a pool of general processing capacity, to which all the complex issues that did not seem to be directly or specifically related to the two sub-systems were assigned. The first attempt to advance the concept came with the pro- posal105 to adopt the Norman and Shallice106 model of attentional control. This divided control between two processes. The first relied on the control of behaviour by habit patterns or schemas, implicitly guided by cues provided by the environment. The second com- prised an attentionally limited controller, the supervi- sory activating system (SAS), that could intervene when routine control was insufficient. Evidence for control by schema came from slips of action in which a familiar action pattern, such as driving to the office, might take over from a less routine intention, such as driving on a Saturday morning to the supermarket, resulting in taking the wrong route. Evidence for the SAS came principally from the study of patients with frontal lobe damage, which was assumed to result in impaired SAS function and which led to inappropriate
McDougall proposed the term ‘conative’ to denote the activity of mental striving or the will, as opposed to cognitive and affective or emotional processes.
Fluid systems Crystallized systems
Figure 5 | The multi-component working memory revision. The dark purple areas represent long-term or crystallized knowledge. The episodic buffer provides an interface between the sub-systems of working memory and long-term memory (LTM)125.
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executive and to be accessible to conscious awareness. Its multi-dimensional coding allows different systems to be integrated, and conscious awareness provides a convenient binding and retrieval process (FIG. 5).
The buffer is therefore regarded as a crucial feature of the capacity of working memory to act as a global work- space that is accessed by conscious awareness along the lines suggested by Baars126,127 (neuropsychological sup- port for such a system is reviewed by Dehaene and Naccache128). The buffer was presented as an entirely sep- arate subsystem, but could be regarded as the storage component of the executive. This proposal differs from related proposals4,129 in assuming that long-term infor- mation is downloaded into a separate temporary store, rather than simply activated in LTM. It therefore empha- sizes the capacity of working memory to manipulate and create new representations, rather than simply activating old memories. The concept of a common unified store has the further advantage of making the multi- component model more compatible with approaches to working memory based on individual differences, which have emphasized executive processes rather than subsystems such as the loop and sketchpad (BOX 1).
Anatomical localization of working memory. Studies based on lesion location in patients and neuroimaging in normal subjects indicate that the three basic compo- nents of working memory are localized in different brain regions (FIG. 6). The case is most clear-cut for the phonological loop, for which lesion studies have indi- cated the involvement of the left temporoparietal region19,130,131. Subsequent neuroimaging studies have reinforced this conclusion, identifying BA 40 as the locus of the storage component of the loop, and Broca’s area (BA 6/44) as involved in the rehearsal compo- nent132–135. Vallar and Papagno19 review the neuropsy- chological evidence for the anatomical localization of the phonological loop, while Smith and Jonides136
review the neuroimaging evidence. A direct comparison of phonological and visuospatial working memory137
identified visuospatial working memory as primarily localized in the right hemisphere, in agreement with earlier lesion studies85,138,139. Other areas that were broadly analogous to the left hemisphere activation of verbal working memory, namely right inferior parietal cortex (BA 40), right premotor cortex (BA 6) and right inferior frontal cortex (BA 47), were also involved, although there was also activation in the anterior extra- striate occipital cortex (BA 19), which Kosslyn et al.140
have suggested is associated with visual imagery137,141. Finally, neuroimaging studies are beginning to throw light on the nature of rehearsal processes in spatial working memory, a topic that has proved difficult to tackle using purely behavioural methods142.
A number of studies have produced evidence for a separation between spatial and visual or object coding, analogous to the ‘what’ versus ‘where’ distinction in visual processing88,143,144. In general, neuroimaging studies tend to support the claim that a dorsal stream processes and stores object information, with spatial coding depending on the inferior parietal cortex87,145.
sentences of about 15 words, compared to five or six unrelated words. Finally, our model had no mechanism for allowing the phonological and visuospatial subsys- tems to interact123, and offered no mechanism for the role of working memory in conscious awareness, which is assumed to depend crucially on working memory124.
To account for these and other issues, a fourth component was proposed — the episodic buffer125. This is assumed to be a limited capacity store that binds together information to form integrated episodes. It is assumed to be attentionally controlled by the
Figure 6 | A tentative mapping of the working memory model components onto the brain. The functions associated with each component are not meant to represent the only functions that are served by the associated brain regions, nor are these labels supposed to represent all brain regions associated with a particular function. The central executive (CE), for example, is likely to engage multiple brain regions in a functionally coherent network, including dorsolateral prefrontal cortex. AR, articulatory rehearsal; IS, inner scribe (spatial rehearsal); PS, phonological store; VC, visual cache (storage). Modified, with permission, from REF. 29 (2001) Psychology Press. Anatomical image adapted, with permission, from REF. 183 (1996) Appleton & Lange.
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relate it to the extensive work that exists on executive control — much of it being concerned with analysis of frontal lobe function. The proposed link between work- ing memory and conscious awareness also represents a lively and exciting interface.
There is, however, still a paucity of research on what drives the system. The conative, emotional and motiva- tional control of working memory is crucial, but largely ignored. The multi-component working memory model has a concept of information, but no equivalent to Freud’s concept of mental energy, nor indeed to the concept of arousal. Without this the system is clearly incomplete. Damasio157 makes this point cogently, prin- cipally using neuropsychological evidence both from adynamic patients who are essentially cognitively inert rather than intellectually impaired, and from patients with frontal lobe damage, who can show apparently normal cognitive capacity coupled with a total inability to make sensible life decisions. Damasio suggests, as do the philosopher Hume158 and the social psychologist Lewin159, that it is useful to conceptualize action as ultimately steered by emotion. This process operates indirectly through learning, which associates positive or negative valence to objects, actions and goals.
Our behaviour is clearly determined not by simple chains of cause and effect, but rather by a range of controlling factors that operate simultaneously at many different levels, often implicit, but sometimes explicit. The multi-level nature of the control of action was observed in the nineteenth century by Hughlings- Jackson160, and emphasized in the twentieth century by Craik161 and Broadbent162. An excellent recent review of the evidence for the multi-level control of action is pro- vided by Frith, Blakemore and Wolpert163. Many levels of control are implicit and independent of working memory. It is, however, only within this broader picture that the role of working memory as an attentionally limited but crucial system for thought, planning and action can be fully understood.
In conclusion, working memory continues to provide a highly productive general theory: human thought processes are underpinned by an integrated system for temporarily storing and manipulating information. The multi-component model that is described here offers one account of its underlying processes.
Lesion studies and functional neuroimaging have provided extensive evidence for an association between executive functioning and the frontal lobes107,136,146. A commonly used measure of executive function is the N-back task, in which subjects are presented with a continuous stream of items and are instructed to press a key when they detect a repetition at a specified delay. For a delay of N = 0, a sequence such as 7, 9, 3, 1, 1 should evoke a response. The overall load can be increased, typically going up to N = 3, for example 7, 9, 3, 1, 9. This difficult task evokes bilateral dorsolateral prefrontal (BA 9/46), inferior frontal (BA 6/44) and parietal (BA 7/44) activation that increases with load147,148. There is also evidence for bilateral activation in the dorsal frontal regions when subjects are required to produce a random sequence of digits or key presses149,150, a task that is known to be executively demanding151.
There have been extensive attempts to associate different executive processes with specific anatomical locations. This is proving to be a challenging task, with different attempts to summarize existing data producing different conclusions29,107,152. For example, D’Esposito et al.153 identified a dorsolateral prefrontal area that was assumed to be involved when two tasks had to be performed simultaneously. However, Klingberg154 found no such area when subjects were combining two other tasks, whereas other studies have found that dorsolateral activation was reduced by the requirement to perform a concurrent task155,156. It seems likely that these results reflect changes in strategy in response to the additional demand. The problem of adequately controlling strategy in imaging studies is important but, I suspect, frequently underestimated. Purely behavioural studies typically use a series of experiments to rule out the possible strategic interpretations of the data, an approach that might be seen as impractically expensive in neuroimaging. It is to be hoped that this crucial variable will increasingly be studied and controlled.
What else is needed for a theory of working memory? Clearly, there is need for a closer link between research on the phonological loop and work on language percep- tion and production, and between work on the sketch- pad, on visual processing and on motor control. In the case of the central executive, there is a clear need to
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