This article provides a concise summary of the development of the concept of working memory as a multicomponent system, a conceptual approach driven by the seminal and continuing work of the author. The text is clear and refers to key experimental findings associated with specific aspects of the multicomponent model. One notable feature of the article is the balance of text given to describing the phonological loop component relative to the visual, spatial, and executive components. The cross referencing to the Luck article on visual short-term memory is useful, but it would have been helpful to set rather more of the experimental and theoretical developments on visual and spatial working memory in the context of the current article. Here, I have in mind the work of Mary Smyth and colleagues on working memory for movement sequences, the work by Logie and Della Sala showing deficits of visuo-spatial working memory in representational neglect patients who have no verbal or visual-perceptual difficulties, Logie & Baddeley's work showing that visual codes can support serial ordered recall, and the studies by Andrade and colleagues showing that visual imagery is affected by irrelevant visual input, but visual short-term memory is not. This could be seen as compatible with the views expressed in Luck's article, but here the reliance is on evidence collected in the context of the multiple component model of working memory rather than within the general framework of visual attention, and the topic offers a clear bridge across the traditional divide between attention and short-term memory.
The concept of the central executive has often been difficult to characterize as something other than an homunculus, and was described by Baddeley himself as a 'conceptual ragbag' at one stage. In addition to the work by Shallice and others on executive deficits in patients with frontal lobe damage, there have been several other serious attempts to tackle the executive theoretically. Notable among these is Barnard's Interacting Cognitive Subsystems model which views executive control as arising out of multiple mutual constraints between a range of specialized subsystems. This could be seen as more complex than, but perhaps analogous to self-organizing systems such as insect colonies in which executive control is an emergent property of the different roles of different members of the colony. Another approach to understanding executive functions is through the study of individual differences, such as in the work of Miyake and colleagues who have identified task switching, updating, and inhibition as separable executive functions rather than reflecting the operation of a single executive. So too, the work by Della Sala, Logie and Baddeley showing that people with Alzheimer's disease have a specific problem in performing two tasks concurrently has led to the suggestion that dual task co-ordination might be a further executive function in the healthy brain.
This review by Alan Baddeley, the preeminent scholar in the field of working memory, is a reasonable and knowledgeable summary of the advances in understanding that can be attributed largely to his own fruitful research career. It summarizes well his own theoretical view of the working memory system and the evidence behind it from a range of investigators.
The review’s approach is a little more historical than it needs to be; the first figure, which describes the view of Baddeley for many years, should be eliminated and replaced with the current second figure, which is a more up-to-date version that includes the new episodic buffer component. Otherwise, readers may mistakenly take the first figure to be an illustration of what the author still believes. Nor is that first figure an accurate summary of what was asserted in the seminal chapter by Baddeley and Hitch (1974). That chapter made mention of a central storage capacity (in addition to code-specific stores) but that central storage capacity was eliminated for the sake of parsimony in more recent work, such as Baddeley (1986), before something like it was re-introduced as the episodic buffer (Baddeley, 2000).
I can find three important issues in the field that were, however, not covered sufficiently in the review, given other work and recent progress: the modularity of stores, the nature of storage and processing capacity, and the decay of memory. I will explain each of these briefly.
Baddeley and colleagues offered good evidence that there is a difference between visual and verbal materials in working memory. Visual processing interferes with memory for visual items more than it interferes with memory for verbal items and, conversely, verbal processing interferes with memory for verbal items more than it interferes with memory for visual items. This may, however, not be sufficient evidence for a modular system in which there are two passive stores, a visual one and a phonological one. There are many forms of memory that do not seem well-suited for either store, such as memory for touch, memory for the spatial location of tones, and memory for concepts. The episodic buffer was established to handle some of these issues (especially concepts and associations) but not all of them. An alternative solution was developed by Cowan (1988, 1995, 1999). The passive memory is based on the activated portions of long-term memory. Within it, stimuli with similar features interfere with each other. Some of the activated items are in the focus of attention, which can bind together features that have not been bound before and help to store them in long-term memory. There seems to be some physiological evidence for that kind of approach (Chein et al., 2003; Ruchkin et al., 2003).
2. Storage and Processing Capacity
Baddeley’s approach developed as it did, without a central processing capacity limit, because he was impressed by how little interference there was between storage and processing. It may be, though, that there is much more interference between storage and processing when certain mnemonic strategies are not possible (most importantly, verbal rehearsal) and passively held memory features cannot be used. For example, memory for visual arrays suffers strong interference from a tone identification task (Stevanovski & Jolicoeur, 2007). Barrouillet and colleagues (2007) have found that memory span decreases linearly as a function of the proportion of time between items occupied by an attention-demanding task, regardless of the details of that task. Chen and Cowan (in press) found that the performance of a simple but speeded nonverbal spatial task gets markedly worse as a memory load builds up across serial positions of a verbal list to be recalled. These studies do suggest that storage and processing share a general resource.
The resource shared between storage and processing could be the focus of attention (Cowan, 1988, 1995, 1999). It is important to understand its storage limit when processing is minimal. Cowan (2001) summarized evidence that the limit is generally 3 to 5 meaningful units or chunks in normal adults, and less in children or the elderly. Cowan et al. (2005) suggested that it is indeed the focus of attention that is so limited. Provided that sensory-phonological memory and rehearsal are eliminated, there appears to be a fixed capacity for abstract information of 3 to 5 units that is shared between verbal-and visual materials (Cowan & Morey, 2007; Saults & Cowan, 2007). Are these the qualities of Baddeley’s episodic buffer? They may well be.
There is currently a controversy about whether the capacity of working memory is limited to a fixed number of items (Awh et al., 2007; Cowan & Rouder, 2009; Rouder et al., 2008; Zhang & Luck, 2008) or whether attention can be spread thinly and fluidly among all presented items (Bays & Husain, 2008, 2009). This controversy may be too recent to be considered resolved one way or the other.
In a slightly different theory of capacity limits, the focus of attention can handle only one item at a time but there is still a capacity-limited region of about 4 items surrounding that focus (Oberauer, 2002). An alternative account for Oberauer’s data is that the capacity-limited region is in the focus of attention but that some items in that focus are more active or privileged than other items, a theory that seems consistent with what Bays and Husain would expect.
3. Decay of Memory
Working memory’s key distinction is that it is temporary. Memory can be lost because of a capacity limit, as in #2 above, or because the memory that is not rehearsed or refreshed quickly fades away or decays. Lewandowsky et al. (2008) have claimed that the evidence shows no loss of information over a number of seconds as time passes; only effects of interference. Yet, the theory of Barrouillet et al. (2007) is predicated on the existence of decay. Perhaps the solution to this problem is that there is a loss of memory over time, but not a gradual loss as one might expect. The information may persist full force in memory for a number of seconds and then drop to zero or die all of a sudden (Winkler et al., 2001; Zhang & Simon, 2009).
In short, three of the most important current issues were under-discussed in the review by Baddeley. If the field is to live, breath, and change like a hard science, these key issues must be highlighted so that future investigators will be inspired to solve the remaining big questions.
Baddeley, A. D. (1986). Working Memory. Oxford: Oxford University Press.
Baddeley, A. D. (2000). The episodic buffer: A new component of working memory? Trends in Cognitive Sciences, 4(11), 417-423.
Baddeley, A. D., & Hitch, G. J. (1974). Working memory. In G. A. Bower (Ed.), The psychology of learning and motivation: Advances in research and theory. (Vol. 8, pp. 47-89). New York: Academic Press.
Barrouillet, P., Bernardin, S., Portrat, S., Vergauwe, E., & Camos, V. (2007). Time and cognitive load in working memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 33, 570-585.
Bays, P.M., & Husain, M. (2008). Dynamic shifts of limited working memory resources in human vision. Science, 321, 851-854.
Bays, P.M., & Husain, M. (2009). Response to comment on “Dynamic shifts of limited working memory resources in human vision. Science, 323, 877d .
Chen, Z., & Cowan, N. (in press). How verbal memory loads consume attention. Memory & Cognition.
Cowan, N. (1988). Evolving conceptions of memory storage, selective attention, and their mutual constraints within the human information processing system. Psychological Bulletin, 104, 163-191.
Cowan, N. (1995). Attention and memory: An integrated framework. Oxford Psychology Series, No. 26. New York: Oxford University Press. (Paperback edition: 1997)
Cowan, N. (1999). An embedded-processes model of working memory. In A. Miyake & P. Shah (eds.), Models of Working Memory: Mechanisms of active maintenance and executive control. Cambridge, U.K.: Cambridge University Press. (pp. 62-101)
Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, 87-185.
Cowan, N., Elliott, E.M., Saults, J.S., Morey, C.C., Mattox, S., Hismjatullina, A., & Conway, A.R.A. (2005). On the capacity of attention: Its estimation and its role in working memory and cognitive aptitudes. Cognitive Psychology, 51, 42-100.
Cowan, N., & Morey, C.C. (2007). How can dual-task working memory retention limits be investigated? Psychological Science, 18, 686-688.
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Lewandowsky, S., Oberauer, K., & Brown, G.D.A. (2008). No temporal decay in verbal short-term memory. Trends in Cognitive Sciences, 13, 120-126.
Oberauer, K. (2002). Access to information in working memory: exploring the focus of attention. Journal of Experimental Psychology: Learning, Memory, and Cognition, 28, 411-421.
Rouder, J.N., Morey, R.D., Cowan, N., Zwilling, C.E., Morey, C.C., & Pratte, M.S. (2008). An assessment of fixed-capacity models of visual working memory. Proceedings of the National Academy of Sciences (PNAS), 105, 5975-5979.
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END OF REVIEWER C
Overall, this is an excellent exposition of working memory, a concept that the author and his colleagues introduced in 1974 and have developed in numerous studies since then. I have only a few comments and suggestions, listed below.
1. Function of the phonological loop: Although the language-learning hypothesis is reasonable re the evolution of the phonological loop, its function at present goes well beyond word learning: we use the loop to remember phone numbers as we dial them, to review the last few words we heard to reanalyze them, to learn a song, to take dictation, to "hear" what we read, and so on. The wider uses are mentioned in the individual differences section.
2. The episodic buffer: it would be helpful to give more information about what this buffer does and what evidence there is for it, including why it was concluded that it is a passive store, not an active processor.
3. The figure with a separation between fluid and crystallised systems has no explanation of this distinction.
Finally, I suggest that the author consider mentioning "conceptual short term memory" (CSTM), as described in the following references and in a forthcoming Scholarpedia entry. The idea is related in some respects to the episodic buffer, but is seen as an active but short-lasting processing system.
Potter, M. C. (1993). Very short-term conceptual memory. Memory & Cognition, 21, 156-161. Potter, M. C. (1999). Understanding sentences and scenes: The role of Conceptual Short Term Memory. In V. Coltheart (Ed.), Fleeting memories: Cognition of brief visual stimuli (pp.13-46). Cambridge, MA: MIT Press.
Response to reviewers
[from Dr. Baddeley -ed]
I enclose a revision of our paper on working memory, in which we have tried to take account of the points made by the various referees. The extent to which we were able to do so is however somewhat limited by space, given that for the most part, the suggestions were for including other aspects of the literature, which they themselves felt should be reported more extensively.
The suggestion here is to include more on visuo-spatial working memory, relying less on cross referencing the article by Luck. We have done so, although perhaps not in detail that this referee would prefer. However, is should now be possible for the reader to access this area more readily.
A second point concerns the characterisation of the central executive. Here, a whole range of possible approaches are suggested. Barnard’s Interacting Cognitive Subsystems model is certainly one possibility. However, it is highly complex, and possibly for this reason has not been widely influential. Others have come up with suggestions as to what is the important characteristics of executive control; we have now cited a number of these, without going into great detail since there is no generally accepted leading interpretation in this area.
- We have now mentioned the importance of the loop in controlling action.
- The episodic buffer. We have now explained why we feel it is a passive rather than active system.
- The caption now explains the difference between fluid and crystallised systems.
- We have now made reference to “conceptual short-term memory”, which we understand will feature elsewhere in Scholarpedia.
This referee, Nelson Cowan, makes a number of constructive suggestions as to his own model and what he sees as being the crucial current controversies, suggesting that our own approach is too historically based. We have responded to these suggestions and now have an extended discussion of Cowan’s approach. However we have taken as our remit the task of presenting what is currently reasonably widely accepted about working memory, while at the same time pointing to other possible interpretations. Our experience over the years is that what seems exciting at a given time changes rapidly, and that our task was to present what was likely to continue to be broadly accepted beyond the next couple of years. Given that the basic model has survived for over 30 years now as a broad general framework, we have opted to concentrate on that, rather than paying too much attention to the very many exciting developments that are taking place within that framework. We have however changed the balance somewhat and removed Figure 1 which presented the early model which has now been substantially elaborated and is reflected in Figure 2.
Finally we would like to thank our referees for their constructive suggestions.