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HE MULTI-COMPONENT MODEL OF WORKING MEMORY:
XPLORATIONS IN EXPERIMENTAL COGNITIVE PSYCHOLOGY
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. REPOVŠa,b* AND A. BADDELEYc

Department of Psychology, Washington University, Campus Box 1125,ne Brookings Drive, St. Louis, MO 63130, USA

Department of Psychology, University of Ljubljana, Ljubljana, Slovenia

Department of Psychology, University of York, York, UK

bstract—There are a number of ways one can hope to de-cribe and explain cognitive abilities, each of them contrib-ting a unique and valuable perspective. Cognitive psychol-gy tries to develop and test functional accounts of cognitiveystems that explain the capacities and properties of cogni-ive abilities as revealed by empirical data gathered by aange of behavioral experimental paradigms. Much of theesearch in the cognitive psychology of working memory haseen strongly influenced by the multi-component model oforking memory [Baddeley AD, Hitch GJ (1974) Workingemory. In: Recent advances in learning and motivation, Vol.(Bower GA, ed), pp 47–90. New York: Academic Press;

addeley AD (1986) Working memory. Oxford, UK: Clarendonress; Baddeley A. Working memory: Thought and action.xford: Oxford University Press, in press]. By expanding theotion of a passive short-term memory to an active systemhat provides the basis for complex cognitive abilities, theodel has opened up numerous questions and new lines of

esearch. In this paper we present the current revision of theulti-component model that encompasses a central execu-

ive, two unimodal storage systems: a phonological loop andvisuospatial sketchpad, and a further component, a multi-odal store capable of integrating information into unitary

pisodic representations, termed episodic buffer. We reviewecent empirical data within experimental cognitive psychol-gy that has shaped the development of the multicomponentodel and the understanding of the capacities and propertiesf working memory. Research based largely on dual-taskxperimental designs and on neuropsychological evidenceas yielded valuable information about the fractionation oforking memory into independent stores and processes, theature of representations in individual stores, the mecha-isms of their maintenance and manipulation, the way theomponents of working memory relate to each other, and theole they play in other cognitive abilities. With many ques-ions still open and new issues emerging, we believe that theulticomponent model will continue to stimulate researchhile providing a comprehensive functional description oforking memory. © 2006 Published by Elsevier Ltd on behalff IBRO.

ey words: dual task, central executive, phonological loop,isuospatial sketchpad, episodic buffer.

Correspondence to: G. Repovš, Department of Psychology, Wash-ngton University, Campus Box 1125, One Brookings Drive, St. Louis,

O 63130 USA. Tel: �1-314-935-8459; fax: �1-314-935-8790.-mail address: [email protected] (G. Repovš).bbreviations: AD, Alzheimer’s disease; DVN, dynamic visual noise;

wMRI, functional magnetic resonance imaging; PET, positron emissionomography; SAS, supervisory activating system.

306-4522/06$30.00�0.00 © 2006 Published by Elsevier Ltd on behalf of IBRO.oi:10.1016/j.neuroscience.2005.12.061

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any disciplines within cognitive neuroscience have indi-idually and in synergy substantially contributed to theathering of empirical results and the development of the-retical models that constitute our understanding of work-

ng memory. The aim of this review is to present insightshat have been enabled by experimental behavioral stud-es within cognitive psychology. Specifically, we will focusn recent empirical findings that relate to the multi-compo-ent model of working memory, a functional model oforking memory developed by Baddeley and Hitch (1974;addeley, 1986, 2000) that has introduced the conceptnd inspired decades of research into the capacities, prop-rties and mechanisms of working memory.

The subject of working memory, like any other withinognitive neuroscience, can be and is approached fromany different levels of description (see Repovš and Bres-

anac, 2006). Each level of description is a valid one andontributes importantly to a complete understanding of thehenomenon under investigation. While neuroscience pro-ides a glimpse into the structural underpinnings of theognitive system and computational cognitive neuro-cience addresses the question of how the informationrocessing is actually carried out, the role of cognitivesychology is to provide a detailed description of the prop-rties and the capacities of the system, to map out a modelf its functional components and the way they relate toach other.

A promise of novel insights and important advance-ents in understanding working memory provides a strong

ncentive to combine the findings of different disciplinesnd levels of description. In doing so, however, one has toe wary of a seductive mistake, namely to equate or con-ate the functional and structural levels of description or tossume a one-to-one mapping between them. Many cog-itive functions and processes are carried out by a networkf brain regions, and individual regions can take part in thexecution of a number of functional components of theystem. The exact relation between them is often far fromtraightforward. To be able to avoid mistakes one has to beble to distinguish the levels clearly and be conscious of,nd specific about, the mappings proposed. To facilitatehat, we will purposefully limit this review to a functionalccount of working memory as explored and revealedhorough behavioral experimental studies. Our goal is toescribe the behavioral properties and capacities of work-

ng memory and to outline a model that hopes to explainhem. We are not of course pretending that this is thehole story. Indeed, many features of the presented model
ere inspired or tested by neuropsychological findings.

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G. Repovš and A. Baddeley / Neuroscience 139 (2006) 5–216

hile we refrain from citing and discussing findings fromther disciplines in the main body of the paper, we whole-eartedly support the ultimate goal of relating and combin-

ng levels of description in a comprehensive multidisci-linary model of working memory. Some possible comple-entary lines of research and findings will be presented in

he last part of the paper, while more complete accounts ofuch integration are given in other papers presented in thispecial issue.

The outline of the paper follows the structure of theaddeley and Hitch (1974; Baddeley, 1986, 2000) multi-omponent model of working memory. First, we willresent a short introduction and a sketch of the model. Weill follow this by focusing on each individual proposedomponent of working memory, reviewing recent empiricalndings that have expanded our empirical knowledge andnfluenced the further development of the theoretical

odel. For a more detailed analysis of the development ofhe model and related empirical findings, please refer toaddeley and Hitch (1974) and Baddeley (1986, 2000, inress). Readers interested in alternative models of workingemory are encouraged to consult Miyake and Shah

1999) as well as other papers in this special issue.

he multi-component model of working memory

here are a number of ways in which the temporary stor-ge of information can be realized within a cognitive sys-em. One can for instance envision a distributed systemith a set of independent processors that communicateith each other. Baddeley and Hitch (1974) instead argued

or the concept of a common system that is “limited in

ig. 1. The current multi-component model of working memory representing “flhange by learning and their proposed relations to “crystallized” cognitive syst

apacity and operates across a range of tasks involvingifferent processing codes and different input modalities”Baddeley, 1986, p. 35). To support their claim, they de-ised and carried out a set of experiments designed to testsingle, but important hypothesis. Namely, if a common

ystem of limited capacity is employed in a range ofognitive tasks, then absorbing a substantial amount of

ts capacity by a concurrent supplementary task shouldave deleterious effects on performing those tasks,ven when they do not have an obvious short-termemory component.

In a range of experiments, the simple concurrent taskf holding and speaking out loud a sequence of six digitsroved to have important effects on learning, comprehen-ion and reasoning (for a thorough overview see Baddeleynd Hitch, 1974, and Baddeley, 1986). The results pro-ided both a persuasive argument in support of the generaloncept of a working memory system as well as informa-ion that led to the formulation of a specific multi-compo-ent model of working memory (Baddeley and Hitch,974). The reported results and the method developedave led to a wealth of empirical research that has enabledurther testing and development of the model.

The initial model presented by Baddeley and Hitch1974) proposed the existence of three functional compo-ents of working memory (Fig. 1). A central executive wasnvisioned as a control system of limited attentional ca-acity that is responsible for the manipulation of informa-ion within working memory and for controlling two subsid-ary storage systems: a phonological loop and a visuospa-ial sketchpad. The phonological loop was assumed to be

uid” capacities (such as attention and temporary storage) that do notems capable of accumulating long-term knowledge.

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G. Repovš and A. Baddeley / Neuroscience 139 (2006) 5–21 7

esponsible for the storage and maintenance of informa-ion in a phonological form, while the visuospatial sketch-ad was dedicated to the storage and maintenance ofisual and spatial information. Based on a number of em-irical findings a fourth component, the episodic buffer,as added recently (Baddeley, 2000). The episodic buffer

s assumed to be a limited capacity store that is capable ofulti-dimensional coding, and that allows the binding of

nformation to create integrated episodes. The paper willddress each of the proposed components in turn, firstresenting the theoretical model and then reviewing theelevant empirical findings.

he phonological loop

he phonological loop comprises two components, a pho-ological store, which holds memory traces in acoustic orhonological form that fade in a few seconds, and anrticulatory rehearsal process analogous to subvocalpeech (Baddeley, 1983). The function of the articulatoryehearsal process is to retrieve and re-articulate the con-ents held in this phonological store and in this way toefresh the memory trace. Further, while speech inputnters the phonological store automatically, informationrom other modalities enters the phonological store onlyhrough recoding into phonological form, a process per-ormed by articulatory rehearsal. As the articulation oper-tes in real time, the capacity of the phonological store is

imited by the number of items that can be articulated in theime available before their memory trace has faded away.

number of important empirical findings support the as-umptions of the model.

Limited span. The most basic finding related to theerbal short-term store is the limited amount of informationt can hold. Measured by a simple task of immediate serialecall, it can hold from about five to eight items (Brener,940). However, as many experiments show, the numberf items held, depends on their characteristics. Thesendings further reveal the structure of the verbal short-termtore.

The phonological similarity effect. Research predat-ng the model (e.g. Conrad, 1964; Conrad and Hull, 1964)ad already shown that sequences of dissimilar sounding

etters such as W, X, K, R, Y and Q are easier to rememberhan sequences of similar sounding letters, such as V, B,, T, P and C. This finding has been replicated many timesnd research has shown that while similarity of soundffects the number of words recalled, similarity of meaningas little effect (Baddeley, 1966a). As the degree of pho-ological similarity within the sequence crucially deter-ines the number of items recalled, the items are mostrobably stored in a phonological code. In contrast, the

ong-term learning of such material is influenced by simi-arity of meaning, but not of sound (Baddeley, 1966b).

The irrelevant sound effect. Exposure to irrelevantpeech either concurrent or subsequent to presentation ofist items significantly reduces serial recall of verbal mate-
ial. First reported by Colle and Welsh (1976) with visually t
resented items, the effect was demonstrated both withisually presented stimuli (e.g. Ellermeier and Zimmer,997; Jones, 1994; Jones and Macken, 1995; Jones et al.,992; Salamé and Baddeley, 1982; Surprenant et al.,000) as well as with auditorily presented items (e.g. Han-

ey and Broadbent, 1987; Neath et al., 1998). Furtheresearch has shown that the effect is not limited to speechnd music, but appears with other forms of fluctuation inhe state of the irrelevant stimulus stream such as variableones (Jones, 1993; Jones et al., 1996). Furthermore theffect of irrelevant speech is equal for phonologically sim-

lar and dissimilar remembered items (Salamé and Badde-ey, 1986) and is also unaffected by phonological similarityetween the irrelevant speech and the material to-be-emembered (Jones and Macken, 1995; Larsen et al.,000; LeCompte and Shaibe, 1997), which speaks againstn account in terms of the phonological masking of theemory trace.

A number of theoretical accounts were proposed forhe explanation of the irrelevant speech effect, based onemporal distinctiveness theory (TDT, LeCompte, 1996),he feature model (Nairne, 1990; Neath, 2000), and thebject-oriented episodic record (O-OER) model (Jones,993). However, additional research has shown that the

rrelevant sound effect is a) additive to the phonologicalimilarity effect (Hanley and Bakopoulou, 2003), b) absenthen the to-be-remembered items are not encoded into

he phonological store (Norris et al., 2004), c) present evenf irrelevant sounds are presented only during a post-pre-entation retention interval, and at that d) even when sub-ocal rehearsal is prevented (Hanley and Bakopoulou,003, Norris et al., 2004). These findings can not be readilyxplained by the alternative models. The fact that the

rrelevant sound effect seems to be brought about by in-erfering with the representation while it is being held withinhe phonological store is, however, consistent with thehonological loop account. The exact mechanism of irrel-vant speech effect is still uncertain, but the evidenceuggests an effect based on the representation of serialrder within the phonological store (Norris et al., 2004). Aandidate account is provided by the Page and Norris2003) primacy model which suggests that the irrelevantound effect comes about through a competition for re-ources between the representation of list order in theo-be-remembered list and the competing cue order withinhe irrelevant sounds list. This interpretation has someimilarity with that proposed by Jones (1993), but is con-ained within a computationally explicit model of the pho-ological loop.

The word length effect. Immediate memory for wordequences declines as the spoken length of words in-reases (Baddeley et al., 1975). This robust finding was

nitially interpreted as reflecting the decay of a memoryrace over time, with long words taking longer to rehearseence allowing more decay than short. Alternative inter-retations have been proposed in terms of word complex-

ty (Caplan et al., 1992; Service, 1998, 2000). In an attempt
o rule out this interpretation, a number of studies have

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ompared the retention of disyllabic words comprising ei-her rapidly spoken short vowel sounds (e.g. bishop,icket) or long (e.g. harpoon, Friday). An effect of durationas found by Baddeley et al. (1975), but not by otherssing different items (Lovatt et al., 2000, 2002; Caplan andaters, 1994). A recent study by Mueller et al. (2003) took

pecial care to assess the extent of phonological similaritymong words and their articulatory duration, concludinghat “phonological complexity per se may have no reliableffects on memory spans over and above those attribut-ble to mean articulatory durations and phonological dis-imilarity” (p. 1363). We suspect, however, that the con-roversy will continue.

Articulatory suppression. When participants are in-tructed to repeatedly articulate an unrelated word, theunction of the articulatory rehearsal process is disabled,eading to a number of consequences that provide addi-ional tests of the model. First, in the presence of suchrticulatory suppression, the word length effect is abol-

shed (Baddeley et al., 1984), which further supports theodel’s assumption that subvocal articulation in real time

erves to refresh the decaying memory traces within thehonological store. The ability to remember items, thoughignificantly impaired, is however not nonexistent, whichuggests that there are other possible ways of storingerbal information, one candidate being the episodicuffer.

Second, articulatory suppression during visual presen-ation of items to be remembered disables the transfer ofnformation to phonological store as evidenced by the re-

oval of the effects in that condition of either phonologicalimilarity (e.g. Baddeley et al., 1984; Longoni et al., 1993;urray, 1967, 1968) or irrelevant sound (Salamé and Bad-eley, 1982; Hanley, 1997). The presence of the effects of

rrelevant speech (Hanley and Broadbent, 1987; Hanleynd Bakopoulou, 2003) and of a phonological similarityffect (Murray, 1968) despite articulatory suppression inhe case of auditory presentation of list items on the otherand implies that speech indeed has automatic and privi-

eged access to the phonological store, bypassing therticulatory rehearsal process.

Conclusion. The phonological loop was the first andhe most studied component of the multicomponent modelf working memory. The initial assumptions of the modeleem to have withstood the vigorous onslaught of empiri-al testing and proved the model to be robust and wellapable of explaining phenomena related to verbal work-ng memory. We can expect further research to provideovel challenges to the model and help map the detailedechanisms employed in the maintenance of phonolog-

cal information in serial order.

he visuospatial sketchpad

hile the phonological loop is specialized to hold verbalnformation, the visuospatial sketchpad is assumed to beapable of maintaining and manipulating visual and spatialnformation, a process that is crucial for performing a range
f cognitive tasks. While initially, most working memory a
esearch focused on verbal material and therefore thehonological loop, recently a number of studies have pro-ided a wealth of interesting results relating to the func-ional structure and properties of visuospatial workingemory. In the following sections we will address recent

ndings related to the fractionation of the visuospatialketchpad and then focus on the form of representationnd mechanisms of maintenance in its visual and spatialubcomponents respectively. We will conclude the sectionith an integration of the current ideas and findings related

o the visuospatial sketchpad.

Fractionation of visuospatial working memory. Fromfunctional point of view, a range of experiments has

rovided evidence for both domain and process divisionsithin visuospatial working memory. Encouraged by aumber of experimental findings (Baddeley, 1996; Logie,986; Logie et al., 1990) and neuropsychological findingsBaddeley et al., 1991b; De Renzi and Nichelli, 1975;hallice and Warrington, 1970), Della Sala et al. (1999)ave shown that a spatial interference task significantlyisrupts performance on the Corsi block tapping test ofpatial working memory, while it has no effect on the visualatterns test, a test of visual working memory, while aisual interference task has the opposite effect. The resultsherefore provided clear evidence for the existence of sep-rate visual and spatial subcomponents of non-verbalorking memory.

To account for possible alternative explanations of theesults obtained by Della Sala et al. (1999) as well as othertudies investigating the visual–spatial division of workingemory (e.g. Hartley et al., 2001), Klauer and Zhao (2004)esigned and performed a number of experiments thaturther explored the distinction between the two sub-ystems and provided a persuasive set of double dissoci-tions between the two proposed subsystems. The resultsot only supported the existence of separate visual andpatial stores, but also provided evidence for separateehearsal mechanisms for visual and spatial information,ndependent of the central executive. The latter resultsgree with the study performed by Bruyer and Scailquin1998), which has shown that only manipulation and notaintenance shares resources with the central executive.

Additionally, research into the role of working memoryn visual search has shown that visual and spatial working

emory tasks differentially interact with visual search (Ohnd Kim, 2004; Woodman et al., 2001; Woodman anduck, 2004). In a dual-task condition, a concurrent visualorking memory task did not affect the efficiency of visualearch as demonstrated by lack of change in the searchate, nor was its accuracy affected by visual search (Wood-an et al., 2001; Oh and Kim, 2004). Spatial workingemory tasks on the other hand reduce the efficiency of

isual search as shown by an increase in the slope of theunction relating reaction time to the number of items in theearch array, while the accuracy of the working memoryask was reduced as well (Oh and Kim, 2004; Woodman
nd Luck, 2004).

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While the research described has provided strong ev-dence for separate visual and spatial storage and main-enance components of working memory, Mohr and Linden2005) suggest that passive and active processes in visualorking memory should be distinguished as well. Passiverocesses are recruited by tasks that require recall of

nformation in the same format as it was memorized, whilective processes are recruited by tasks that require the

nformation to be modified, transformed, integrated or oth-rwise manipulated. In a series of experiments, these au-hors have shown a lack of interference between concur-ent color and spatial manipulation tasks in comparison tosingle task condition, whereas interference was present

n dual task conditions within the same domain. Addition-lly and in accordance with previous results (Bruyer andcailquin, 1998), the manipulation tasks in both domains

nterfered with a concurrent random generation task that isssumed to rely heavily on the central executive, while

nterference was absent in the case of maintenance tasks.he results provide evidence of independent resources foranipulation within the visual and spatial components oforking memory with both sharing resources with the cen-

ral executive.Taken together these studies show that visuospatial

orking memory is not a unitary system, but can be furtherivided into spatial and visual subsystems each with its

ndependent storage, maintenance and manipulation pro-esses. Of these, maintenance seems to be independentf executive processes while manipulation depends on

hem.A formal fractionation of the visuospatial sketchpad

nalogous to the phonological loop was proposed by Logie1995). Logie (1995) proposes a distinction between aassive visual storage component, termed the visualache, and a dynamic spatial retrieval and rehearsal pro-ess, termed the inner scribe. While the proposal providesgood account of the neuropsychological data (e.g. Dellaala and Logie, 2002), the model does not allow for sep-rate maintenance mechanisms for the contents of visualnd spatial stores as suggested by the reviewed empiricalata.

Representation and maintenance of information in vi-ual working memory. A number of varied studies bearn the question of representation within visuospatial work-

ng memory. Having established the existence of separatetores for visual and spatial information, we can assumehat the representations used differ as well. In a series ofxperiments aimed at measuring the capacity of visualorking memory Luck and Vogel (1997) and Vogel et al.

2001) established that observers are able to retain infor-ation about three to four different features within a singleimension (e.g. color, orientation) but that these can beurther combined with an additional three to four featuresrom another dimension when integrated into objects. Inhis fashion, observers were able to retain 16 individualeatures when distributed across four objects, each de-ned by a conjunction of four features. Based on the
nding that visual working memory is constrained by the t
umber of objects and not by the number of distinguish-ble features that make up those object, the authors con-luded that information in visual working memory is re-ained in the form of integrated objects.

Relating to the feature integration theory (Treisman,993), Wheeler and Treisman (2002) pointed out that theesults of Luck and Vogel (1997) and Vogel et al. (2001)ould also be explained by assuming parallel feature-spe-ific memory stores of independent capacity. If capacity is

imited only within a specific feature store, then the numberf distinct features retained can be doubled, tripled or evenuadrupled when features differ over independent dimen-ions, without the need for the information to be bound into

ntegrated objects. To explore the alternative explanationheeler and Treisman (2002) specifically tested whether

nly the features are retained, or the specific conjunctionsre retained as well. Across a number of change detectionxperiments, the authors showed that the features areaintained independently of their binding, which the par-

icipants often failed to retain. While the retention of spe-ific features was rather robust, the retention of bindingas vulnerable and seemed to depend on the participants’

imited attentional resources. It is worth noting at this pointhat Wheeler and Treisman (2002) tested performancesing an array of items, which required the subjects tocan before responding, whereas Luck and Vogel (1997)ypically probe with a single item. Subsequent work byllen et al. (in press) suggests that the binding together of

eatures may require little in the way of additional attention,ut that such bindings may be more readily disrupted byhe processing of subsequent items.

Further insight relating to the form of representationsetained in visual working memory was provided by Al-arez and Cavanagh (2004), who tested the capacity oforking memory for objects of varying complexity. Their

esults revealed a strong linear relation between searchate and the number of objects retained and showed thatthe upper storage limit of four or five items is attainablenly by the very simplest objects; as the visual information

oad per item increases, the storage limit drops to substan-ially lower levels” (p. 110). The results are congruent withhe proposal by Wheeler and Treisman (2002) that theumber of objects retained depends on the maximumumber of distinct features that can be retained within apecific dimension. For simple objects that consist only ofingle features within separate dimension (e.g. color, ori-ntation) the number can be equal to the number of distincteatures that can be retained within those dimensions,nabling four to five objects to be retained. Objects thathemselves combine conjunctions of features within theame dimensions, quickly exhaust the available number ofetained distinct features within a specific dimension, sig-ificantly limiting the total number of retained objects.

Another study by Barnes et al. (2001), made use of theingle object advantage—a finding that two attributes cane more effectively discriminated when being part of aingle object as compared with being a part of two differentbjects (Duncan, 1984; Baylis and Driver, 1992). In a dual
ask paradigm, Barnes et al. (2001) showed that only an

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bject working memory task significantly reduced the sin-le object advantage while verbal, and spatial workingemory tasks had no effect. Based on these results theuthors concluded that the same attentional mechanismsre engaged in maintaining objects in working memory andelecting perceptual objects in a visual scene.

Based on the empirical results gathered so far we canssume that the retention of integrated objects is accom-lished by a binding mechanism depending on limitedttentional resources. The exact form of representationnd mechanisms of maintenance of individual features inisual working memory, on the other hand, is still unclear.urther insight might come from lines of research relatingisual working memory to perception and visual imagery.t the same time more attention will have to be paid to theistinction between information held in visuospatial work-

ng memory and the episodic buffer, as some studies (e.g.addeley and Andrade, 2000; Zimmer et al., 2003) alreadyuggest that episodic buffer might be involved in the rep-esentation and storage of integrated visual information.

Visual working memory, perception and experience.ore than other components of working memory, visualorking memory seems to be closely related to perception.he visual world itself, with its relatively stable and persis-

ent character provides a form of external continuing mem-ry record, which makes detailed visual retention some-hat redundant (O’Regan, 1992). Relying on a relatively

imited capacity, visual working memory is geared towardffectively representing the most relevant features of theisual world. When perception of a scene is briefly inter-upted, viewers often fail to detect quite major modifica-ions in the scene, the phenomenon of change blindness.ased on this research, Rensink et al. (1997) suggested

hat the knowledge of the structure of visual scenes accu-ulated in long-term memory is used to detect the regionsf central interest, which then guide attention and theransfer of information into visual working memory. Theetection of any changes in the visual scene is thereforeost likely to be limited to those regions of interest.

The top-down influences on the transfer of informationo visual working memory were recently investigated inelation to the representation of objects. Wagar and Dixon2005) explored how the relevance of specific features forategory judgment, affects their encoding and mainte-ance in working memory. Their findings showed that in ahange detection paradigm, a change in features that weremportant for categorization (diagnostic features) was sig-ificantly more likely to be detected than a change inon-diagnostic features. The results suggest that previousxperience importantly affects the encoding of information

nto working memory.However, the transfer of information to working mem-

ry is not only mediated by top-down perceptual experi-nce, but also by bottom-up features of visual informationuch as visual cues (Schmidt et al., 2002), perceptualrouping (Woodman et al., 2003) and object-based featureinding (Xu, 2002). Schmidt et al. (2002) showed that their
ubjects were more accurate in a visual working memory s
ask when the item to be later probed was preceded by aisual cue, even when the cue was not predictive of the

ocation that was probed. The authors concluded that theue automatically influences the transfer of visual informa-ion into working memory. Expanding on the findings bychmidt et al. (2002), Woodman et al. (2003) exploredhether bottom-up perceptual grouping cues, such as ge-talt principles of proximity and connectedness, may biashe entry of items into visual working memory. In a changeetection task the subjects were indeed more likely toetect a change in objects perceptually grouped with theued object, than a change in ungrouped objects, leadingo the conclusion that the perceptual organization of visualnput influences its transfer into visual working memory.inally, Xu (2002) showed in a change detection task, that

he features of objects are best retained when they belongo the same part of an object, less well when they belongo a different part of an object and worst when they formpatially separated objects.

A direct link between visual working memory and per-eptual input is provided by the irrelevant picture effect. Aumber of studies (e.g. Della Sala et al., 1999; Logie andarchetti, 1991; Quinn and McConnell, 1996) have shown

hat the presentation of irrelevant pictures disrupts theaintenance of information in visual working memory (butot spatial working memory). Further studies by McConnellnd Quinn (2000) showed that there has to be a dynamicspect to the interfering display. In a subsequent studycConnell and Quinn (2004) showed that aspects of visual

omplexity such as the number of dots, their density andhe overall size of the visual noise field determine theegree of interference by visual noise fields with visualorking memory. Based on their results they concluded

hat the passive visual store is directly accessible by ex-ernally presented interference, bypassing higher-levelnowledge-based analysis.

There have however been some problems in reliablyeplicating the interference effect based on the dynamicisual noise (DVN) paradigm developed by Quinn andcConnell (1996). Andrade et al. (2002) tested the inter-

erence effect of DVN on the recall of static matrix patterns,ecognition of arrays of matrix patterns and recognition ofhinese characters, as well as on the peg-word mnemonic

ask used by Quinn and McConnell (1996; McConnell anduinn, 2004). While the peg-word mnemonic task showedrobust effect of interference, none of the other tasks did.imilar findings were reported by Zimmer and Speiser

2002). Andrade et al. (2002) explained their results byifferentiating between visual imagery, which in their opin-

on is employed in the peg-word mnemonic task, and visualhort-term memory, which is probed by the other taskssed in their study. They conclude that the two types ofask either load different components of working memory,r else that DVN disrupts only the processing included inisual imagery without disrupting the storage of the under-

ying representation. As the peg-word mnemonic task in-olves the creation of new visual images based on long-erm memory, it is also possible, that it involves the epi-
odic buffer rather than the visual subcomponent of

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isuospatial sketchpad. With conscious awareness provid-ng the mechanism for retrieval from the episodic bufferBaddeley, 2000), presentation of salient changes in theisual input presents a good candidate for interference withhe contents of the buffer.

Representation and maintenance in spatial workingemory. Building on an analogy with the separation ofassive store and active rehearsal mechanism within thehonological loop, and a number of studies showing thatoluntary eye movements disrupt spatial working memory,addeley (1986) initially proposed that covert eye move-ents, visiting the locations to be remembered, might

erve as an active rehearsal mechanism. Noting that focusf attention moves with the eyes, Baddeley (1986; c.f.,ostle et al., 2006) also suggested that the system thatontrols visual attention, and not the control of eye move-ents itself, might be crucial for the rehearsal of spatial

nformation.In addition to voluntary eye movements, studies have

hown that other forms of concurrent body movement suchs sequential tapping of keys (e.g. Logie and Marchetti,991; Pearson et al., 1999; Smyth et al., 1988; Smyth andendleton, 1989) and arm movements across an unseenatrix (Quinn, 1994; Quinn and Ralston, 1986) also pro-uce interference with spatial working memory. Interfer-nce was additionally shown not to be dependent on actualerformance of movement, but is present even when thearticipants are asked only to imagine making an armovement (Johnson, 1982). It seems that the planning ofovements and not the actual execution is the source of

nterference with spatial working memory. A possible ex-lanation of the findings was proposed by Logie (1995)ho assumed that the inner scribe component of visuo-patial sketchpad, otherwise responsible for active re-earsal of information held within passive visuospatialache, is also involved with extraction of information usedor planning and execution of voluntary movements, caus-ng the interference when both tasks have to be performed.

As already hinted by Baddeley (1986), both voluntaryye and arm movements are accompanied by shifts ofpatial attention. Could it be that it is not the movementtself, but rather a more general process of shifting spatialttention, common both to the eye and arm movement,hat is causing the interference observed? A study bymyth and Scholey (1994) showed that attention is indeed

nvolved in producing the interference, while a follow-uptudy by Smyth (1996) controlling for eye movements fur-her ascertained that the shifts in spatial attention can byhemselves produce interference with the spatial workingemory span.

Building on results by Smyth (1996), Awh et al. (1998)howed that visual processing is facilitated at the locationsaintained in spatial working memory compared with lo-

ations not maintained in spatial working memory. Addi-ionally, in line with the results by Smyth (1996), they havehown that if subjects are forced to direct attention awayrom locations held in working memory their ability to re-
ember those locations is impaired. Based on their em- s
irical data, Awh and Jonides (2001) concluded that aunctional overlap exists between the mechanisms of spa-ial working memory and spatial selective attention. In theirpinion it is the mechanisms of spatial attention, such asocal shifts of attention to memorized locations that provide

rehearsal-like function of maintaining information activen spatial working memory.

A close relation between spatial working memory andttention was also revealed by studies of visual search.hile the effectiveness of visual search as measured by

earch rate was shown to be unaffected by a concurrenterbal (Logan, 1978) or visual material load on workingemory (Woodman et al., 2001; Oh and Kim, 2004), a

patial working memory task significantly slowed visualearch and reduced its accuracy (Woodman and Luck,004; Oh and Kim, 2004). These results point to a commonesource, employed by both visual search and mainte-ance of information in spatial working memory. The au-hors point to a number of possible candidates for a com-on resource. Both tasks could rely on the allocation of

patial attention, they could share a common system forepresenting spatial information, or spatial working mem-ry could be actively involved in keeping track of alreadyisited locations in visual search.

While attention has been shown to affect spatial work-ng memory (Smyth, 1996; Awh et al., 1998), that does noty itself prove that it is only the shifting of attention thatontributes to the interference of voluntary eye and limbovements with spatial working memory, an issue thatas more directly addressed in recent studies. A study byawrence et al. (2001) examined the effect of saccadic eyeovement, limb movement, and saccade inhibition onemory span for spatial locations. As all three tasks pro-uced comparable interference with spatial span, the au-hors concluded that all spatial movements irrespective ofhe type produce similar effects on spatial working mem-ry, congruent with a hypothesis of a common mechanismf interference, such as the shifting of spatial attentionroposed by Awh and Jonides (2001). Later studies byearson and Sahraie (2003) and Lawrence et al. (2004)uggested rather different conclusions, both showing thatye movements resulted in an interference effect that wasignificantly stronger than shifts of attention alone. Pear-on and Sahraie (2003) additionally showed that thereater interference in eye movement tasks was mostlyue to mistakes in spatial and not temporal coding. Theuthors of both studies concluded that the interferencessociated with eye movements cannot be explained only

n terms of shifts of attention. Lawrence et al. (2004) furtheruggest that changes in the retinal coordinates of theo-be-remembered locations, and the cognitive suppres-ion of spatial processing during the execution of eyeovement might be the causes of the additional interfer-nce effect observed, while Pearson and Sahraie (2003)ropose that oculomotor control processes play a crucialole in short-term rehearsal of location-specific represen-ations in working memory.

Further exploring the link between eye movements and
patial working memory, Theeuwes et al. (2005) con-

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ucted an experiment in which they observed eye move-ents directed either toward or away from the to-be-re-embered location. Based on the strong link between

isuospatial working memory, spatial attention and eyeovements, and on previous findings that eye movementsay deviate away from visible stimuli that need to be

gnored (e.g. Doyle and Walker, 2001; Sheliga et al.,994), the authors suggested that a remembered locationight induce a similar deviation in eye movements away

rom the to-be-remembered location. These results sup-orted the hypothesis of a strong link between visuospatialorking memory and eye movements and gave support to

he assumption that locations in spatial working memoryight be represented also at the oculomotor level.

Conclusion. The empirical data reviewed have pro-ided a wealth of information on the capacities and prop-rties of the visuospatial working memory. The studiesave not only confirmed the original assumption of theulticomponent model (Baddeley and Hitch, 1974) that

isuospatial memory forms a distinct component of theorking memory, but have also provided evidence for

urther fractionation. The empirical evidence suggests thatisuospatial working memory can be further divided intoisual and spatial subcomponents, each with separate andndependent passive storage, representations, mecha-isms of maintenance, and manipulation. Both subcompo-ents have been shown to be closely related to forms ofisual attention.

Representation in the visual subsystem seems to beased on the relatively robust retention of a small numberf distinct basic features (e.g. color, shape, orientation)

hat are independently stored in a set of parallel feature-pecific stores. The retained individual features can thene bound together into integrated object representationsnd maintained through more vulnerable attentional mech-nisms. The encoding of information in visual workingemory has been shown to be significantly affected byoth bottom up perceptual features, and by top down

nfluences based on previous experience such as categoryearning.

While visual working memory is closely related to per-eption and visual imagery, spatial working memory showsloser connection to attention and action. The exact naturef the relation is not yet established, but the empiricalndings suggest that spatial working memory shares im-ortant resources with spatial attention and oculomotorontrol.

The recent empirical findings seem to have overtakenhe theoretical model. The extension of the original Bad-eley and Hitch (1974) model proposed by Logie (1995)eparating the visuospatial sketchpad in a manner that isnalogous to the phonological loop, into a passive visualache and a dynamic spatial inner scribe is not able toccount for all the data. The theoretical model clearlyeeds to be further elaborated. Two possible and promis-

ng directions of development seem to be related on oneand to the feature integration model of visual attention
Treisman, 1993) and to a model of visual imagery (Koss- o
yn, 1994) on the other. An important future task for theulticomponent model is to distinguish clearly between

epresentations held in the visuospatial sketchpad andhose held in the episodic buffer. This is especially relevantn the case of visual imagery, which integrates visual in-ormation from various sources, including long-term mem-ry, a task for which the episodic buffer would seem to beost suitable.

he central executive

he central executive has been the most important buteast understood and least empirically studied componentf the multi-component working memory model (Baddeley,986, 1996). Initially, it was conceived in rather vague

erms as a limited capacity pool of general processingesources. As such, it functioned as a homunculus anderved as a convenient ragbag for unanswered questionselated to the control of working memory and its two slaveubsystems. While such a homunculus cannot provide ancceptable explanation of the phenomena, it can serve aseful function by identifying the functions and propertieshat still need to be explained. They can then be system-tically investigated until there is nothing left to explain andhe homunculus effectively vanishes (Baddeley, 2001).

To sketch a functional model of the central executive,wo general questions need to be answered. First, what ishe role of the central executive in the functioning of work-ng memory? Specifically, when and how does the centralxecutive interact with the slave subsystems? Second,hat other cognitive functions and abilities are dependentn the central executive? In recent years a number oftudies have helped advance our understanding of theentral executive, mapping out its role in the control of thelave subsystems, in the manipulation of information withinorking memory and in attentional control. In this sectione will first present efforts to define the processes andapacities of the central executive. We will then move on toiscuss the rather scarce findings relating to the role ofentral executive within the performance of working mem-ry. Next, we will touch upon recent findings that explorehe involvement of working memory in attentional pro-esses employed in distractor interference tasks and inisual search. We will finish the section with some con-luding thoughts.

Fractionating the functions of the central executive.he first attempt at replacing the homunculus (Baddeley,986) came with the adoption of the Norman and Shallice1986) model of attentional control, which assumes twoasic control processes. Most human action consists ofoutine tasks that can be controlled by schemata andabits employing environmental cues. Different cues fre-uently contradict each other, but most conflicts can beasily resolved using fairly automatic conflict-resolutionrocesses. In the Norman and Shallice (1986) model theseituations are resolved using a process they termed con-ention scheduling. Novel situations and problems how-ver, cannot be resolved using automatic processes based
n previous experience. In these cases a novel solution

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eeds to be planned and followed through, based on thective combination of existing stimuli and informationtored in long-term memory. In the Norman and Shallice1986), model, this is assumed to depend on a limitedapacity attentional component they termed the supervi-ory activating system (SAS).

Adopting the SAS as a model of the central executiveid not dispel the homunculus, but it did provide a frame-ork for specifying the processes and capacities neededy such an attentional controller. Four basic capacitiesere postulated and explored (Baddeley, 1996): the ability

o focus, to divide and to switch attention, and the ability toelate the content of working memory to long-term mem-ry. The capacity to focus attention was explored using aandom digit generation task, which was argued to place aeavy load on the central executive (Baddeley et al.,998). In a study by Robbins et al. (1996) it has beenhown that while chess playing is not disrupted by articu-atory suppression, it is significantly disrupted by a concur-ent visuospatial task and even more so by a random digiteneration task. The random digit task has also been showno significantly disrupt category generation tasks (Baddeley,966c), mental arithmetic (Logie et al., 1994) and syllogisticeasoning (Gilhooly et al., 1993) making a strong case for themplication of the central executive in a range of complexognitive tasks requiring focused attention.

The capacity to divide attention was explored throughhe work with patients with Alzheimer’s disease (AD). Suchatients typically suffer from both a pronounced episodic

ong-term memory deficit and attentional deficits (Perrynd Hodges, 1999), which led Baddeley et al. (1991a) touggest that they might suffer from a central executivempairment. In a study exploring the capacity for dual taskerformance, the patients were required to combine tasksmploying the phonological loop and the visuospatialketchpad. In both cases the difficulty of the tasks wasitrated so that the level of performance on a single tasklone matched that of both elderly and young control par-icipants. While the manipulation of level of difficulty of theingle task performed alone did not differentially affect ADatients, their dual task performance was dramatically im-aired, while it was not affected by age (Logie et al., 2000).hese results support the assumption that the capacity toivide attention presents a separable executive capacity.

The relation of the capacity to switch attention to ex-cutive processes was extensively tested by Allport et al.1994). The authors argued that if the capacity to switchttention is an important component of executive control,hen switching cost should interact with the executive de-and of the tasks that were being switched. The observedattern of results did not confirm the hypothesis, showing aelatively constant cost of switching across conditions.

Another test of the capacity of attention switching as anxecutive process was carried out by Baddeley et al.2001). The authors studied the performance of subjectsn a switching task under dual-task conditions employingarious articulatory suppression and central executiveasks. The results showed a consistent though small role of
he central executive in attentional switching. Perhaps (
ore importantly, the study revealed a significant effect ofrticulatory suppression on attention switching in someonditions. The results seem to reveal an important con-ribution of the phonological loop to the control of a specificerbally-based action, possibly through maintenance of anction switching program. The same pattern of results wasecently obtained by Saeki and Saito (2004). These au-hors also reported a significant increase in switch costshen task switching was accompanied by articulatory sup-ression in the absence of external task cues, while con-urrent tapping had no effect.

Taken together the results imply that task switchingight be better considered as a result of a number ofifferent processes rather than a single executive process.ost authors agree with such a multi-component notion of

ask switching (e.g. De Jong, 2000; Goschke, 2000; Ru-instein et al., 2001; Saeki and Saito, 2004). Two capac-

ties are assumed: the maintenance of a task switch pro-ram, and the capacity to execute or activate the appro-riate task. Of these, the central executive seems more

ikely to be involved in the latter, while the phonologicaloop may be a useful means of storing and operating aask-specific internal program in the absence of externalues.

The fourth component capacity of the central executives proposed by Baddeley (1996) is the ability to relate theontent of working memory to long-term memory. The

nterface between the working memory subsystems andong-term memory has been subsequently transferred to aew component of working memory, the episodic buffer,hich will be addressed separately.

The role of the central executive in working memoryasks and processes. In exploring multicomponent work-ng memory, most of the early studies focused on theroperties of the slave subsystems, probing the forms ofepresentation and mechanisms of maintenance they uti-ize. At least implicitly, most studies assumed that someorm of executive control was involved in the performancef the tasks employed, but few addressed the specificontribution of the central executive directly. Studying theffect of concurrent visual span for letters or patterns on aisual matrix or verbally based imagery task, Logie et al.1990) found strong differential interference. A visuospatialmagery task was much more severely disrupted by con-urrent memory span for visually presented patterns thany span for visually presented letter sequences while theonverse was true for a verbal task. In addition however,oth the visual and verbal memory span tasks interferedith both the visuospatial and the verbal concurrent tasks,lthough to a smaller degree. The authors concluded thathe dual task design revealed the existence of both spe-ialist resources that are differentially employed in differentersions of the tasks, and a general-purpose resourcemployed in all the tasks used. The study therefore impli-ates the central executive in the dual task performance ofhe tasks studied.

A question that remained open in the Logie et al.
1990) study was whether the central executive is involved

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n coordinating the dual task performance, or do the imag-ry tasks themselves draw on general processing re-ources in addition to specialist resources. To tackle thisuestion Salway and Logie (1995) used a random lettereneration task in addition to articulatory suppression andpatial suppression tasks, designed to disrupt the centralxecutive, the phonological loop and the visuospatialketchpad, respectively. Results showed that random gen-ration produced notably greater interference than eitherpatial or articulatory suppression for both visual and ver-al versions of the imagery task. Further analysis addition-lly revealed a tendency for stronger interference with theisual than the verbal task. The authors concluded that inomparison to simple temporary storage of visual or spatialnformation, the generation of mental images from heardnstructions requires general purpose resources ascribedo the central executive.

The contribution of the central executive to image ma-ipulation was further explored in a dual task study byruyer and Scailquin (1998). In this study the authorsxplored the impact of random letter generation on imageeneration and maintenance. The results revealed thathile there was no effect on passive maintenance, both

mage manipulation tasks showed strong interference fromhe concurrent random letter generation task. A more re-ent study by Mohr and Linden (2005) explored the effectf random word generation on manipulation of colors andngles in working memory. The results confirmed that theentral executive task significantly interferes with botholor and angle manipulation while it does not affect theirimple maintenance. An additional important finding washat while both color and angle manipulation tasks seem toraw on processing resources related to the central exec-tive, the subjects were able to perform them in parallelithout noticeable decline in performance.

Working memory and visual selective attention. Aseported in the section on spatial visual attention, a numberf recent studies have explored the relation of workingemory to visual selective attention. A point worth noting

s that the majority of studies did not specifically addresshe role of the central executive. There are a number ofays in which working memory might be involved in visualelective attention tasks. Most authors agree that the stim-li presented in the task need to be stored in workingemory to be able to perform the task (Bundesen, 1990).he representations in working memory are assumed toerve as templates that enable the selective activation ofargets and inhibition of distractors (Duncan and Hum-hreys, 1989; Desimone, 1996) based on active mainte-ance of stimulus priorities (Lavie, 2005).

As already mentioned, initial dual task explorations ofisual search showed that concurrent maintenance of ver-al (Logan, 1978) or visual material (Woodman et al.,001; Oh and Kim, 2004) does not impair the efficiency ofisual search. Does that mean that working memory is noteeded in tasks employing selective visual attention?hree lines of research provide evidence for a resounding
no” in reply to this question. First, studies employing a m
oncurrent spatial working memory task show that main-enance of spatial information significantly interferes withisual search (Woodman and Luck, 2004; Oh and Kim,004). It seems that spatial working memory and visualearch are intimately related. The exact nature of relationowever still needs to be determined.

Second, a study by Han and Kim (2004) shows thatanipulation within working memory significantly inter-

eres with concurrent visual search. In their study partici-ants were asked to either count backwards from a givenumber or to sort a given string of letters while performingvisual search task. The dual-task condition in both cases

educed the speed of the visual search task as reflected byignificantly steeper search slopes compared with theearch-alone condition. No effect of interference was ob-erved when visual search was combined with a simpleorking memory maintenance task. The authors con-luded that visual search might require working memoryesources related to the executive functions.

Third, in a series of studies, Lavie (2005; De Fockert etl., 2001; Lavie et al., 2004; Lavie and De Fockert, 2005)ave shown that even a simple working memory mainte-ance task significantly interferes with visual attentionasks when potent distractors that strongly compete withargets are present. The authors have found higher inter-erence from distractors under high vs. low working mem-ry load (memory for digit order) in a Stroop-like taskequiring subjects to classify famous written names as poptars or politicians while ignoring distractor faces (De Fo-kert et al., 2001). The same pattern of results was ob-erved in a flanker response-competition task combinedith digit set maintenance (Lavie et al., 2004). Similarly,

he authors have shown greater interference from a salientut task irrelevant color singleton in a visual search task incondition of high vs. low working load (Lavie and De

ockert, in press). Based on this cumulative evidence theuthors argue that working memory is crucial for maintain-

ng task-processing priorities between relevant and irrele-ant stimuli. This enables goal-directed control of selectivettention and the rejection of irrelevant distractors. Suchctive control, however, only seems to be needed when aonflict between targets and a salient competing distractoreeds to be resolved.

Conclusion. While the central executive may havenitially seemed to be simply a convenient homunculus,ecent empirical work clearly demonstrates that a numberf potentially separable executive functions and capacitiesan be distinguished. These in turn are importantly in-olved both in the functioning of the storage components oforking memory, and in a number of more general cogni-

ive processes. In the realm of working memory tasks,xecutive processes seem to be involved whenever infor-ation within the stores needs to be manipulated. Simple

epresentation and maintenance on the other hand may bendependent of the central executive, unless it requires theomplex binding and integration of information. In complexognitive abilities, the central executive seems to be
ostly involved as a source of attentional control, enabling

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he focusing of attention, the division of attention betweenoncurrent tasks and as one component of attentionalwitching. In many of these functions central executive isupported by other components of working memory. Thehonological loop seems to provide one form of convenienttorage of execution programs, while the visuospatialketchpad seems to be involved in guiding visual andpatial attention. Assessing the contribution of the centralxecutive to the performance of complex tasks has onlyecently gained in popularity and requires further explora-ion, but the results gathered so far represent a useful firsttep in elucidating the contribution of working memory toeneral cognition.

he episodic buffer

he episodic buffer is the latest addition to the multi-omponent model of working memory (Baddeley, 2000). Itepresents a separate storage system of limited capacitysing a multi-modal code. It is episodic by virtue of holding

nformation that is integrated from a range of systemsncluding other working memory components and long-erm memory into coherent complex structures: scenes orpisodes. It is a buffer in that it serves as an intermediaryetween subsystems with different codes, which it com-ines into unitary multi-dimensional representations. The

ntegration and maintenance of information within the ep-sodic buffer depends on a limited capacity attentionalystem, namely the central executive. The retrieval of

nformation is based on conscious awareness, which bindsogether complex information from multiple sources andodalities. Together with the ability to create and ma-ipulate novel representations, it creates a mental mod-ling space that enables the consideration of possibleutcomes, hence providing the basis for planning futurection.

The episodic buffer was postulated to account for aange of empirical data that could not be explained usinghe original tripartite model. In this section we shall there-ore first present the issues that episodic buffer was pro-osed to address. Next we will consider some of the recenttudies that have approached the episodic buffer in differ-nt ways. We will conclude with the challenges and pros-ects that the exploration of the new component of workingemory is facing.

The problems addressed by the episodic buffer.hroughout the years of exploration of working memory, aumber of empirical findings accumulated that failed to beatisfactorily accounted for by the existing three-compo-ent model of working memory. While the model assumed

hat the verbal information in the phonological loop istored in a purely phonological code, early research al-eady showed that immediate memory for words is sensi-ive to semantic similarity when the words can be readilyombined into meaningful pairs (Baddeley and Levy,971). Comparing unrelated verbal material to immediateemory for prose yielded a substantial difference in recall.hile subjects successfully recalled about five unrelated
ords, they were able to recall up to 16 words when tested g
sing sentences (Baddeley et al., 1987). The existingodel offered no explanation for the advantage in recallrovided by the meaningful relation between words; norid it provide a mechanism that would enable aggregationf individual items into larger units: the process or chunk-

ng (Miller, 1956).The model provided no explanation of how the sub-

ystems of working memory relate to and interface withong-term memory, even though memory span for unre-ated words proved to be affected by variables that arerdinarily related to long-term memory, such as word fre-uency and imageability (Hulme et al., 1995).

The original model also did not explain how informationrom the two slave subsystems might be bound together,ven though a number of studies had shown that simpleerbal span can show evidence of combined verbal andisual encoding (Chincotta et al., 1999, Logie et al., 2000).s the two slave subsystems provide separate and inde-endent stores, the question arises how and where is the

nformation combined.Addressing the question of conscious awareness, Bad-

eley and Andrade (2000) exposed a similar problem. Theuthors conducted a study requiring the participants toorm images either of a novel array comprising shapes orones, or images based on long-term knowledge (such as

familiar market scene), while performing concurrent ar-iculatory or spatial suppression. The study revealed thathe expressed judgments of vividness reflected a combi-ation of working memory and long-term memory factors.t seems that information from both sources was combinedn a way that the tripartite model was not able to explain.he same problem is expressed in the ability to combine old

mages in novel ways such as imagining a “swan shopping orn ice-hockey-playing elephant” (Baddeley, 2001, pp. 857).

Results from other areas of research were also instruc-ive. A study of densely amnesic patients (Baddeley and

ilson, 2002) revealed that while their delayed recall ofrose paragraphs is effectively zero, a few patients stillhowed excellent immediate recall. Such patients showedhigh level of general intelligence and typically had well-

reserved executive capacities. It seems that while theelayed recall of prose critically depends on intact long-erm memory, immediate recall may be achieved by aeparate system, closely related to working memory.

Last but not least, the model was unable to appropri-tely account for the wealth of research exploring individ-al differences in working memory as reflected in the work-

ng memory span measure developed by Daneman andarpenter (1980, 1983). The working memory span taskas devised to assess the capacity of working memory toimultaneously process and store information. It requireshe participants to read and/or verify a sequence of sen-ences, storing the last word of each sentence, which theyust then recall. The measure was shown to correlate

trongly with a performance on a wide variety of tasks (seengle, 1996; Jarrold and Towse, 2006) and was suggested

o be virtually equivalent to a measure of general intelli-
ence (Kyllonen and Christal, 1990). It was however, un-

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lear how the task and related findings could be explainedy the existing multicomponent model of working memory.

The empirical findings reviewed above presented atrong case for the ability of working memory to integratend store information from various sources, including thexisting subsidiary working memory systems and long-

erm memory, in a way that would allow their active main-enance and manipulation. The addition of a fourth com-onent, the episodic buffer, enabled the multicomponentodel to account for these findings.

Exploration of the episodic buffer. While the abovendings motivated the postulation of a novel component oforking memory, further research is needed to test andlucidate the model. To explore the episodic buffer and itsole in cognition in the same way as the rest of the com-onents of the working memory model, two classes ofasks need to be developed, namely measures of capacitynd interference tasks.

Two tasks that seem to employ the use of episodicuffer were used in recent neuroimaging studies (Prab-akaran et al., 2000; Zhang et al., 2004). Prabhakaran etl. (2000) devised a task that required concurrent mainte-ance of presented letters and locations, both of themeing tested independently. When the four locations to beemembered were each represented by one of the fouretters, that is when the letter and location were bound, theccuracy of responses to the test stimulus was higher and

he reaction times shorter than when letters were pre-ented in a separate position from the locations. In addi-ion, when the probe was congruent (the probe letter wasresented in the same position as at presentation), theubject responded faster and with greater accuracy than inhe case of incongruent probes. The imaging results,howed right prefrontal activation in the bound condition.he authors proposed that the study provided evidence formemory buffer that is distinct from the phonological loop

r the visuospatial sketchpad, and which allows for theemporary retention of integrated information.

In a similar study, Zhang et al. (2004) asked partici-ants to recall a series of auditorily presented digits andisual locations given either in pseudorandom mixed orderr in a separate order, with digits following the positions.revious research (Penney, 1989; Zhang et al., 1997,999) has shown that when auditory and visual stimuli areresented in a mixed order, separate recall of items wasuch better (12–13 items) than when subject were re-uired to recall the items in the exact order presented (sixo seven items). Zhang et al. (2004) replicated the findingsy Prabhakaran et al. (2000) in showing greater right pre-rontal activation in the task that requested or encouragedntegrated representation in working memory.

An additional candidate for a task requiring involve-ent of the episodic buffer was reported by Zimmer et al.

2003). The authors tested participants’ short-term mem-ry for the spatial location of objects. Using a dual-taskaradigm, the authors showed that neither DVN nor apatial tapping task interfered with memory for object lo-
ations, while the spatial task significantly interfered with w
he Corsi spatial short-term memory task and with memoryor the location of nonsense figures. The authors con-luded that the configuration of the objects is probablyeconstructed from perceptual records in the episodicuffer. It should however be noted that DVN might not ben appropriate interference task for visual working memorysee the above section on visual working memory).

The studies reviewed here represent only the first stepn the empirical exploration of the episodic buffer. They doowever, broadly support the assumption of a separateorking memory store that enables the maintenance of

nformation in an integrated multidimensional form andelies upon the processing resources of the central exec-tive.

Conclusion. The proposal of a new component oforking memory, the episodic buffer, seems timely, al-

hough research on the buffer is still in its infancy and mayell prove more challenging than was the study of thehonological loop and visuospatial sketchpad. While be-avioral studies of the other two subcomponents of work-

ng memory focused on developing simple tasks that wouldarget their basic elements and mechanisms, the study ofhe episodic buffer by definition depends on tasks thatequire complex integration of information. Progress in thenderstanding of the episodic buffer may therefore dependo a larger degree upon multidisciplinary research, as al-eady witnessed by studies in neuropsychology (e.g. Bad-eley and Wilson, 2002; Gooding et al., 2005; Kittler et al.,004), neuroimaging (Prabhakaran et al., 2000; Zhang etl., 2004) and individual differences (Alloway et al., 2004).ue to its close connections to both the phonological loopnd the visuospatial sketchpad, specific attention will haveo be paid to maintaining a clear conceptual and opera-ional distinction between the proposed subsystems.

ognitive neuroscience: the connection betweenrain and cognitive function

hen describing a TV remote control, an electrician woulde concerned with the exact circuitry employed that en-bles the emission of either the radio or infrared signal, ahemist would be interested in the compounds that enablehe lightness and rigidity of the casing, while the user onlyants to know which button to press for the desired effect.very description of the remote control is valid and needed

n its relevant context. Each contributes to the understand-ng of the remote control and its function. In just the sameay, brain and mind can be observed and described atery different levels, each contributing a part of the story,ulfilling different functions and roles. To be able to reach aull understanding of the mind, different levels of descrip-ion need to be brought together in a congruent fashion.hey need to inspire, inform and constrain one another. In

rying to achieve this, one needs to be aware of the differ-nces between these levels of analysis, recognizing their

ndividual strengths and weaknesses, and avoiding theangerous lure of oversimplification.

Our intention was to present a functional model of
orking memory along with the behavioral properties and

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apacities it tries to explain. The present review was pur-osely limited to experimental behavioral data, mapping aunctional description of working memory. Experimentalehavioral research, however, is not the only line of re-earch that bears upon a functional description of workingemory. Human working memory is a system imple-ented in the brain and therefore constrained by its prop-rties. Carefully planned and executed, studies that in-lude the brain dimension can contribute important testnd insights to the development of functional description ofny cognitive ability.

Among the most influential approaches to relatingunctional description of cognitive processes to the brainuilds on systematic mapping of mental processes to braintructures. The approach was pioneered by cognitive neu-opsychology, relating specific brain damage to accompa-ying cognitive dysfunctions. Knowing which cognitive pro-esses are disturbed by damage to a specific brain area,llows one to assume not only that a specific function isubserved by that region, but also that the function itselfresent a distinct functional component of the system.tudies that described patients with severely disruptedbility for immediate recall, but otherwise intact long-termemory and general cognition (e.g. Shallice and War-

ington, 1970; Vallar and Baddeley, 1984) supported bothhe idea of a separate short-term memory store as well asts fractionation into multiple components. Study of brainamaged patients has since supported many other disso-iations between subcomponents of working memory andffered additional important insights (for recent reviewsee Vallar and Papagano, 2002; Della Sala and Logie,002; Mueller and Knight, 2006).

A number of other methods also allow systematic func-ion to structure mapping, among them single cell studiessee Shintaro, 2006), virtual lesioning by transcranial mag-etic stimulation (see Mottaghy, 2006), and functionalrain imaging techniques such as positron emission to-ography (PET), functional magnetic resonance imaging

fMRI) and source localization using multichannel electro-ncephalography (EEG) or magnetoencephalographyMEG). While some might be doubtful about the value ofuch function to structure mapping, comparing neuroimag-ng to modern day phrenology (e.g. Uttal, 2001), the abilityo observe the brain regions being activated by a specificask or condition does present another valuable dependentariable that can be used to augment behavioral studies inesting competing theories and/or generating new onesHenson, 2005).

Henson (2005) describes two types of inference of-ered by neuroimaging studies. First, when two experimen-al conditions result in a qualitatively different pattern ofctivity over the brain, we can conclude that the two con-itions give rise to different functional processes. Second,ctivity of the same brain regions under different conditions

mplies the existence of functional processes that are com-on to both conditions. Both types of inference can be

ound in the working memory literature. Initial PET andMRI studies, for instance, found support for the functional
eparation of the phonological loop and the visuospatial s
ketchpad by confirming that verbal and spatial workingemory tasks activate different brain regions (Paulesu etl., 1993; Jonides et al., 1993; Smith et al., 1996). Suchtudies also identified separate anatomical regions con-erned with short-term phonological storage and re-earsal, in line with the phonological loop model (for aomprehensive review see Baddeley, in press).

When combined with detailed knowledge gained fromrevious neuroimaging, brain lesion and single cell stud-

es, neuroimaging studies employing well-designed cogni-ive tasks can provide important contributions to under-tanding the functional mechanisms and representationssed in working memory. One example is a recent study byurtis et al. (2004; see also Curtis, 2006), which showed

hat small differences in spatial working memory task canead to significantly different pattern of brain activation.oupled with specific knowledge of the regions involved,

he authors concluded, that based on the demands of theask, the participants can maintain spatial information ei-her in the form of prepared eye movements or as a per-eptual memory of stimulus position. The results compli-ent and extend the results obtained by behavioral exper-

mental studies (see the section on representation andaintenance in spatial working memory).

Mapping cognitive processes onto brain regions is notf course, the only possible way of informing and con-training functional models of cognition. Knowledge ofrain structure and physiology shapes our understandingf computational properties of the brain, allowing us touild comprehensive computational models. Exploration ofrain dysfunctions in neurological and psychiatric condi-ions can also help explain the observed patterns of cog-itive impairments, further testing and constraining theunctional models of the impaired functions (see Barch,006; Honey and Fletcher, 2006). With existing and newethods and research paradigms relating the brain and

he mind being constantly developed, the future of multi-isciplinary research of working memory indeed seemsright.

eneral conclusions

e suggest that working memory has proved to be anmportant part of the cognitive system, providing the abilityo maintain and manipulate information in the process ofuiding and executing complex cognitive tasks. It can beractionated into a number of independent subsystems,rocesses and mechanisms. It can usefully be describeds a multicomponent system guided by an executive com-onent consisting of a number of processes that providettentional control over other components of working mem-ry as well as other cognitive abilities. The subordinateomponents provide limited capacity memory stores thatnable the representation and maintenance of information.wo of the subcomponents are domain specific, providing

he ability to hold phonological and visuospatial informationn separate stores. A third subcomponent enables thentegration of information into complex multi-modal repre-
entations linking working memory to long-term memory.

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The functional model of working memory we haveescribed might not prove easy to map onto the underlyingeuroanatomy. Some have argued (see Postle, 2006;azy et al., 2006), that working memory might prove to ben emergent property, a product of the interaction of aighly distributed neuronal system. On a functional level,owever, working memory provides a well-defined concep-ual system that fulfills its role of presenting, organizing andxplaining the existing empirical evidence and, impor-antly, continues to be fruitful in generating further empiri-ally tractable questions.

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(Accepted 4 December 2005)(Available online 6 March 2006)

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Cognitive Psychology- Working memory