P.C.Fletcher and R.N.A.Hensor (A)Typical Verbal Maintenance Task Maintenance BHR☐b 阿□丽 (B)Typical Spatial Maintenance Task Maintenance 口可□□0 .□可9□ C)Typical Object Mainte Task Maintenance 回□￥ ✉回☐啊 D)n-back Task 2-hack Task 0-back Task(target=X) BHCRC... BHC RX Alphabetize BHC2H Fig.2 Schematic representation of working memory tasks. vell as the ventral FC in the m WM.Howe ver the differ (Smith and Jonides,1995a.b).The areas more active in the object and spatial information appears more likely to reflect spatial task k were the right VLFC and the right posterior a left-right lateralization than a ventral-dorsal one.spatial tasks (Belger).the spatial task activated the righ arise when the objects are faces.for which obiect tasks tend to produce VLFC activation and spatial tasks DLFCactivation ry si (e.g.Courtney er )One possit that face the partici nsiomainainmoreha1gdhni2o ical studies suggest that face-selective sha pes,which is beyond the normal visuospatial memor neurones are restricted to ventral FC regions (O'Scalaidhe span)(MeCa rthy et al.,1996).Finally,in a study compar etal,19971. esp 【a objec has proved difficul to isolate storage activation was observed in the former and left DLFC in the latter.These studies suggest a role for the dorsal as movements has little support.because activations of frontal
854 P. C. Fletcher and R. N. A. Henson Fig. 2 Schematic representation of working memory tasks. task were the left posterior parietal cortex and left inferior well as the ventral FC in the maintenance of information in temporal cortex, a subset of the areas implicated in the study WM. However, the difference between the maintenance of (Smith and Jonides, 1995a, b). The areas more active in the object and spatial information appears more likely to reflect spatial task were the right VLFC and the right posterior a left–right lateralization than a ventral–dorsal one, spatial parietal, right anterior occipital and right premotor cortices. tasks activating the right FC and object tasks activating the In another study comparing spatial and object Sternberg left or bilateral FC. One exception to this pattern appears to tasks (Belger et al., 1998), the spatial task activated the right arise when the objects are faces, for which object tasks tend DLFC, whereas the object task activated bilateral DLFC and to produce VLFC activation and spatial tasks DLFC activation left VLFC. A very similar pattern was reported by McCarthy (e.g. Courtney et al., 1996). One possibility is that faces and colleagues (though in this case the memory task required constitute a special class of visual objects [e.g. the participants to maintain more than 18 different locations/ electrophysiological studies suggest that face-selective FC shapes, which is beyond the normal visuospatial memory neurones are restricted to ventral FC regions (O’Scalaidhe span) (McCarthy et al., 1996). Finally, in a study comparing et al., 1997)]. a spatial delayed response task with an object delayed It has proved difficult to isolate storage from rehearsal matching task (Baker et al., 1996a), greater right DLFC processes in spatial and object maintenance tasks. The hypoactivation was observed in the former and greater left DLFC thesis that visuospatial rehearsal corresponds to planned eye in the latter. These studies suggest a role for the dorsal as movements has little support, because activations of frontal
Frontal lobes and human memory 855 mply to heneve T a prespe of information viewed as propo rtional to theworking memory load'the istent with neuroimaging studies of spatial attention total demand placed on the maintenance and/or manipulation of the pro nd colle Coull and 1998)A tentativ hypothesis is tha visuospatial information is stored as abstract or object visua representations in the occipital cortex and inferor tempora linearly increasing functi aps com well as nted by as between these areas and the righ rea On the ad parietal cortex. associations that may be refreshed by a process above.the VLFC.posterior parietal and motor activations sequenti network of areas involved in th arsal of the mo In summary,imaging studies have produced good evidence findings implicate the additional bilateral activation of DLFC for material-specific in manipulation (e.g.updating of the particular letters being udy Smit y to sus studies ver ther ted bilat DLFC/AFC activations in both a verbal is little imaging evide e for ventral-dorsal object -spatia no task in human vation i obiect inf left-lat tudy owen Sn6 nd c and that for the maintenance of spatial information.Th back tasks (Owen).Although differences betv een gion mos consistently the spatial and object memory-relate we as th p infor for the object task the coordinates of the DLFC is also sometime peaks of the bilateral DLFC/AFC activations for the two et al.,1996a:Belger et al..1998) SKS within a pr ght al int Manipulation in working memory ses in vlEC but that maninulation p he Manipulation of the contents of WM involves n array of nmon to visual- tial and visual-obiect WM.These two processe that may b unde the headin que the M sed and a huse nge of diffe onmateral-specine en tasks have been examined.Without attempting a precis FC lateralization of different executive processes. we Cohen and colleagues attempted to dissociate maintenanc on broad have be 'dual'and planning tasks.We emphasize that these term (Cohen et al 1997)Brain regions involved in transien are descriptive of the type of task employed and are not such ceiving stimu meant to imply different sets of executive processe isual and m N-back tasks such as maintenance,were predicted to show an uask that combine and manipulation isthe These regions inclu ask (Fig tas requires the right DLF Region whe ever the current stimulus matches the stimulusp mantain.were predicted to show an inter on between load tions back in the quence.For n 0.this task requires t an I time (i.e.greater transient effects at higher loads).The maintenance of the last n stimuli (in order)and updating of only lateral prefront region to shov this pattem was
Frontal lobes and human memory 855 eye fields, the pulvinar nucleus or superior colliculus are not these stimuli each time a new stimulus occurs (for n 0 the typically observed in neuroimaging studies of visuospatial task is simply to respond whenever a prespecified target WM. Another possibility, that rehearsal of visuospatial occurs, thus no updating is required). The value of n is often information involves an internal attentional mechanism, is viewed as proportional to the ‘working memory load’—the consistent with neuroimaging studies of spatial attention, total demand placed on the maintenance and/or manipulation which activates similar areas of the right superior parietal processes. cortex independently of eye movement (Corbetta et al., 1993; Braver and colleagues varied the verbal WM load by Coull and Nobre, 1998). A tentative hypothesis is that increasing n from 0 to 3 in a letter version of the n-back visuospatial information is stored as abstract or object visual task (Braver et al., 1997). Areas in which activity was a representations in the occipital cortex and inferior temporal linearly increasing function of load included DLFC, VLFC cortex, respectively (perhaps corresponding to visual caches). and the parietal cortex, bilaterally in each case, as well as a The (egocentric) spatial organization of the stimuli may be number of left motor, premotor and supplementary motor represented by associations between these areas and the right areas. On the basis of the maintenance studies reviewed parietal cortex, associations that may be refreshed by a process above, the VLFC, posterior parietal and motor activations of sequential, selective attention (perhaps corresponding to are likely to reflect the network of areas involved in the an inner scribe) that engages the right superior parietal cortex, maintenance of verbal information (e.g. the storage and right premotor cortex and right FC. rehearsal of the most recent n letters). If this is so, then these In summary, imaging studies have produced good evidence findings implicate the additional bilateral activation of DLFC for material-specific stores in posterior brain regions and in manipulation (e.g. updating of the particular letters being some evidence for a left–right lateralization of FC regions for maintained). the rehearsal of verbal and spatial information, respectively. In another study, Smith and colleagues (Smith et al., 1996) Contrary to suggestions from primate studies, however, there reported bilateral DLFC/AFC activations in both a verbal is little imaging evidence for ventral–dorsal object–spatial and spatial 3-back task, though there is a tendency for greater distinction in non-verbal maintenance tasks in humans. left DLFC activation in the former and greater right DLFC Rather, FC activation associated with the maintenance of activation in the latter (Smith and Jonides, 1997). In a similar object information appears to be more left-lateralized relative study, Owen and colleagues compared spatial and object 2- to that for the maintenance of spatial information. The back tasks (Owen et al., 1998). Although differences between FC region most consistently associated with the simple the spatial and object memory-related activations were maintenance of verbal material is the left VLFC. The VLFC observed in posterior regions, such as the posterior parietal is often associated with the maintenance of spatial and object cortex for the spatial task and the middle and anterior information (on the right for spatial information), though temporal cortex for the object task, the coordinates of the DLFC is also sometimes activated in these cases (e.g. Baker peaks of the bilateral DLFC/AFC activations for the two et al., 1996a; Belger et al., 1998). tasks were within 2 mm of each other. These data suggest that manipulation processes in DLFC are left–right lateralized for verbal versus spatial information, as for maintenance Manipulation in working memory processes in VLFC, but that manipulation processes may be Manipulation of the contents of WM involves an array of common to visual–spatial and visual–object WM. These two processes that may be loosely grouped under the heading studies again question the specific dorsal–ventral spatial– of executive processes. Many different types of executive object FC dissociation suggested by Goldman-Rakic, processes have been proposed and a huge range of different though support a material-specific left–right verbal–spatial tasks have been examined. Without attempting a precise FC lateralization. definition of different executive processes, we concentrate Cohen and colleagues attempted to dissociate maintenance below on broad categories of manipulation task that have been and manipulation in an n-back task by using event-related used in neuroimaging: ‘n-back’, ‘reordering’, ‘generation’, fMRI to measure activity at four intervals after each trial ‘dual’ and ‘planning’ tasks. We emphasize that these terms (Cohen et al., 1997). Brain regions involved in transient are descriptive of the type of task employed and are not processes, such as perceiving stimuli and producing meant to imply different sets of executive processes. responses, were predicted to show an effect of time but no effect of load (n). As expected, these regions included the visual and motor cortices. Regions involved in sustained N-back tasks processes, such as maintenance, were predicted to show an A task that combines maintenance and manipulation is the effect of load but not time. These regions included bilateral N-back task (Fig. 2D). This task requires the monitoring of VLFC and right DLFC. Regions associated with transient a continuous sequence of stimuli; a positive reponse occurs manipulation processes, such as updating the n items to whenever the current stimulus matches the stimulus n posi- maintain, were predicted to show an interaction between load tions back in the sequence. For n 0, this task requires both and time (i.e. greater transient effects at higher loads). The maintenance of the last n stimuli (in order) and updating of only lateral prefrontal region to show this pattern was left
856 P.C.Fletcher and R.N.A.Hensor VLFC.Though this was not the FC region that might have to detect which digit was omitted,the same bilateral DLFC DLCthisd on the bas of the above ee th activation was observed (Petrides er al..1993b) re (Baddeley as well as condition. forming to any rule or pattern.Tasks like these involve no only internal monitoring of previous responses (as in self but also The use of eventa clated fMRI to distinguish transient and random ke sustained effects in wM tasks is clearly an important method pressing was compared with reactive.stimulus-driven key ological advance.and one that is likely to prove valuable in pressing(Frith et al. 1991).Jahanshahi and colleagues apart pen on anc ma on when ra technique to isolate brain regionsre nsive during the s at higher of WM trials (see also (ahanshahi e) (D'Eposi whe for ex with a sea of fiv indices of random s or the ation rate orting the letters.followed by either a 'forward or an'alphabetize instruction.After a d ented tha are required by,but not related to,random generation and h as ih gen ney.a com osition denoted by the ce of fiv stimuli from much lar sets.The ve task ters were maintained in the (original)forward order (in requires generation without repetition of.for example.as Oin th the s).or five lette ge alpha of five as the latter additional manipulation GpTcntoncWtaceiestoaidgeaernlion(eg ordering).Both VLFC resp sive durin en a subject is require to gene rate as many animals a del D sho may by the consistent with the ecific FC modelo left di fC activation when letter fluency was compared with Petride these word repetition () are invo CO rable only DL and the evi Ge Dual asks task m tasks simulta rated withoutr tition one at a tim y 1986)mo e This task has been explored in neuropsychological (Petride between information appropriate for one or other task.patient and Milner.1982 and neur maging studies Petrides with frontal lesions may be disproportiona es er a 199 ersus singl Dowell er a durins contro task in which participants res nded WM.D'Esposito and collea exte nally produced stim h participants performed two brain activit tasks concurrently with ab was ting the atial totatio task and a semantic iude nt task roduced lateralization of manipulation ses in visuos patial wm ignificant activation of DLFC when performed alone:only s),DLFC activation as bi they were combir activa of did ed was significant bilate not dep activation w when an extemally ordered condition was tested in which omhined hecause a second exneriment in which perform participants listened to a random sequence of digits in orde ance of the rotation task was impaired by decreasing the
856 P. C. Fletcher and R. N. A. Henson VLFC. Though this was not the FC region that might have to detect which digit was omitted, the same bilateral DLFC been expected on the basis of the above studies (i.e. the activation was observed (Petrides et al., 1993b). DLFC), this experiment illustrates the opportunity afforded A related task is random number generation (Baddeley, by event-related studies to dissociate FC processes by time 1966), in which numbers must be generated without conas well as condition. forming to any rule or pattern. Tasks like these involve not only internal monitoring of previous responses (as in selfordering tasks), but also inhibition of prepotent responses Reordering tasks and well-learned routines. Frith and colleagues reported The use of event-related fMRI to distinguish transient and bilateral DLFC activations when generative, random key sustained effects in WM tasks is clearly an important method- pressing was compared with reactive, stimulus-driven key ological advance, and one that is likely to prove valuable in pressing (Frith et al., 1991). Jahanshahi and colleagues teasing apart perception and maintenance, and maintenance observed left DLFC activation when random number generaand manipulation. D’Esposito and colleagues have used this tion was compared with counting, and this activity was technique to isolate brain regions responsive during the negatively related to indices of randomness at higher generapresentation, delay and probe phases of WM trials (see also tion rates (Jahanshahi et al., 2000). Interestingly, VLFC Courtney et al., 1997). D’Esposito and colleagues (D’Esposito activation was also seen when random number generation et al., 1999) and Postle and colleagues (Postle et al., 1999), was compared with counting, but did not correlate with for example, presented subjects with a sequence of five indices of randomness or the generation rate, supporting the letters, followed by either a ‘forward’ or an ‘alphabetize’ proposal that this region is involved in maintenance processes instruction. After a delay of 8 s, a probe was presented that that are required by, but not related to, random generation. consisted of a letter and a digit (Fig. 2E). The subject’s task Other generation tasks, such as verbal fluency, a common was to indicate whether the probe letter would appear in the clinical test of frontal lobe damage, involve the selection of position denoted by the probe digit if the sequence of five stimuli from much larger sets. The verbal fluency task letters were maintained in the (original) forward order (in requires generation without repetition of, for example, as the ‘forward’ trials), or if the five letters were rearranged many animal names (category fluency) or words beginning into alphabetical order (in the ‘alphabetize’ trials). The former with a specified letter (letter fluency) as possible in a short trials require only the maintenance of five letters in order, period of time. This task involves not only monitoring but whereas the latter trials require additional manipulation (i.e. also the development of new strategies to aid generation (e.g. reordering). Both VLFC and DLFC were responsive during when a subject is required to generate as many animals as the delay period, but DLFC showed a greater response they can, they may begin by thinking of pets, then safari during the alphabetize trials (bilaterally in all cases). Though animals, etc.). The PET study of Frith and colleagues found broadly consistent with the process-specific FC model of left DLFC activation when letter fluency was compared with Petrides and colleagues, these studies suggest a nested word repetition (Frith et al., 1991). organization in which both VLFC and DLFC are involved Considerable evidence thus exists for a role of DLFC, on in maintenance, but only DLFC is additionally involved in the left for verbal and the right for visuospatial information, manipulation. in the manipulation processes necessary for generation tasks. Generation tasks Dual tasks In the self-ordering task mentioned earlier, stimuli must be Performing two tasks simultaneously makes demands on generated without repetition, one at a time, from a finite set. WM (Baddeley, 1986), most probably reflecting the switching This task has been explored in neuropsychological (Petrides between information appropriate for one or other task. Patients and Milner, 1982) and neuroimaging studies. Petrides and with frontal lesions may be disproportionately impaired in colleagues (Petrides et al., 1993a, b) compared brain activity dual-task versus single-task performance (McDowell et al., during the performance of a self-ordering task with activity 1997), again suggesting a frontal role in these aspects of during a control task in which participants responded to WM. D’Esposito and colleagues compared brain activity externally produced stimuli, without the requirement to order when participants performed two tasks concurrently with their own responses. When abstract figures were used, the the brain activity when each task was performed alone self-ordering task produced greater activation in right DLFC, (D’Esposito et al., 1995). Neither of the two tasks, a as predicted (Petrides et al., 1993a), supporting the right spatial rotation task and a semantic judgement task, produced lateralization of manipulation processes in visuospatial WM. significant activation of DLFC when performed alone; only With verbal stimuli (digits), DLFC activation was bilateral when they were combined was significant bilateral activation (Petrides et al., 1993b). This FC activation did not depend of this area observed. This activation was unlikely to be due solely on the self-generated nature of the ordering task: simply to the impaired performance of both tasks when when an externally ordered condition was tested in which combined, because a second experiment in which performparticipants listened to a random sequence of digits in order ance of the rotation task was impaired by decreasing the
Frontal lobes and human memory 857 bilateral id while when a stimulus was of lower luminance or pitch than the achieving subgoals.Imp y the previous stimulus.Klingberg and colleagues found no cortical AFC activation was selective to the branching condition,and rea that was a al in the dua-task in compa ble control ng. Card-Sorting task was actually diminished when combined with an auditory verbal shadowing task (Goldberg and et al..1998).participants watched a sequence of words and and Plc che kept a rnning count of the number of w that were name was diminished when the task was combined with a visuo condition.activations were seen in both right DLFC and motor secondary task (Fletcher).One possible right AFC.Like the branching task.this task might also be explan tha or even when evaluation)is pe activating DLFC.This might leave less scope for additional In a recent PET study,Burgess and colleagues observed sa set of different prospective wh the performance of b k (Burges 【asks aga1 whil differ ent task Thus AFC activation may reflect a third of dual-tasking.see Adcock et al..2000:Bunge et al.2000) level of executive control,beyond the manipulation in DLFC and m intenance Though this level Planning tasks ing.it is likely to nt of yday life Shallice introduced the Tower of London task in order to (Burgess er al.2000b).such as when we are inter test planning deficits in patients with frontal lesions(Shallice with a question while performing a complex task like reading. 98 must ge a set constraints on legal movements of the balls.this task re Other working memory tasks er te Yet more complex problem-solving tasks have been g02 Sta aedwith this gu AFC a simple visual-motor control,as well as several regions in DLFC (for a review,see Christoff and Gabrieli.2000).The the right premotor and parietal cortices that may be a compon ent processes of such complex tasks remain even less nainte ance (Owen er al. understood,however,and we do not discuss them requirins no movement (particinants were shown an initial state and a goal state and simply indicated the minimum number of moves from the initial to the goal state)(Bake Summary 199 s)The that subtrac FC suserve This evidence is summarized right AFC.These s are at least suggestive of a (perhap n Table 1.VLFC,for example,is more often activated FCinmepulhtion.cvenifmaipulai durin吗as srequiring maintenance anc DLF The study by Baker and collea es (1996h)is also or the few WM studies we have considered thus far,apart from that of Goldman-Rakic (Goldman-Rakic.1987).Nonetheless. 1996:0we here also appears to be a lateralization of FC processes hough the of London task.which includes up and maintainin ons of verbal and spatial tasks suggest that left multiple subgoals at the same time as making (or imagining movements between states.A more recent study showed an
Frontal lobes and human memory 857 interval between stimuli did not reveal any significant increase association between bilateral AFC activation and a in DLFC activity. However, in another dual-task study, using ‘branching’ task (Koechlin et al., 1999). This task also a visual and an auditory task in which participants indicated required the participant to maintain an overall goal while when a stimulus was of lower luminance or pitch than the concurrently setting and achieving subgoals. Importantly, the previous stimulus, Klingberg and colleagues found no cortical AFC activation was selective to the branching condition, and area that was activated specifically in the dual-task condition was not seen in comparable control conditions that required (Klingberg et al., 1998). Moreover, Goldberg and Berman either switching attention between goals (dual-tasking, which found that the DLFC activation associated with the Wisconsin activated right DLFC instead) or simply maintaining a single Card-Sorting task was actually diminished when combined goal. In another WM study that activated AFC (MacLeod with an auditory verbal shadowing task (Goldberg and et al., 1998), participants watched a sequence of words and Berman, 1998), and Fletcher and colleagues found that the kept a running count of the number of words that were names DLFC activation associated with elaborative verbal encoding of dangerous animals. Relative to a passive word-viewing was diminished when the task was combined with a visuo- condition, activations were seen in both right DLFC and motor secondary task (Fletcher et al., 1998b). One possible right AFC. Like the branching task, this task might also be explanation for these results is that one or both tasks, viewed as entailing the maintenance and periodic updating unlike the tasks used by D’Esposito and colleagues, included of one type of goal information while a demanding task manipulation requirements even when performed alone, (semantic evaluation) is performed concurrently. activating DLFC. This might leave less scope for additional In a recent PET study, Burgess and colleagues observed DLFC activation when the tasks are combined, or even a bilateral AFC activation across a set of different prospective decrease in DLFC activation when the performance of both memory tasks (Burgess et al., 2000a). These tasks again tasks suffers under dual-task conditions (for arguments against required delayed realization of an intention while performing the association of specific regions with the executive demands a different task. Thus AFC activation may reflect a third of dual-tasking, see Adcock et al., 2000; Bunge et al., 2000). level of executive control, beyond the manipulation in DLFC and maintenance in VLFC. Though this level of executive control is difficult to isolate and control in the laboratory Planning tasks setting, it is likely to be a vital component of everyday life Shallice introduced the Tower of London task in order to (Burgess et al., 2000b), such as when we are interrupted test planning deficits in patients with frontal lesions (Shallice, with a question while performing a complex task like reading. 1982). Participants in this task must rearrange a set of balls in order to match a specified goal state. Because of the constraints on legal movements of the balls, this task requires Other working memory tasks advance planning of a number of separate moves in order to Yet more complex problem-solving tasks have been attain the goal state, often via various subgoals, in the investigated with functional imaging, such as Wisconsin minimum number of moves. Owen and colleagues found Card-Sorting, Raven’s matrices, and inductive reasoning. activation of left DLFC associated with this task relative to These tasks have also tended to activate AFC as well as a simple visual–motor control, as well as several regions in DLFC (for a review, see Christoff and Gabrieli, 2000). The the right premotor and parietal cortices that may be associated component processes of such complex tasks remain even less with visuospatial maintenance (Owen et al., 1996). Baker well understood, however, and we do not discuss them and colleagues used a version of the Tower of London task further here. requiring no movement (participants were shown an initial state and a goal state and simply indicated the minimum number of moves from the initial to the goal state) (Baker Summary et al., 1996b). They found that subtraction of easy (two or Functional imaging of human WM has provided considerable three moves) from difficult (solutions involving four or five evidence that broad anatomical divisions within the lateral moves) conditions revealed activation in bilateral DLFC and FC subserve different processes. This evidence is summarized right AFC. These studies are at least suggestive of a (perhaps in Table 1. VLFC, for example, is more often activated bilateral) role of DLFC in manipulation, even if manipulation during tasks requiring maintenance and DLFC is more often was not completely dissociated from maintenance in this task. activated during tasks requiring manipulation. This is more The study by Baker and colleagues (1996b) is also one of consistent with the view of Petrides (Petrides, 1994) than with the few WM studies we have considered thus far, apart from that of Goldman-Rakic (Goldman-Rakic, 1987). Nonetheless, some n-back tasks with large n (Smith et al., 1996; Owen there also appears to be a lateralization of FC processes et al., 1998), in which AFC was activated. This activation is according to the type of material. Though the FC activations perhaps related to the complex planning required in the Tower are often bilateral (relative to baseline tasks), direct of London task, which includes setting up and maintaining comparisons of verbal and spatial tasks suggest that left multiple subgoals at the same time as making (or imagining) VLFC is primarily concerned with the maintenance of verbal movements between states. A more recent study showed an information and right VLFC with the maintenance of spatial
P.C.Fletcher and R.N.A.Henson Table 1 Working memory studies VLFC DLFC AFC Left Right Left Right Left Right Sn th et al. verba-p ds(199 h er af.(19 Be er et al (1998 ct-spatial Sternberg ne spa nngs Baker et al (996a Spatialob Br vera-back Spatial 3-back ,1997 ng m al. erbal n-back Frith er al.(1991) a2000 Random ke -tasking n er at.(19 et al.(19 96a Ko chlin etal.(1999 B ss et al (2000a) Semantic monitoring son is descriptive ony
858 P. C. Fletcher and R. N. A. Henson Table 1 Working memory studies VLFC DLFC AFC Left Right Left Right Left Right Awh et al. (1996) Verbal Sternberg Paulesu et al. (1993) Verbal Sternberg Jonides et al. (1993) Spatial Sternberg Smith et al. (1996) Spatial–verbal Sternberg Verbal–spatial Sternberg Smith and Jonides (1994) Object Sternberg Smith et al. (1995b) Spatial–object Sternberg Belger et al. (1998) Spatial–object Sternberg Object–spatial Sternberg McCarthy et al. (1996) Spatial–object running span Object–spatial running span Baker et al. (1996a) Spatial–object delayed Object–spatial delayed Braver et al. (1997) Increasing n in verbal n-back Smith et al. (1996) Verbal 3-back Spatial 3-back Owen et al. (1998) Spatial 2-back Object 2-back Cohen et al. (1997) Increasing n in verbal n-back D’Esposito et al. (1999) Alphabetization Petrides et al. (1993a) Spatial reordering Petrides et al. (1993b) Verbal reordering Frith et al. (1991) Random key-pressing Letter fluency Jahanshahi et al. (2000) Random key-pressing D’Esposito et al. (1995) Dual- versus single-tasking Owen et al. (1996) Planning versus difficult control Baker et al. (1996a) Planning versus control Koechlin et al. (1999) Branching versus dual-tasking Burgess et al. (2000a) Prospective memory MacLeod et al. (1998) Semantic monitoring Significant activation detected; no significant activation detected. Note that the label for each comparison is descriptive only; for more details see text. Studies are ordered according to their order of appearance in the text.