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Mapping Task Switching in Frontal Cortex Through Neuropsychological Group Studies

Mapping Task Switching in Frontal Cortex Through Neuropsychological Group Studies FOCUSED REVIEW Mapping task switching in frontal cortex through neuropsychological group studies 1,2 3,4,† 3,4 Tim Shallice , Donald T. Stuss , Terence W. Picton , 5,6 3 Michael P. Alexander and Susan Gillingham Cognitive Neuroscience Sector, SISSA, Trieste, Italy. Institute of Cognitive Neuroscience, University College, London, UK. Rotman Research Institute, Baycrest, Toronto, ON, Canada. Departments of Psychology and Medicine, University of Toronto, Toronto, ON, Canada. Behavioral Neurology Unit, KS 253, Beth Israel Deaconess Medical Center, Boston, MA, USA. Harvard Medical School, Boston, MA, USA. Edited by: Robert T. Knight, University of California Berkeley, USA Reviewed by: This paper considers evidence provided by large neuropsychological group studies and meta-analyses Marcel Brass, Ghent of functional imaging experiments on the location in frontal cortex of the subprocesses involved in the University, Belgium carrying out of task-switching paradigms. The function of the individual subprocesses is also considered Leon Deouell, The Hebrew University of Jerusalem, Israel; in the light of analyses of the performance of normal subjects. Interdisciplinary Center for Neural Computation, Keywords: task-switching, switch-cost, anterior attentional system, frontal lobes, focal lesions, reaction time, Hebrew University, Israel errors, review Correspondence: Task switching is the area of research concerned of their shapes, in their colour and in how many with how a subject effects a change in the rules that there are. The clinician has another set of cards each govern how he or she responds to incoming stimuli. of which matches one of the key cards in colour, As an area of research, it is situated at the confl uence another in shape and so on. The patient is given one of two intellectual traditions. Within human exper- card at a time and has to place it with the key card Donald T. Stuss, with Tim imental psychology the study of cognitive control which has the same value on one of the dimensions. Shallice, Michael P. Alexander and Terence W. Picton began processes in normal subjects had seen important The correct match is dictated by a rule in operation, a formal collaboration on theoretical and experimental advances with the which can be to match by colour or by shape or by understanding the anterior work of Atkinson and Shiffrin (1968) and Schneider number. Every so often, the rule changes and the (frontal) attentional system over 15 years ago. Their and Shiffrin (1977). In complementary fashion, patient must discover the new rule by trial and error. approach was somewhat clinical neuropsychological investigations of lateral The cognitive component that is held to be impaired of a gamble. They started prefrontal functions had seen the development of by lateral frontal lesions in such a task is set shifting. with simple tasks to isolate simple processes, and tests for assessing executive functions including the The prototypical lateral prefrontal patient contin- increased task diffi culty and Wisconsin Card-Sorting (Milner, 1963) and the ues to use the previously relevant rule after the rule included other processes. Five Extra-Dimensional Intra-Dimensional Shift test of changes, even though he or she is being told on each years to screen and test all participants, and an equal the CANTAB battery (Owen et al., 1990). trial that the match is incorrect. What appears to be time for analysis, led to the As an example of a relevant clinical test, in lost is the ability to shift from seeing one attribute of identifi cation and localization Wisconsin Card-Sorting, the patient is presented the stimulus as being relevant to seeing another. of multiple attentional functions of the frontal lobes, with four cards on each of which there is a set of col- Performance on such clinical tests, however, is highlighted in eight separate oured shapes, all the shapes on any one card being very complex to analyse as there are many relevant publications since 2005. [email protected] identical. These four ‘key’ cards differ in the form cognitive components; Wisconsin Card-Sorting, Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 79 Shallice et al. for example, is affected in a qualitatively different moves consistently round the panes in turn; in the fashion by lesions in different parts of prefrontal two upper positions one task is operative, in the two cortex (Stuss et al., 2000). Moreover, such prefron- lower positions the other. Shortly thereafter, Meiran tal clinical tests are poorly adapted to the isolation (1996) introduced the procedure of the cueing of the by purely behavioural means of critical subcom- task by a distinctive warning signal occurring before ponents. So exploration of the performance of stimulus presentation; this enabled the relevant task normal subjects in these paradigms in the fashion to be varied randomly across trials. His paradigm typical of human experimental psychology, virtu- also used four panes of a square (see Figure 1). ally never occurred. Cognitive psychology and neu- A circle appears in one of the four panes. The ropsychology remained separate to their mutual response key to be pressed depends on which dimen- disadvantage. sion of the square is currently operative. If it is the In the 1990s, developments in so-called ‘task- vertical dimension, then the subject must make a switching’ experimental paradigms provided an top-bottom decision. If, instead, it is the horizontal avenue to overcome this separation between the dif- dimension then the subject must make a left–right ferent research traditions. These experimental para- decision. Which of the two tasks is operative on a digms are also beginning to allow us to distinguish particular trial is indicated through a warning sig- key components that contribute to performance in nal just before the stimulus. This means that the normal subjects, a key step in designing clinical tests task that is relevant can be randomised across trials which isolate different components of prefrontal unlike on the Rogers and Monsell design. functioning. The central cognitive component is This original group of experiments showed again set-shifting. Typically on each trial of a task- that if stimuli require different responses depend- switching experiment a stimulus is presented which ing on which task is operative, then trials where can be conceived of in one of a number of ways, the task switches from the previous trial are slower typically two (see Figure 1). Two studies essentially than those where the task repeats – the difference initiated the modern use of this type of paradigm between the two reaction times is known as the in normal subjects, those of Allport et al. (1994) switch effect. Moreover, the size of the switch effect and Rogers and Monsell (1995). In the second of generally decreases with an increase in the time these, subjects are presented with a letter and a digit. available in which the subject can prepare for the On some trials it is the letter which is relevant and specifi c task. This led Rogers and Monsell to pro- the subject must respond by pressing one of two pose that there is a top-down process of task recon- keys as quickly as possible as to whether the letter is fi guration which takes appreciable time and which a consonant or a vowel. On other trials the relevant is measurable by the decrease in switch effect. The part of the stimulus is the digit and the subject must paradigm appeared to provide a measure of the make the corresponding decision between odd and time to switch the dimension of the stimulus that even. In the Rogers and Monsell type of procedure should control behaviour. However, Allport et al. it is possible for the subject to predict in advance (1994) found that the switch effect was less when which task is relevant. The letter-digit combina- switching is into a more diffi cult task than into an tion appears in one of four panes (quadrants) of easier one, and argued that the switch effect was a square on each trial, and the position of the pair due instead to the need to inhibit a previously active Figure 1 | The experimental paradigm used in Shallice et al. (2008b). It is derived from the paradigm developed by Meiran (1996). 80 | July 2008 | Volume 2 | Issue 1 www.frontiersin.org Task switching and frontal lesions task-set or action schema, namely the control proc- noted in the inferior frontal gyrus, which was the ess that determines which task is undertaken. These largest region and the second most signifi cant in two types of explanation are not necessarily in con- the meta- analysis. In a second meta-analysis by fl ict, since they can be represented in different levels Buchsbaum et al. (2005), using many of the same of a relatively simple computational model of task- source studies, the set of distinct regions found by switching of the interactive activation type (Gilbert Derrfuss et al. (2005) essentially clustered into two and Shallice, 2002). On such a model, though, the prefrontal regions in each hemisphere. Buchsbaum switch cost as measured from reaction times, no et al. (2005) related their fi ndings to studies involv- longer corresponds quantitatively to the time that ing simple go/no-go tasks which require response the internal process of task reconfi guration takes. inhibition, and so suggested that the right frontal The incorporation of this type of experimental regions involved in the switching task might medi- paradigm into the behavioural literature led to an ate the inhibition of a previous response rule. explosion of studies. However, it became rapidly Given the potentially large number of prefron- realised that the paradigm, apparently so simple, tal regions suggested to be relevant from the func- had many complexities and involved many differ- tional imaging meta-analyses, neuropsychological ent processes (e.g. Monsell, 2003). For instance, studies of patients with focal frontal lesions seem Allport and Wylie (2000) discovered that presen- potentially important. In particular, they have the tation of a stimulus associated with a previously advantage that the impairment of a component active task-set would tend to reactivate it. At much can produce signature effects which help to nar- the same time Meiran et al. (2000) in somewhat row down hypotheses on the underlying functional confl icting fashion showed that task-set effects dis- processes. This is particularly the case if neuropsy- sipate over time. Despite these complications the chological studies of task-switching are carried out basic paradigms, together with the scores of sub- in conjunction with parallel studies of related para- sequent variants (e.g. Brass and von Cramon, 2002; digms, which potentially involve different combi- Braver et al., 2003), while complex, remain much nations of the same set of subsystems. simpler than the clinical tests from which they were A number of studies of task switching have originally derived. The complexity of the under- been carried out in neurological patients (see for lying processes, however, means that a return to instance, Mayr et al., 2006; Mecklinger et al., 1999), brain-based studies – functional neuro-imaging and but they have mainly involved small numbers of lesion studies – becomes critically necessary if the patients and so are not suitable for differential components are to be isolated. This is based on the localisation of subcomponents within prefrontal assumption that the different cognitive resources cortex. Our paper, that of Shallice et al. (2008b), involved are differently localised; if so the degree to is only the second study of task-switching with a which a particular cognitive resource is used in a sizeable number of patients with lesions in differ- given condition can be measured or the effect of its ent parts of prefrontal cortex, the fi rst being that loss discovered. of Aron et al. (2004). These two studies reported Interpretation of the functional imaging stud- fi ndings on 41 and 36 frontal patients respectively. ies, though, has not been straightforward. Many Aron et al. (2004), using a version of the Rogers– regions of cortex can be involved including pari- Monsell paradigm, found that both patients with etal and even the anterior insula cortex (Wager left frontal lesions and ones with right frontal et al., 2004). Even restricting attention to prefrontal lesions showed a signifi cantly larger switch cost cortex, some studies (e.g. DiGirolamo et al., 2001; than controls. The right frontal group also showed Luks et al., 2002) have obtained a large number of a dramatically elevated error rate on switch trials. activation maxima (>10) for relevant contrasts. In a The Shallice et al. (2008b) study which instead meta-analysis of task-switching, set-shifting and used the Meiran paradigm also found a left frontal stimulus-response reversal studies, Derrfuss et al. effect, but this time on errors. The major reaction (2005) found many regions in frontal cortex that time effect we found was a striking slowing on both showed relatively consistent effects; these include switch and repeat trials for patients with superior the inferior frontal junction – a region in the vicin- medial prefrontal lesions. An increase in errors in ity of the junction of the inferior frontal sulcus conditions where two tasks are potentially relevant and the inferior precentral sulcus – the inferior on any given trial was also found for patients with frontal sulcus itself, the inferior frontal gyrus, the inferior medial prefrontal lesions. medial superior frontal gyrus, the anterior cingu- The results of the two studies, like the functional late and the pre-SMA, activations generally involv- imaging studies, suggest that many parts of pre- ing both hemispheres. However the consistency frontal cortex can potentially play distinct roles in was only relative; for instance, of the seven pure task-switching. The divergence of results across the task- switching studies, in only two was a maximum two studies highlights the importance of differences Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 81 Shallice et al. in both the specifi c behavioural paradigms and For the fi ne-grain analyses Aron et al. (2004) methodology. used the percentage of particular gyri affected Both studies followed an appropriate two-level (superior, middle, and inferior frontal, orbital and method for localising patterns of defi cits, a fi rst- medial) and Shallice et al. (2008b) used difference pass coarse-grain and then a second-pass fi ne-grain between presence and absence of damage to partic- analysis. However, there was a difference between the ular frontal areas as defi ned by Petrides and Pandya two studies in both the coarse-grain and the fi ne- (1999). Both studies obtained similar results on the grain procedure employed. Thus, the coarse-grain coarse grain procedure as far as localisation is con- analysis used by Aron et al. (2004) involved compar- cerned in the left lateral frontal region. The fi ne- ing the effects of left frontal and right frontal lesions grain effects were consistent across the two studies, with control performance, but in the Shallice et al. with the Aron et al. (2004) study suggesting the (2008b) case a contrast was drawn between each middle frontal gyrus is the critical locus and the of four groups – left lateral, right lateral, superior Shallice et al. (2008b) one an area within it, 9/46v, medial and inferior medial – and the controls. In the but only as a trend, so there is no strong sugges- fi ne-grain analysis, Aron et al. (2004) took patients tion that either fi ne-grain procedure is inferior to with lesions to one or other frontal lobe and corre- the other. As far as the functional imaging meta- lated particular aspects of their performance with the analyses are concerned, in the Derrfuss et al. (2005) proportion of each gyrus that was damaged. More paper this region does not seem to be activated but simply, Shallice et al. (2008b) took each Petrides in the Buchsbaum et al. (2005) it is. There is thus and Pandya (1999) region of frontal cortex, and a convergence in the pattern of results across all compared the performance of patients with damage studies as far as the involvement of the left lateral to that region with that of all the other patients with prefrontal cortex is concerned, although there are frontal lesions. Both types of neuropsychological possible differences in the specifi c region involved. fi ne-grain analysis procedures make strong assump- Even the apparently similar left lateral effects tions and both are subject to the possible problem obtained in the two neuropsychological studies, of associated defi cits from lesions extending into though, differed in terms of the variable involved. In neighbouring regions. Therefore it is argued in our the Aron et al. (2004) study, patients with left fron- paper that it is critical that the fi ne-grain effects are tal lesions were slowed down more by a switch than consistent with the coarse-grain ones; if not, this were control subjects (a larger reaction time switch increases the possibility that fi ne-grain effects could cost). In contrast, in our study the increased switch result from associated defi cits. cost in patients with lateral frontal lesions occurred At the coarse-grain level Aron et al. (2004) com- for the errors rather than reaction time. However, pared left frontal and right frontal patients; the in addition, our paper points to the importance of Shallice et al. (2008b) study compared left lateral, a variable not considered in many task-switch- right lateral and two medial groups, superior and ing studies, namely the degree of practice. The left inferior. On all reaction time measures in our study lateral effect on errors concerned this variable; this it is only the Superior Medial group that is slowed. group showed a signifi cantly greater change in error Moreover, this slowing interacts with many variables rate from the fi rst half to the second half of the such as switch trials versus repeat trials, incongruent experiment than the controls or the other patient trials (where the two tasks lead to different response groups. The left lateral patients had over double the outcomes given the same stimulus) versus congruent error rate of the controls in the fi rst half, but were trials, harder versus easier tasks. This slowing with the virtually at the control rate in the second half. This Superior Medial lesions is closely analogous to effects is a similar effect to that shown by Alexander et al. found in a variety of other reaction time studies of (2005) in a study of the acquisition of a serial reac- frontal patients (e.g. Alexander et al., 2005, 2007; Stuss tion time paradigm. As in our study the left lateral et al., 2002, 2005). It fi ts with the idea that systems group were as fast as any other group including the in the Superior Medial region, probably the anterior control group from the start. However, they made cingulate and/or the pre-SMA (see Rushworth et al., signifi cantly more errors on the fi rst 100 trials 2002), are critical for the energisation (cognitive whereas on the next 400 trials their error rate was effort) necessary to activate operations not directly normal. This pair of studies point to the importance triggered in an overlearned fashion by perceptual and of the left lateral region in task-setting, the proc- motivational inputs. Both functional imaging meta- esses which allow task performance to move from analyses of task-switching produce activation of the a novel to a routinised state (see Stuss et al., 1995). anterior cingulate. Thus the four-group coarse-grain This is somewhat related to the argument presented procedure is likely to be preferable to the two-group by Aron et al. (2004) for their left frontal effects, coarse grain procedure as far as localisation of pre- namely that there is weaker endogenous control of frontal functional systems is concerned. task-set in left frontal patients and to the argument 82 | July 2008 | Volume 2 | Issue 1 www.frontiersin.org Task switching and frontal lesions of Derrfuss et al. (2005) that the left inferior fron- in avoiding capture errors (Reverberi et al., 2005), tal junction is involved in the updating of task rep- in judging the passage of time (Picton et al., 2006) resentations. Our position is that such updating is or in counting sounds (Shallice et al., 2008a). operating throughout the whole process of learning Impairments in performance monitoring (e.g. not the task and not just on a single trial. noticing that switching is required or had not been In addition to the difference concerning the carried out) might also explain the fi ndings in the Superior Medial fi ndings, other fi ndings also differed Aron et al. (2004) study. between the two studies. The Aron et al. (2004) study The differences in patterns of performance found effects of right inferior frontal gyrus lesions between the two studies may derive from the com- and the Shallice et al. (2008b) one of orbital frontal position of the two patient populations and from the lesions, neither of which was found in the other study. group study analysis methodology used. However, as This points to the importance of group composition, important a factor is likely to be the specifi c version and to the specifi c behavioural characteristics of the of task-switching employed. Our study differed from task in addition to the analysis procedures. Thus as that of Aron et al. (2004) and was unusual but far far as group composition is concerned, the absence from unique in not fi nding a reduction in switch of a Superior Medial effect in the Aron et al. (2004) costs with an increased preparation interval. In our study may well be due to the fact that their patients paradigm, preparation effects would be expected to had few sizeable lesions involving the medial region, be less as, unlike most other studies, we maintained especially in the critical left frontal group. the interval between stimuli in successive trials con- An effect which was found in our study but not in stant whatever the preparation interval, so as to the Aron et al. (2004) study was a highly signifi cant avoid differences between conditions in task-set dis- increase in errors compared with controls in the sipation which Meiran et al. (2000) had shown to be Inferior Medial group. This effect was found both an important variable for the size of the switch cost. for repeat as well as switch trials and it was found An increased preparation interval did, however, have for both preparation intervals, for the 1500 ms gap a major effect. It led to an overall reduction in reac- between cue and stimulus as well as for the 200 ms tion time on both switch and repeat trials, of 287 ms gap. The Inferior Medial region includes the orbital in the control group and a dramatic 613 ms in the prefrontal cortex. Patients with orbital prefrontal Superior Medial group. This suggests that the specifi c lesions make much higher error rates on memory paradigm used led patients, and possibly also con- tasks where the chance of error is increased by the trols, to prepare when to respond rather than how to use of category cueing (Turner et al., 2007). Such a respond according to a specifi c task-set, a possibility lesion may well lower the subjective cost of an error that has been considered by Meiran et al. (2000) and (Rolls, 2004); the subject is less careful. In a behav- Altmann (2005) in other task-switching situations. ioural task switching study where there are compet- It is relevant in this respect that the patient groups in ing task-sets such a tendency could well produce a the Aron et al. (2004) study had if anything smaller higher error rate. The effects may also be similar to preparation effects than the control group. This sup- the inability of patients to evaluate risk in gambling ports the possibility that there was a population dif- paradigms (Floden et al., 2008). ference between the two studies with medial patients, One contrasting effect that was found in the study and in particular that Superior Medial patients were of Aron et al. (2004) but where no analogous effect less well represented in the Aron et al. (2004) study. was found in our study is an increased error rate on Overall our study shows the importance of switch trials in their right frontal group. Aron et al. a number of frontally located processes in task (2004) link the error phenomenon they observed in switching paradigms (see Figure 2). These proc- their coarse-grain analysis with a reaction time effect esses, which were analysed within the framework they found only in their fi ne-grain analysis. This lat- of a Supervisory system approach (see Shallice and ter effect was that the larger the lesion in the region Burgess, 1996; Stuss et al. 1995) are, however, held of the pars opercularis in the right inferior frontal to have much wider relevance than task-switching gyrus the larger was the reaction time switch cost. per se. So-called task-setting may be conceived as They argue that this region contains a subsystem operating over the short-term or the longer term; involved in inhibiting erroneous responses (see also analogous effects to those obtained here in the left Aron and Poldrack, 2006). There was no analogous lateral prefrontal cortex have been found in a neu- effect in our study. We have previously considered ropsychological group study employing a rather that regions of the right lateral frontal cortex might different type reaction time task – that of Alexander be involved in monitoring task performance – in et al. (2005) (see also Stuss et al., 2002). The ener- reacting rapidly and accurately to multidimensional gisation effects found in the Superior Medial group stimuli (Stuss et al., 2002), in increasing prepara- are consonant with those found in a range of tion while a cued interval passes (Stuss et al., 2005), studies using other paradigms, such as Stuss et al. Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 83 Shallice et al. Figure 2 | The conceptual framework of the paper based on the Supervisory System framework (Shallice and Burgess, 1996; Stuss et al., 1995). The inner green boxes and yellow arrows represent on-line processes which (following learning) would be realised in contention scheduling. The outer blue boxes and arrows represent supervisory control processes. These include (in the early stages of learning) task rules operating as explicit if-then contingencies. The four patient groups discussed – left lateral, right lateral, superior medial and inferior medial – are held to have impairments in different supervisory processes. (2002, 2005), Alexander et al. (2005, 2007), Picton simplifi cation of set-switching processes known to et al. (2006, 2007), Shallice et al. (2008a); they had be critical for effective performance on the most the properties to be expected of an impairment to famous clinical test of prefrontal impairment, the energisation process. Finally, analogous error Wisconsin Card-Sorting. During the early years of effects to those obtained in the Inferior Medial the 21st century neuropsychological group stud- group attributable to impairments to an attentive- ies have demonstrated how many processes are ness process have not previously been found to our involved even in this simplifi ed paradigm! knowledge in reaction time paradigms with such CONFLICT OF INTEREST STATEMENT patients. 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Component processes in task switching. Cogn. Shallice, T., and Burgess, P. W. (1996). Domains of Psychol. 41, 211–253. supervisory control and the temporal organisa- Citation: Front. Neurosci. (2008) 2, 1: 79–85, Milner, B. (1963). Effects of different brain lesions tion of behaviour. Philos. Trans. R. Soc. Lond., B, doi: 10.3389/neuro.01.013.2008 on card sorting: the role of frontal lobes. Arch. Biol. Sci. 351, 1405–1412. Copyright © 2008 Shallice, Stuss, Picton, Alexander Neurol. 9, 90–100. Shallice, T., Stuss, D. T., Alexander, M. P., Picton, T. W., and Gillingham. This is an open-access article Monsell, S. (2003). Task switching. Trends Cogn. Sci. and Derkzen, D. (2008a). The multiple dimen- subject to an exclusive license agreement between 7, 134–140. sions of sustained attention. Cortex 44, 794–805. the authors and the Frontiers Research Foundation, Owen, A. M., Downes, J. J., Sahakian, B. J., Polkey, C. E., Shallice, T., Stuss, D. T., Picton, T. W., Alexander, M. P., which permits unrestricted use, distribution, and and Robbins, T. W. (1990). Planning and spatial and Gillingham, S. (2008b). Multiple effects of reproduction in any medium, provided the original working memory following frontal-lobe lesions prefrontal lesions on task-switching. Front. Hum. authors and source are credited. in man. Neuropsychologia 28, 1021–1034. Neurosci. 1, 1–12. Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 85 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Frontiers in Neuroscience Pubmed Central

Mapping Task Switching in Frontal Cortex Through Neuropsychological Group Studies

Shallice, Tim; Stuss, Donald T.; Picton, Terence W.; Alexander, Michael P.; Gillingham, Susan
Frontiers in Neuroscience , Volume 2 (1) – Jul 7, 2008
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FOCUSED REVIEW Mapping task switching in frontal cortex through neuropsychological group studies 1,2 3,4,† 3,4 Tim Shallice , Donald T. Stuss , Terence W. Picton , 5,6 3 Michael P. Alexander and Susan Gillingham Cognitive Neuroscience Sector, SISSA, Trieste, Italy. Institute of Cognitive Neuroscience, University College, London, UK. Rotman Research Institute, Baycrest, Toronto, ON, Canada. Departments of Psychology and Medicine, University of Toronto, Toronto, ON, Canada. Behavioral Neurology Unit, KS 253, Beth Israel Deaconess Medical Center, Boston, MA, USA. Harvard Medical School, Boston, MA, USA. Edited by: Robert T. Knight, University of California Berkeley, USA Reviewed by: This paper considers evidence provided by large neuropsychological group studies and meta-analyses Marcel Brass, Ghent of functional imaging experiments on the location in frontal cortex of the subprocesses involved in the University, Belgium carrying out of task-switching paradigms. The function of the individual subprocesses is also considered Leon Deouell, The Hebrew University of Jerusalem, Israel; in the light of analyses of the performance of normal subjects. Interdisciplinary Center for Neural Computation, Keywords: task-switching, switch-cost, anterior attentional system, frontal lobes, focal lesions, reaction time, Hebrew University, Israel errors, review Correspondence: Task switching is the area of research concerned of their shapes, in their colour and in how many with how a subject effects a change in the rules that there are. The clinician has another set of cards each govern how he or she responds to incoming stimuli. of which matches one of the key cards in colour, As an area of research, it is situated at the confl uence another in shape and so on. The patient is given one of two intellectual traditions. Within human exper- card at a time and has to place it with the key card Donald T. Stuss, with Tim imental psychology the study of cognitive control which has the same value on one of the dimensions. Shallice, Michael P. Alexander and Terence W. Picton began processes in normal subjects had seen important The correct match is dictated by a rule in operation, a formal collaboration on theoretical and experimental advances with the which can be to match by colour or by shape or by understanding the anterior work of Atkinson and Shiffrin (1968) and Schneider number. Every so often, the rule changes and the (frontal) attentional system over 15 years ago. Their and Shiffrin (1977). In complementary fashion, patient must discover the new rule by trial and error. approach was somewhat clinical neuropsychological investigations of lateral The cognitive component that is held to be impaired of a gamble. They started prefrontal functions had seen the development of by lateral frontal lesions in such a task is set shifting. with simple tasks to isolate simple processes, and tests for assessing executive functions including the The prototypical lateral prefrontal patient contin- increased task diffi culty and Wisconsin Card-Sorting (Milner, 1963) and the ues to use the previously relevant rule after the rule included other processes. Five Extra-Dimensional Intra-Dimensional Shift test of changes, even though he or she is being told on each years to screen and test all participants, and an equal the CANTAB battery (Owen et al., 1990). trial that the match is incorrect. What appears to be time for analysis, led to the As an example of a relevant clinical test, in lost is the ability to shift from seeing one attribute of identifi cation and localization Wisconsin Card-Sorting, the patient is presented the stimulus as being relevant to seeing another. of multiple attentional functions of the frontal lobes, with four cards on each of which there is a set of col- Performance on such clinical tests, however, is highlighted in eight separate oured shapes, all the shapes on any one card being very complex to analyse as there are many relevant publications since 2005. [email protected] identical. These four ‘key’ cards differ in the form cognitive components; Wisconsin Card-Sorting, Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 79 Shallice et al. for example, is affected in a qualitatively different moves consistently round the panes in turn; in the fashion by lesions in different parts of prefrontal two upper positions one task is operative, in the two cortex (Stuss et al., 2000). Moreover, such prefron- lower positions the other. Shortly thereafter, Meiran tal clinical tests are poorly adapted to the isolation (1996) introduced the procedure of the cueing of the by purely behavioural means of critical subcom- task by a distinctive warning signal occurring before ponents. So exploration of the performance of stimulus presentation; this enabled the relevant task normal subjects in these paradigms in the fashion to be varied randomly across trials. His paradigm typical of human experimental psychology, virtu- also used four panes of a square (see Figure 1). ally never occurred. Cognitive psychology and neu- A circle appears in one of the four panes. The ropsychology remained separate to their mutual response key to be pressed depends on which dimen- disadvantage. sion of the square is currently operative. If it is the In the 1990s, developments in so-called ‘task- vertical dimension, then the subject must make a switching’ experimental paradigms provided an top-bottom decision. If, instead, it is the horizontal avenue to overcome this separation between the dif- dimension then the subject must make a left–right ferent research traditions. These experimental para- decision. Which of the two tasks is operative on a digms are also beginning to allow us to distinguish particular trial is indicated through a warning sig- key components that contribute to performance in nal just before the stimulus. This means that the normal subjects, a key step in designing clinical tests task that is relevant can be randomised across trials which isolate different components of prefrontal unlike on the Rogers and Monsell design. functioning. The central cognitive component is This original group of experiments showed again set-shifting. Typically on each trial of a task- that if stimuli require different responses depend- switching experiment a stimulus is presented which ing on which task is operative, then trials where can be conceived of in one of a number of ways, the task switches from the previous trial are slower typically two (see Figure 1). Two studies essentially than those where the task repeats – the difference initiated the modern use of this type of paradigm between the two reaction times is known as the in normal subjects, those of Allport et al. (1994) switch effect. Moreover, the size of the switch effect and Rogers and Monsell (1995). In the second of generally decreases with an increase in the time these, subjects are presented with a letter and a digit. available in which the subject can prepare for the On some trials it is the letter which is relevant and specifi c task. This led Rogers and Monsell to pro- the subject must respond by pressing one of two pose that there is a top-down process of task recon- keys as quickly as possible as to whether the letter is fi guration which takes appreciable time and which a consonant or a vowel. On other trials the relevant is measurable by the decrease in switch effect. The part of the stimulus is the digit and the subject must paradigm appeared to provide a measure of the make the corresponding decision between odd and time to switch the dimension of the stimulus that even. In the Rogers and Monsell type of procedure should control behaviour. However, Allport et al. it is possible for the subject to predict in advance (1994) found that the switch effect was less when which task is relevant. The letter-digit combina- switching is into a more diffi cult task than into an tion appears in one of four panes (quadrants) of easier one, and argued that the switch effect was a square on each trial, and the position of the pair due instead to the need to inhibit a previously active Figure 1 | The experimental paradigm used in Shallice et al. (2008b). It is derived from the paradigm developed by Meiran (1996). 80 | July 2008 | Volume 2 | Issue 1 www.frontiersin.org Task switching and frontal lesions task-set or action schema, namely the control proc- noted in the inferior frontal gyrus, which was the ess that determines which task is undertaken. These largest region and the second most signifi cant in two types of explanation are not necessarily in con- the meta- analysis. In a second meta-analysis by fl ict, since they can be represented in different levels Buchsbaum et al. (2005), using many of the same of a relatively simple computational model of task- source studies, the set of distinct regions found by switching of the interactive activation type (Gilbert Derrfuss et al. (2005) essentially clustered into two and Shallice, 2002). On such a model, though, the prefrontal regions in each hemisphere. Buchsbaum switch cost as measured from reaction times, no et al. (2005) related their fi ndings to studies involv- longer corresponds quantitatively to the time that ing simple go/no-go tasks which require response the internal process of task reconfi guration takes. inhibition, and so suggested that the right frontal The incorporation of this type of experimental regions involved in the switching task might medi- paradigm into the behavioural literature led to an ate the inhibition of a previous response rule. explosion of studies. However, it became rapidly Given the potentially large number of prefron- realised that the paradigm, apparently so simple, tal regions suggested to be relevant from the func- had many complexities and involved many differ- tional imaging meta-analyses, neuropsychological ent processes (e.g. Monsell, 2003). For instance, studies of patients with focal frontal lesions seem Allport and Wylie (2000) discovered that presen- potentially important. In particular, they have the tation of a stimulus associated with a previously advantage that the impairment of a component active task-set would tend to reactivate it. At much can produce signature effects which help to nar- the same time Meiran et al. (2000) in somewhat row down hypotheses on the underlying functional confl icting fashion showed that task-set effects dis- processes. This is particularly the case if neuropsy- sipate over time. Despite these complications the chological studies of task-switching are carried out basic paradigms, together with the scores of sub- in conjunction with parallel studies of related para- sequent variants (e.g. Brass and von Cramon, 2002; digms, which potentially involve different combi- Braver et al., 2003), while complex, remain much nations of the same set of subsystems. simpler than the clinical tests from which they were A number of studies of task switching have originally derived. The complexity of the under- been carried out in neurological patients (see for lying processes, however, means that a return to instance, Mayr et al., 2006; Mecklinger et al., 1999), brain-based studies – functional neuro-imaging and but they have mainly involved small numbers of lesion studies – becomes critically necessary if the patients and so are not suitable for differential components are to be isolated. This is based on the localisation of subcomponents within prefrontal assumption that the different cognitive resources cortex. Our paper, that of Shallice et al. (2008b), involved are differently localised; if so the degree to is only the second study of task-switching with a which a particular cognitive resource is used in a sizeable number of patients with lesions in differ- given condition can be measured or the effect of its ent parts of prefrontal cortex, the fi rst being that loss discovered. of Aron et al. (2004). These two studies reported Interpretation of the functional imaging stud- fi ndings on 41 and 36 frontal patients respectively. ies, though, has not been straightforward. Many Aron et al. (2004), using a version of the Rogers– regions of cortex can be involved including pari- Monsell paradigm, found that both patients with etal and even the anterior insula cortex (Wager left frontal lesions and ones with right frontal et al., 2004). Even restricting attention to prefrontal lesions showed a signifi cantly larger switch cost cortex, some studies (e.g. DiGirolamo et al., 2001; than controls. The right frontal group also showed Luks et al., 2002) have obtained a large number of a dramatically elevated error rate on switch trials. activation maxima (>10) for relevant contrasts. In a The Shallice et al. (2008b) study which instead meta-analysis of task-switching, set-shifting and used the Meiran paradigm also found a left frontal stimulus-response reversal studies, Derrfuss et al. effect, but this time on errors. The major reaction (2005) found many regions in frontal cortex that time effect we found was a striking slowing on both showed relatively consistent effects; these include switch and repeat trials for patients with superior the inferior frontal junction – a region in the vicin- medial prefrontal lesions. An increase in errors in ity of the junction of the inferior frontal sulcus conditions where two tasks are potentially relevant and the inferior precentral sulcus – the inferior on any given trial was also found for patients with frontal sulcus itself, the inferior frontal gyrus, the inferior medial prefrontal lesions. medial superior frontal gyrus, the anterior cingu- The results of the two studies, like the functional late and the pre-SMA, activations generally involv- imaging studies, suggest that many parts of pre- ing both hemispheres. However the consistency frontal cortex can potentially play distinct roles in was only relative; for instance, of the seven pure task-switching. The divergence of results across the task- switching studies, in only two was a maximum two studies highlights the importance of differences Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 81 Shallice et al. in both the specifi c behavioural paradigms and For the fi ne-grain analyses Aron et al. (2004) methodology. used the percentage of particular gyri affected Both studies followed an appropriate two-level (superior, middle, and inferior frontal, orbital and method for localising patterns of defi cits, a fi rst- medial) and Shallice et al. (2008b) used difference pass coarse-grain and then a second-pass fi ne-grain between presence and absence of damage to partic- analysis. However, there was a difference between the ular frontal areas as defi ned by Petrides and Pandya two studies in both the coarse-grain and the fi ne- (1999). Both studies obtained similar results on the grain procedure employed. Thus, the coarse-grain coarse grain procedure as far as localisation is con- analysis used by Aron et al. (2004) involved compar- cerned in the left lateral frontal region. The fi ne- ing the effects of left frontal and right frontal lesions grain effects were consistent across the two studies, with control performance, but in the Shallice et al. with the Aron et al. (2004) study suggesting the (2008b) case a contrast was drawn between each middle frontal gyrus is the critical locus and the of four groups – left lateral, right lateral, superior Shallice et al. (2008b) one an area within it, 9/46v, medial and inferior medial – and the controls. In the but only as a trend, so there is no strong sugges- fi ne-grain analysis, Aron et al. (2004) took patients tion that either fi ne-grain procedure is inferior to with lesions to one or other frontal lobe and corre- the other. As far as the functional imaging meta- lated particular aspects of their performance with the analyses are concerned, in the Derrfuss et al. (2005) proportion of each gyrus that was damaged. More paper this region does not seem to be activated but simply, Shallice et al. (2008b) took each Petrides in the Buchsbaum et al. (2005) it is. There is thus and Pandya (1999) region of frontal cortex, and a convergence in the pattern of results across all compared the performance of patients with damage studies as far as the involvement of the left lateral to that region with that of all the other patients with prefrontal cortex is concerned, although there are frontal lesions. Both types of neuropsychological possible differences in the specifi c region involved. fi ne-grain analysis procedures make strong assump- Even the apparently similar left lateral effects tions and both are subject to the possible problem obtained in the two neuropsychological studies, of associated defi cits from lesions extending into though, differed in terms of the variable involved. In neighbouring regions. Therefore it is argued in our the Aron et al. (2004) study, patients with left fron- paper that it is critical that the fi ne-grain effects are tal lesions were slowed down more by a switch than consistent with the coarse-grain ones; if not, this were control subjects (a larger reaction time switch increases the possibility that fi ne-grain effects could cost). In contrast, in our study the increased switch result from associated defi cits. cost in patients with lateral frontal lesions occurred At the coarse-grain level Aron et al. (2004) com- for the errors rather than reaction time. However, pared left frontal and right frontal patients; the in addition, our paper points to the importance of Shallice et al. (2008b) study compared left lateral, a variable not considered in many task-switch- right lateral and two medial groups, superior and ing studies, namely the degree of practice. The left inferior. On all reaction time measures in our study lateral effect on errors concerned this variable; this it is only the Superior Medial group that is slowed. group showed a signifi cantly greater change in error Moreover, this slowing interacts with many variables rate from the fi rst half to the second half of the such as switch trials versus repeat trials, incongruent experiment than the controls or the other patient trials (where the two tasks lead to different response groups. The left lateral patients had over double the outcomes given the same stimulus) versus congruent error rate of the controls in the fi rst half, but were trials, harder versus easier tasks. This slowing with the virtually at the control rate in the second half. This Superior Medial lesions is closely analogous to effects is a similar effect to that shown by Alexander et al. found in a variety of other reaction time studies of (2005) in a study of the acquisition of a serial reac- frontal patients (e.g. Alexander et al., 2005, 2007; Stuss tion time paradigm. As in our study the left lateral et al., 2002, 2005). It fi ts with the idea that systems group were as fast as any other group including the in the Superior Medial region, probably the anterior control group from the start. However, they made cingulate and/or the pre-SMA (see Rushworth et al., signifi cantly more errors on the fi rst 100 trials 2002), are critical for the energisation (cognitive whereas on the next 400 trials their error rate was effort) necessary to activate operations not directly normal. This pair of studies point to the importance triggered in an overlearned fashion by perceptual and of the left lateral region in task-setting, the proc- motivational inputs. Both functional imaging meta- esses which allow task performance to move from analyses of task-switching produce activation of the a novel to a routinised state (see Stuss et al., 1995). anterior cingulate. Thus the four-group coarse-grain This is somewhat related to the argument presented procedure is likely to be preferable to the two-group by Aron et al. (2004) for their left frontal effects, coarse grain procedure as far as localisation of pre- namely that there is weaker endogenous control of frontal functional systems is concerned. task-set in left frontal patients and to the argument 82 | July 2008 | Volume 2 | Issue 1 www.frontiersin.org Task switching and frontal lesions of Derrfuss et al. (2005) that the left inferior fron- in avoiding capture errors (Reverberi et al., 2005), tal junction is involved in the updating of task rep- in judging the passage of time (Picton et al., 2006) resentations. Our position is that such updating is or in counting sounds (Shallice et al., 2008a). operating throughout the whole process of learning Impairments in performance monitoring (e.g. not the task and not just on a single trial. noticing that switching is required or had not been In addition to the difference concerning the carried out) might also explain the fi ndings in the Superior Medial fi ndings, other fi ndings also differed Aron et al. (2004) study. between the two studies. The Aron et al. (2004) study The differences in patterns of performance found effects of right inferior frontal gyrus lesions between the two studies may derive from the com- and the Shallice et al. (2008b) one of orbital frontal position of the two patient populations and from the lesions, neither of which was found in the other study. group study analysis methodology used. However, as This points to the importance of group composition, important a factor is likely to be the specifi c version and to the specifi c behavioural characteristics of the of task-switching employed. Our study differed from task in addition to the analysis procedures. Thus as that of Aron et al. (2004) and was unusual but far far as group composition is concerned, the absence from unique in not fi nding a reduction in switch of a Superior Medial effect in the Aron et al. (2004) costs with an increased preparation interval. In our study may well be due to the fact that their patients paradigm, preparation effects would be expected to had few sizeable lesions involving the medial region, be less as, unlike most other studies, we maintained especially in the critical left frontal group. the interval between stimuli in successive trials con- An effect which was found in our study but not in stant whatever the preparation interval, so as to the Aron et al. (2004) study was a highly signifi cant avoid differences between conditions in task-set dis- increase in errors compared with controls in the sipation which Meiran et al. (2000) had shown to be Inferior Medial group. This effect was found both an important variable for the size of the switch cost. for repeat as well as switch trials and it was found An increased preparation interval did, however, have for both preparation intervals, for the 1500 ms gap a major effect. It led to an overall reduction in reac- between cue and stimulus as well as for the 200 ms tion time on both switch and repeat trials, of 287 ms gap. The Inferior Medial region includes the orbital in the control group and a dramatic 613 ms in the prefrontal cortex. Patients with orbital prefrontal Superior Medial group. This suggests that the specifi c lesions make much higher error rates on memory paradigm used led patients, and possibly also con- tasks where the chance of error is increased by the trols, to prepare when to respond rather than how to use of category cueing (Turner et al., 2007). Such a respond according to a specifi c task-set, a possibility lesion may well lower the subjective cost of an error that has been considered by Meiran et al. (2000) and (Rolls, 2004); the subject is less careful. In a behav- Altmann (2005) in other task-switching situations. ioural task switching study where there are compet- It is relevant in this respect that the patient groups in ing task-sets such a tendency could well produce a the Aron et al. (2004) study had if anything smaller higher error rate. The effects may also be similar to preparation effects than the control group. This sup- the inability of patients to evaluate risk in gambling ports the possibility that there was a population dif- paradigms (Floden et al., 2008). ference between the two studies with medial patients, One contrasting effect that was found in the study and in particular that Superior Medial patients were of Aron et al. (2004) but where no analogous effect less well represented in the Aron et al. (2004) study. was found in our study is an increased error rate on Overall our study shows the importance of switch trials in their right frontal group. Aron et al. a number of frontally located processes in task (2004) link the error phenomenon they observed in switching paradigms (see Figure 2). These proc- their coarse-grain analysis with a reaction time effect esses, which were analysed within the framework they found only in their fi ne-grain analysis. This lat- of a Supervisory system approach (see Shallice and ter effect was that the larger the lesion in the region Burgess, 1996; Stuss et al. 1995) are, however, held of the pars opercularis in the right inferior frontal to have much wider relevance than task-switching gyrus the larger was the reaction time switch cost. per se. So-called task-setting may be conceived as They argue that this region contains a subsystem operating over the short-term or the longer term; involved in inhibiting erroneous responses (see also analogous effects to those obtained here in the left Aron and Poldrack, 2006). There was no analogous lateral prefrontal cortex have been found in a neu- effect in our study. We have previously considered ropsychological group study employing a rather that regions of the right lateral frontal cortex might different type reaction time task – that of Alexander be involved in monitoring task performance – in et al. (2005) (see also Stuss et al., 2002). The ener- reacting rapidly and accurately to multidimensional gisation effects found in the Superior Medial group stimuli (Stuss et al., 2002), in increasing prepara- are consonant with those found in a range of tion while a cued interval passes (Stuss et al., 2005), studies using other paradigms, such as Stuss et al. Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 83 Shallice et al. Figure 2 | The conceptual framework of the paper based on the Supervisory System framework (Shallice and Burgess, 1996; Stuss et al., 1995). The inner green boxes and yellow arrows represent on-line processes which (following learning) would be realised in contention scheduling. The outer blue boxes and arrows represent supervisory control processes. These include (in the early stages of learning) task rules operating as explicit if-then contingencies. The four patient groups discussed – left lateral, right lateral, superior medial and inferior medial – are held to have impairments in different supervisory processes. (2002, 2005), Alexander et al. (2005, 2007), Picton simplifi cation of set-switching processes known to et al. (2006, 2007), Shallice et al. (2008a); they had be critical for effective performance on the most the properties to be expected of an impairment to famous clinical test of prefrontal impairment, the energisation process. Finally, analogous error Wisconsin Card-Sorting. During the early years of effects to those obtained in the Inferior Medial the 21st century neuropsychological group stud- group attributable to impairments to an attentive- ies have demonstrated how many processes are ness process have not previously been found to our involved even in this simplifi ed paradigm! knowledge in reaction time paradigms with such CONFLICT OF INTEREST STATEMENT patients. 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Component processes in task switching. Cogn. Shallice, T., and Burgess, P. W. (1996). Domains of Psychol. 41, 211–253. supervisory control and the temporal organisa- Citation: Front. Neurosci. (2008) 2, 1: 79–85, Milner, B. (1963). Effects of different brain lesions tion of behaviour. Philos. Trans. R. Soc. Lond., B, doi: 10.3389/neuro.01.013.2008 on card sorting: the role of frontal lobes. Arch. Biol. Sci. 351, 1405–1412. Copyright © 2008 Shallice, Stuss, Picton, Alexander Neurol. 9, 90–100. Shallice, T., Stuss, D. T., Alexander, M. P., Picton, T. W., and Gillingham. This is an open-access article Monsell, S. (2003). Task switching. Trends Cogn. Sci. and Derkzen, D. (2008a). The multiple dimen- subject to an exclusive license agreement between 7, 134–140. sions of sustained attention. Cortex 44, 794–805. the authors and the Frontiers Research Foundation, Owen, A. M., Downes, J. J., Sahakian, B. J., Polkey, C. E., Shallice, T., Stuss, D. T., Picton, T. W., Alexander, M. P., which permits unrestricted use, distribution, and and Robbins, T. W. (1990). Planning and spatial and Gillingham, S. (2008b). Multiple effects of reproduction in any medium, provided the original working memory following frontal-lobe lesions prefrontal lesions on task-switching. Front. Hum. authors and source are credited. in man. Neuropsychologia 28, 1021–1034. Neurosci. 1, 1–12. Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 85

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