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Dissociating neural indices of dynamic cognitive control in advance task-set preparation: An ERP study of task switching
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
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Research Report
Dissociating neural indices of dynamic cognitive control in advance task-set preparation: An ERP study of task switching D.E. Astle a,⁎, G.M. Jackson b , R. Swainson a a
School of Psychology, University of Nottingham, University Park, Nottingham, NG7 2RD, UK Division of Psychiatry, University of Nottingham, UK
b
A R T I C LE I N FO
AB S T R A C T
Article history:
Switching between different tasks is associated with performance deficits, or ‘switch costs’,
Accepted 27 September 2006
relative to repeating the same task. Recent evidence suggests that response rather than task
Available online 7 November 2006
selection processes may be a major cause of switch costs [Schuch, S., Koch, I., 2003. The role of response selection for inhibition of task sets in task shifting. J Exp Psychol Hum Percept
Keywords:
Perform, 29 (1) 92–105]. Thus, switch costs are not incurred if on the preceding trial a task has
Task switching
been prepared for but no response required (a ‘no-go’ trial). We investigated the relationship
ERPs
between response selection and the subsequent preparation of an alternative task set.
Task set
While switch costs were absent following ‘no-go’ trials, ERP differences during the precueing
Cognitive control
interval showed that response selection has implications for subsequent task preparation as
Executive function
well as for task performance per se. The results are discussed in relation to the dissociation of intention versus action in behavioural control and the role of inhibition in switching between task sets. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
Although we are able to exert a great degree of control over our behaviour, it seems clear that the pattern and efficiency of the behaviour we exhibit is driven not only by our intentions but also by our recent behaviour. That is, there can be a dissociation of intention and action. Such dissociations are particularly common in patients with lesions of the frontal lobe: such patients' behaviour tends to be ‘perseverative’, with repetition of an action despite its no longer being appropriate (Milner, 1963; Sandson and Albert, 1984); some patients even comment that they are aware that their behaviour does not match their intention (Luria, 1966). A recent example is provided by the patient JR with a lesion of the supplementary eye field, described by Husain et al. (2003). Following a change in the rule governing the appropriate eye movement response
to a visual stimulus, JR tended to respond according to the previous rule before self-correcting the response to reflect the current rule; thus, while he clearly understood the task and was able to execute the appropriate response, his performance, at least initially, was still more in line with his previous actions than with his current intentions. Such instances are not restricted to those with brain damage, however. Duncan et al. (1996) described the phenomenon of ‘goal neglect’ among healthy individuals, especially those with low general intelligence, whereby instructions which are fully understood are simply not acted upon. In that study, subjects failed to switch attention to an alternative stream of visual input in a letter monitoring task, and Duncan et al. pointed out that it is in leaving behind a line of action to switch to another that such failures of intention to control action tend to occur.
⁎ Corresponding author. E-mail address: [email protected] (D.E. Astle). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.09.092
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The experimental ‘task-switching’ paradigm has been much used in recent years to examine the nature and limits of our dynamic control over behaviour. One of the most intriguing findings from this research is the phenomenon of the ‘residual switch cost’ (Rogers and Monsell, 1995). This is a cost to performance in terms of speed and/or accuracy which is incurred on trials where the rule governing responding differs from that used on the previous trial. Crucially, this cost cannot be overcome by voluntary preparation undertaken in advance of the target. The existence of the residual switch cost seems to indicate that performance of a task at least once is necessary to enable subsequent task performance to be efficient, and it appears to demonstrate that even in healthy individuals of high intelligence, our recent actions have an effect upon the efficiency of current performance which is inescapable, despite the strength of our current intentions.1 In their work on cognitive control using the task-switching paradigm, Mayr and Keele (2000) put forward evidence that the residual switch cost is at least partly due to the presence of what they termed ‘backward inhibition’. By this, they referred to the apparent inhibition of a task set, which was abandoned in order that an alternative task set could be used instead. The existence and persisting nature of such inhibition can be inferred from the slowing of response times seen when a recently abandoned task set is switched back to. Mayr and Keele argued that backward inhibition is an executive control process, driven by our intentions to perform a specific task from among a number of competing alternatives. Interestingly, although the presence of backward inhibition appeared to depend upon a high-level intention, such as a verbal representation in working memory, apparently it could not be overcome by an analogous voluntary mechanism. Instead, it survived long preparatory intervals during which the switched-to task was being prepared. This led Mayr and Keele to highlight the apparent functional dissociation between two levels of representation of a task set: a highlevel representation which is necessary for eliciting inhibition but apparently powerless to remove it, and a lowerlevel, ‘effective’ representation, which is itself modulated by inhibition. Despite the existence of such dissociations, most of the research into cognitive control has not attempted to distinguish between the effects of an intention to perform a particular task and the effects of previous performance. In all of the examples above, for instance, performance was measured after a switch from an alternative task (or goal, or action) which was actually carried out, so it is not clear whether the same behavioural effects would occur when a switch was required from an alternative course of action which was only intended rather than actually executed. However, a recent study by Schuch and Koch (2003) cleverly
1
There is evidence that, given appropriate circumstances which maximise the motivation to exercise voluntary control, the cost of switching may not affect a proportion of individual trials (De Jong, 2000); nevertheless, we seem to be limited in the degree to which we can exercise such control trial after trial because overall the switch cost remains, providing evidence of a persisting effect of the previous task upon current performance.
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dissociated preparation (intention) from performance in a task-switching paradigm and showed that it is actually the performance (i.e., selection of a task-specific response in relation to a target stimulus), not simply the preparation, of an alternative task on the previous trial which is responsible for the existence of switch costs on the current trial. Using a combined task-precueing and ‘go/no-go’ paradigm, they induced participants to prepare for a particular task on every trial, but on 25% of trials, without warning, a target was presented which required no response (a ‘no-go’ trial). While robust residual switch costs were observed following ‘go’ trials, they were absent following ‘no-go’ trials; further, there was evidence that these residual costs were the result of overcoming inhibition of the current task, and that this inhibition had been triggered by performance – but not by preparation alone – of the alternative task on the previous trial. So far we have seen that there can be a dissociation between our intentions and actions, in that these can exert differential effects upon subsequent performance. But this only covers the effect of the dissociation upon observed performance—what about the potential effects upon preparation processes? Schuch and Koch (2003) found no costs to performance following ‘no-go’ trials, but it is still an open question as to whether preparation for a subsequent task switch is affected by prior response selection. This question is difficult to address using behavioural measures alone as these necessarily require performance, but it can be tackled using the event-related potential (ERP) technique. In this study, we adopted Schuch and Koch's ‘go/no-go’ task-switching procedure to enable us to dissociate task switches away from alternative tasks which were only prepared and those from tasks which were also performed, and we used ERPs to provide indices of neural activity during the preparatory period separately from that occurring during task performance.
2.
Results
2.1.
Behavioural data
2.1.1.
Preparation
In order to ascertain whether or not participants were using the precue actively to prepare in advance of the target, switch costs at the short-CTI were compared to those at the long-CTI using a two-way ANOVA with factors CTI and switching (including only accurate ‘go’ trials preceded by accurate ‘go’ trials). This revealed a significant interaction [F(1,16) = 4.60, p = 0.048]. Switch-trial RTs were reduced from 754 ms in the short-CTI condition to 683 ms in the long-CTI condition, whereas repeat-trial RTs were reduced from 658 ms to 621 ms. There was a reduction in switch costs with preparation, therefore, from 96 ms in the short-CTI condition to 62 ms in the long-CTI condition. It may be the case that by blocking CTIs and placing the short-CTI trials at consistent positions within a block, subjects may have learned to predict their occurrence and become ‘lazy’, not bothering to prepare rapidly on predicted long-CTI trials (Altmann, 2004). Nevertheless, the significant interaction of the switch and CTI effects shows that on average subjects were making use
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repeat waveforms were compared with each other, separately for each of the response sequences (GGG and GNG).
2.3.
Precue-locked
2.3.1.
Late parietal positivity (LPP) for switches
We observed an increased positivity for switch compared with repeat trials over parietal electrodes. Fig. 2 shows that this effect was present for both the GGG and the GNG response sequences. There was a similar topography for the effect in each response sequence condition, clustered around Pz (electrode 62), where it reached consecutive significance relatively late in the epoch (488–646 ms for GGG, and 472– 716 ms for GNG).
2.3.2.
Fig. 1 – Behavioural data for task switching following a ‘go’ (ggg) and following a ‘no-go’ (gng). (A) Mean RT. (B) Mean proportion error.
Late frontal negativity (LFN) for switches
A precue-locked late right-frontal negativity for switch versus repeat trials was seen for the GGG but not the GNG condition. The effect was most prominent over right frontotemporal electrodes; Fig. 3A (right) shows the effect at FC6 (electrode 117). (N.B. Because its time course overlaps with that of the late parietal positivity, this negativity can also partially be seen in the topographical plot in Fig. 2A.) This effect resembles a superimposed slow wave somewhat like a contingent negative variation (CNV) for GGG-switch trials, with a negativity over frontal electrodes towards the end of the epoch (i.e., towards the time of target onset); consecutive significance at
of the CTI to engage in preparatory switching, and so we would expect to see electrophysiological correlates of such processing in the precue-locked ERPs of the long-CTI trials. Error data were analysed using Wilcoxon matched-pairs tests. There was no difference in the error switch cost between CTI conditions (Z = − 0.43, p = 0.670) (proportion error scores, short-CTI: switch trials, 0.16; repeat trials, 0.12; switch cost 0.04; long-CTI: switch trials, 0.14; repeat trials, 0.08; switch cost, 0.06).
2.1.2. costs
Relationship between response sequence and switch
Long-CTI trials (from which the ERPs waveforms were produced offline) were used to assess the relationship between response sequence and task switching using a twoway ANOVA. This revealed a significant interaction [F(1,16) = 20.308, p < 0.001] (Fig. 1A), with significant switch costs following a ‘go’ [F(1,16) = 33.125, p < 0.001] but not following a ‘no-go’ [F(1,16) = 2.123, p = 0.164]. The difference in performance between the trials following a ‘no-go’ and those following a ‘go’ was apparent both on repeats [F(1,16) = 25.618, p < 0.001] and switches [F(1,16) = 6.508, p = 0.021]. There were no significant switch-related differences for the error data [p > 0.05] (Fig. 1B), with the difference in repeat trials and switch trials between the two response sequences being nonsignificant [p > 0.05]. The pattern of these behavioural data replicates that of Schuch and Koch (2003, Experiment 1b).
2.2.
ERP results
The analysis of the ERP waveforms was split into two separate epochs: one following the precue (‘precue-locked’), and one following the target (‘target-locked’). In each case, switch and
Fig. 2 – The late parietal positivity, time-locked to precue-onset at 0 ms. The effect is present both following a ‘go’ (A) and following a ‘no-go’ (B). Waveforms are shown from electrode Pz, and topographical plots apply to the time period of consecutive significance at Pz. In the topographical plots, dark circles indicate significantly greater positivity for switch than repeat trials, and vice versa for dark squares. Arrows indicate the electrode site at which the accompanying waveforms were recorded.
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Fig. 3 – The late frontal negativity, time-locked to precue-onset at 0 ms. The effect is present following a ‘go’ (A; negative pole at FC6 and positive pole at P3) but not following a ‘no-go’ (B). Other plotting conventions as for Fig. 2.
FC6 was reached from 752 to 1148 ms. There was an associated positivity with a similar time course (reaching consecutive significance from 788 to 904 ms and 952–1080 ms) over left parieto-occipital electrodes; Fig. 3A (left) shows P3 (electrode 53), located within this positive cluster. While the time period of this effect overlapped with the late parietal positivity, its topography and time course were clearly distinguishable. (Fig.
3A shows the topography of the late frontal negativity towards the end of the epoch, during which time it is not overlapped by the late parietal positivity.) More critically, these components dissociated according to response sequence; while the late parietal positivity was present for both response sequences, the late frontal negativity was present in the GGG response sequence but completely absent in the GNG response
Fig. 4 – The centro-parietal positivity, time-locked to target-onset at 0 ms. The effect is present both following a ‘go’ and following a ‘no-go’, with slightly differing topography. The positivity is shown at CPz following a ‘go’ (A, left) and at electrode 86 following a ‘no-go’ (B, left). Following a ‘no-go’, a corresponding negativity is also present with a similar time course, shown in the waveform from AFz (B, right); this was not present following a ‘go’ (A, right). Other plotting conventions as for Fig. 2.
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condition (Fig. 3B). The pattern of waveforms on switch and repeat-trial types across the response sequence conditions is interesting in that it appears (see Fig. 3) that, rather than there being an additional frontal negativity for GGG-switch trials, there may in fact be a reduced negativity (or additional positivity) for GGG-repeat trials relative to all other trial types (and vice versa for the left parieto-occipital positivity). However, additional analyses conducted at electrodes P3 and FC6 showed no significant effect of response sequence on either repeat trials or switch trials.
2.4.
Target-locked
2.4.1.
Centro-parietal positivity (C-PP) for repeats
A centro-parietal positivity for repeats was present in both GGG and GNG sequences. Fig. 4A (left) shows the centroparietal positivity over CPz (electrode 55) in the GGG condition (312–568 ms), and Fig. 4B (left) over electrode 86 in the GNG condition (448–580 ms); comparison of their respective topographies indicates that the GNG centropositivity was shifted in a right-posterior direction. Fig. 4B (right) also shows an associated negativity over left frontal sites in the GNG condition (344–504 ms at AFz, electrode 16), which was absent in the GGG condition (Fig. 4A, right). (N.B. In the topographical plot for 5A, the lateral positive shift (see below and Fig. 5A) is also observable over right hemisphere electrodes.)
2.4.2.
Lateral positive shift
This component was located over right-temporal sensors, starting relatively early in the epoch, with a gradually increa-
Fig. 5 – The lateral positive shift, time-locked to target-onset at 0 ms. The effect is present following a ‘go’ (A), but not following a ‘no-go’ (B). Other plotting conventions as for Fig. 2.
sing switch-versus-repeat difference (with switch more positive than repeat), reaching consecutive significance at 388– 496 ms, 572–640 ms, 716–756 ms and 932–996 ms at T8 (electrode 109). Fig. 5 demonstrates that this was present in the GGG condition but absent in the GNG condition. The lateral positive shift has been reported previously. Swainson et al. (2003) observed something very similar in the target-locked epoch of a predictable task-switching design, where the component was visible as a positivity at lateral electrodes over both hemispheres. Those authors concluded that while this might appear to be a distinct component in its own right, it is likely the result of re-baselining on presentation of the target. In the present data set this is also plausible, with electrode T8, where the target-locked lateral positive shift is clearly seen, also showing a clear precue-locked negativity for switch trials, part of the precue-locked late frontal negativity (shown in Fig. 3).
3.
Discussion
This study explored the effect of dissociating task performance from task preparation upon subsequent task switching. Using a precued design, with ‘no-go’ responses required on 25% of trials, residual switch costs were found to be absent following a trial on which a task was only prepared and not actually performed (i.e., following a ‘no-go’ response). Of more importance, preparatory activity evident in ERPs occurring before target onset was shown to be affected by previous task performance. While a switch-related late parietal positivity (LPP) was present in the precueing interval following both ‘go’ and ‘no-go’ responses, a switch-related late frontal negativity (LFN) in the same interval was only found following a ‘go’ response, with repeat and switch not significantly differing following a ‘no-go’. A target-locked centro-parietal positivity for repeats was present following both ‘go’ and ‘no-go’ responses. We replicated the results of Schuch and Koch (2003), showing that switch costs are absent when a ‘no-go’ response was made on the previous trial (i.e., the task was prepared, but not performed). Our data showed the same pattern as in that original study, with the absence of switch costs following a ‘no-go’ apparently being due both to an additional cost on repeat trials and a benefit on switch trials. A number of factors could explain the additional cost on repeat trials, for instance: elimination of task-set priming (Allport and Wylie, 1999); addition of a ‘restart cost’ (Allport and Wylie, 2000; Altmann, 2002); and lack of priming of the ‘go’ response (Schuch and Koch, 2003). We suggest a further alternative, which is that in this type of paradigm, switching may occur on two levels: the ‘task-set’ level (consonant/vowel versus upper/ lower case judgements) and the ‘response-set’ level (‘go’ versus ‘no-go’ responding). In order to produce the infrequent ‘no-go’ response, subjects probably have to inhibit the tendency to respond immediately upon target onset (the ‘go’ response set). When a subsequent ‘go’ response is required, therefore, such inhibition (if it persists) would have to be overcome, potentially producing a cost to performance (see Koch et al., 2004, for evidence of persisting inhibition of response mode). Whichever of these explanations is true, it
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ought to affect both task-switch and task-repeat trials in a GNG sequence. But critically its effect might be counteracted on switch trials by an additional relative benefit—i.e., absence of the need to overcome task-set inhibition when no response was selected on the previous trial. Clearly there was an additional benefit on switch trials in this study because there was a significant benefit to performance of a preceding ‘no-go’ on switch trials. In addition, the error data ought to be sensitive to effects of switching ‘task set’ rather than those of switching ‘response set’ (in this case, switching from not responding to responding), and these showed effects only on switch trials. Thus, being in the inappropriate ‘task set’ would lead to erroneous responses (responses being driven by the inappropriate task on incongruent trials), so eliminating the need to switch tasks should reduce error rates on switch trials. But inappropriately retaining a tendency to inhibit immediate responding (or remaining in a ‘no-go’ response set) ought to lead only to delayed responding rather than increased errors. While nonsignificant, this is the pattern (i.e., with a larger difference on switch trials than repeat) that we observe in the error data.
3.1. ERP switch effects modulated by prior task performance The important finding in the current study is that the neural correlates associated with preparing to switch versus repeat the current task on the next trial are different following a ‘nogo’ versus a ‘go’ response. The LFN ERP, seen during the preparatory interval for switch trials in comparison to repeat trials, was present only for the GGG sequence and not for the GNG sequence. Therefore, it is clear that when a prepared task is performed (used to produce a task-specific response to a target stimulus) as opposed to only being prepared, the implications of this are evident during subsequent preparation of an alternative task set on the next trial (as shown by the LFN effects) as well as at the time of performance (as shown by the behavioural effects). Although this issue was not explicitly addressed in the current study, there is reason to speculate that the specificity of the LFN effect to GGG sequences relates to the need to overcome inhibition of the switched-to task. Schuch and Koch (2003) showed that following a ‘no-go’ trial, not only were switch costs removed, but also that the specific cost of switching back to a task abandoned on the previous trial (i.e., the cost of performing an ABA relative to a CBA task sequence) was removed. They suggested that ‘backward inhibition’ (Mayr and Keele, 2000) of a previous task is triggered at the time of response selection for the new task and not during the preceding preparation period. The consequence of this would be that following a ‘no-go’ trial (on which a new task was only prepared and never actually performed), there would be no cost of switching back to the original task (which had been abandoned on the previous, ‘no-go’, trial) because there would be no residual inhibition of that task to be overcome. In terms of the dissociation of preparatory and performance-based processes upon task inhibition, then, our behavioural data support the view that inhibition is neither triggered by preparation (switch costs
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being absent following a ‘no-go’ response), nor can it be entirely overcome during the preparation interval (residual switch costs remaining robustly in the GGG condition despite a substantial precue-target interval). However, the ERP findings from our study – specifically, the presence of the precue-locked LFN for GGG but not GNG sequences – suggest that this residual inhibition may nevertheless be encountered during the time when one prepares to return to the previous task set, and not only at the time of preparing a specific response appropriate to that task. Although we found no significant differences between the two response sequences (GGG versus GNG) on either repeat or switch trials in terms of the LFN waveforms, Fig. 3 does suggest that the presence of a task-switch effect on GGG trials may be due to a reduced negativity on GGGrepeat trials at least as much as an increased negativity on GGG-switch trials. How can this be squared with the explanation offered above that the effect on GGG trials may be due to encountering inhibition of a task set on a GGG-switch trial? As discussed above, it may be that the same need to inhibit a contradictory tendency (or ‘set’) applies to both tasks and types of response. Therefore, performing both repeat and switch trials in the GNG condition may entail encountering response suppression (but neither task suppression), whereas – if the GNG sequence does indeed prevent the need for persisting inhibition of the switched-to task – only switch trials in the GGG condition would entail encountering task suppression (but not response suppression). Thus, it is consistent with our hypothesis (that the LFN reflects processes to do with encountering persisting inhibition) that a similar degree of negativity should be present in both GNG-trial types as in GGG-switch trials, and more than on GGG-repeat trials (when neither persisting response suppression nor task suppression are encountered). As yet, we cannot be specific as to a potential role of the LFN. For instance, if it is related to the presence of persisting ‘backward inhibition’ upon the (recently abandoned) switched-to task, it may index an active attempt to overcome inhibition, a passive result of encountering inhibition during preparation, or the recruitment of resources needed to overcome inhibition once the target is presented. The effect resembles a slow wave (negative over frontal scalp) superimposed upon the underlying waveforms on switch trials in the GGG condition. Such slow waves have been associated with the allocation of cognitive resources to a particular processing module in the cortex (Rosler et al., 1997). One such slow wave is the contingent negative variation (CNV)—i.e., a negativity which increased towards the time of onset of a predictable stimulus (Walter et al., 1964). An increased frontal CNV has been shown on trials where subjects made a particular ‘effort’, in anticipation of a target, to perform well and has been interpreted as a reassignment of resources (Falkenstein et al., 2003). The LFN seen here on switch trials relative to repeat, following a ‘go’, may similarly represent the recruitment of resources, ready to deal with generating a response to a difficult type of target—i.e., one which it is known will involve a switch in task and the overcoming of persisting backward inhibition. We have also seen this effect recently (Mueller, Swainson and Jackson, submitted) when
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participants switched between tasks which shared response meanings (i.e., where responses were bivalent with respect to tasks—a left-hand response, for example, being used to indicate either that a letter was a vowel or that a digit was an odd number) but not between tasks which used separate sets of (univalent) responses. If the LFN does relate to overcoming inhibition of a recently abandoned switched-to task, it may index the need specifically to overcome inhibition of the response meanings associated with that task.
3.2.
ERP effects independent of prior task performance
A commonly reported ERP effect associated with task switching is a late positivity over parietal scalp (Barcelo, 2003; Jackson et al., 2001; Karayanidis et al., 2003; Kieffaber and Hetrick, 2005; Nicholson et al., 2005; Rushworth et al., 2002; Swainson et al., 2003, 2006) and this effect was replicated in the present study. Like the LFN effect discussed above, this late parietal positivity (LPP) occurred prior to the target onset during the precue-target interval, but, unlike the LFN, it was evident following both ‘go’ and ‘no-go’ responses. Thus, it appears to index a process which is independent of switching between performing alternative tasks per se. This component appears to occur at the time within a trial when ‘endogenous’ reconfiguration would be expected to take place—i.e., that aspect of task switching which, it is postulated, can be performed prior to target-onset, given certain necessary conditions such as advance information and the expectation of sufficient time in which to use it, but which otherwise occurs after target onset (Rogers and Monsell, 1995). Thus, the component has been seen both prior to target onset (Nicholson et al., 2005; Rushworth et al., 2002) and subsequent to target onset (Jackson et al., 2001; Swainson et al., 2003, 2006). In fact, its timing seems to be relatively consistent in relation to the timing of the task cue in these studies. Therefore, we have suggested that it reflects processing driven by a cue signaling the need for a task switch (Swainson et al., 2006). (N.B. In the study by Karayanidis et al., 2003, tasks switched predictably and the positivity for task switching was observed to be timelocked to the preceding response, but a task-cueing grid was present throughout and the positivity may in fact have been time-locked to participants’ attending to the grid. The effect may prove eventually not to be specific to the processing of an external cueing stimulus, but none of the studies so far have shown this to be the case.) The component has a relatively long latency after precue-onset of around 500–700 ms and in our previous study, where response times were very fast (usually <500 ms), we showed that this component need not precede the response; therefore, we suggested that it may index a nonobligatory switch-related process, such as a shift from controlled to relatively automatic task processing rather than an obligatory reconfiguration allowing performance of the appropriate task (Swainson et al., 2006). The current data are consistent with the effect being related to processing of a task cue. Whether this is generated exclusively by parietal regions, or whether this component is composed of the activity of a number of generators (for instance, including the prefrontal cortex; Brass et al., 2005) remains unclear at this stage. The similarity of the effect between the GGG and the
GNG response sequence conditions indeed implies that the effect reflects switching at the level of the abstract task set, goal or intended task put in place by the task cue on the previous trial whether or not that task was actually performed, rather than at a level corresponding to the ‘effective task’ and which would therefore be dependent upon recent performance (Mayr and Keele, 2000). A second component observed regardless of whether there was a switch in the performed task was the target-locked centro-parietal positivity (C-PP) for task repetition. While this was observed in both the GGG and GNG response sequence conditions, there was nevertheless a difference in topography, although to what extent this difference may reflect different psychological process occurring following a ‘go’ and following a ‘no-go’ response is unclear. We have previously observed this component in the target-locked epoch of a predictable task-switching paradigm (Swainson et al., 2003, 2006) and have interpreted it as indexing the current ‘strength’ of taskspecific stimulus evaluation, such that when a task is performed with the knowledge that it will immediately be repeated, it may be actively consolidated, or maintained, thereby allowing improved performance on the subsequent trial. Wylie et al. (2003) also observed the effect in a predictable task sequence with three trials in each run (…AAABBBAAA…) where they saw its amplitude increase along with the number of repetitions. They attributed this increase to a decreasing conflict between the task sets with task repetition, consistent with a parallel improvement in performance. Similarly, Barcelo (2003) saw that the component increased with task repetition and with improved performance on repeat trials. We have previously found the effect to be specific to a predictable task sequence, with no C-PP evident for repeat versus switch trials in random task sequences (Swainson et al., 2006), and suggested that this was because a task set could potentially be more rapidly consolidated when subsequent repetitions were predictable. The data from the present study and those from Barcelo (2003) indicate, however, that after more than one repeat trial, the effect can be observed without repeat trials being predictable (in the current study, all repeat trials occurred after at least two previous trials with the same task, which was not the case in our previous study). This implies that the same mechanism of task-set consolidation which previous data suggests is available with predictable task repeats still occurs (only more gradually) even without predictability, or at least that the two mechanisms have the same effect in terms of increasing the C-PP. Unlike these previous studies, though, in the present data the component was not seen to have direct behavioural consequences. That is, in the previous studies an increased C-PP was seen to coincide with a behavioural repetition benefit, but in the GNG condition of the current study, despite there being a switch/ repeat modulation in the C-PP, there was no behavioural ‘repeat benefit’ (or in other words, no switch cost). We suggest that this may be because the component indexes consolidation at a perceptual level, producing speeded stimulus evaluation on repeat trials (Kok, 2001). This would not necessarily also lead to a speeding of response times, especially if slowing for another reason – such as the need to overcome a ‘no-go’ response tendency – renders speeding of a parallel process impotent to affect response times. It is unclear
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why the topography of this effect differed between the response sequence conditions. It is plausible that the same consolidation process occurs both following a ‘go’ and ‘no-go’ response yet reaches significance over different electrodes, perhaps due to there being additional processes occurring following a ‘no-go’ which affect its observed scalp distribution such that its negative pole becomes apparent over frontal sites. Conversely, the difference in topographies could indicate that genuinely different psychological processes occur in the two response sequence conditions, for instance that following a ‘no-go’ there is a qualitative difference in the task-consolidation mechanism. As yet either alternative is possible.
3.3. The relationship between response selection and task-set preparation While the LPP seems to be relatively independent from whether or not subjects switch between performing (as opposed to only preparing) alternative tasks on successive trials, this study provides clear evidence that the effects of previous action are encountered during preparation for the subsequent trial (in the form of the LFN). The combination of task-switching and ‘no-go’ paradigms demonstrates that it is possible to dissociate neural indices of cognitive control in task switching, between those that relate to changing performance per se and those involved in changing intended performance.
4.
Experimental procedures
4.1.
Subjects
Eighteen healthy right-handed participants completed a combined EEG and behavioural experiment. Fourteen were female, with an average age of 23.63 ± 3.55 (SD) years. Participants provided written informed consent. Eighteen subjects were included in the EEG data set, and 17 in the behavioural (due to a technical error meaning that some behavioural data were lost for one subject).
4.2.
4.3.
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Experimental design
After practice at performing each task separately and switching between the two (until each participant had achieved 90% accuracy on a ‘switching’ block), the participant performed 17 blocks of 60 trials. Each block contained both short (100 ms) and long (1200 ms) CTI (cue-target interval) trials, with the short-CTI trials being arranged in a short run of 7–9 trials at the beginning of each block, and again at the midpoint. (These trials were blocked rather than interleaved to avoid introducing an additional factor of switching between long and short trials.) Participants were not informed of this pattern, only that the time to prepare could be short or long on each trial. Each cue (in fact, a ‘precue’, as all cues were presented in advance of targets) was on the screen for the full duration of the CTI, and each target was displayed on the screen for 160 ms. Following each target there was a response window of 800 ms, which ended upon response execution. This was followed by a subsequent fixed-length response window of either 800 or 1900, depending upon CTI, such that each trial time lasted 2000 ms + RT regardless of the preparation interval. ERPs were generated only from long-CTI trials which were responded to accurately and which were also preceded by two other accurate long-CTI trials. The short-CTI trials were included to provide subjects with incentive to engage in preparation immediately on presentation of the precue and to provide reaction time information for this to be assessed. As in Schuch and Koch's (2003) study, there were no repeats of ‘nogo’ trials; thus, trials fell at the end of either a go/go/go (GGG) or a go/no-go/go (GNG) ‘response sequence’ triad. But in a modification to their original design, we imposed another constraint that the task pertaining to the third trial in these triads also pertained to the first (see Fig. 6). Thus, there was either a switch back to the task performed two trials ago (switch triads: ABA, where A and B are alternative task sets) or no switch at all within the triad (repeat triads: AAA). The reason for this was in the cases where trial n − 1 required a ‘no-
Behavioural task
Each trial was precued by a fixation cross which changed colour prior to the onset of the imperative stimulus. The colour of the target and fixation cross indicated the task set to be prepared (the assignment of colour to task being counterbalanced across participants). Each target stimulus was selected from a set of vowels and consonants in upper and lower case (A,E,I,U,a,e,i,u,G,B,T,D,g,b,t,d). On a ‘go’ trial a single letter was presented, upon which either of two task sets could be implemented, responses being made with a two-button serial mouse: discrimination of the letter as either a vowel (left button) or a consonant (right button) or discrimination of the letter as lower (left button) or upper (right button) case. On a quasi-randomly selected 25% of trials, a task-irrelevant stimulus (#) appeared instead of the usual target, to which no response could be made (‘no-go’ trials). These trials never appeared in direct succession.
Fig. 6 – The four specific trial sequences used in the main analyses: two repeat and two switch sequences, two with a ‘no-go’ on trial n − 1 and two with a ‘go’ on trial n − 1. White or black precues and stimuli indicate one or other of the two tasks ought to be performed on that trial.
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go’ response, neither the ‘switch’ nor the ‘nonswitch’ triads would entail a switch at the level of performance. The four types of trial included in the analysis were therefore GGGswitch, GGG-repeat, GNG-switch and GNG-repeat. In order to ensure that enough of the these specific sequences were produced, the trials were generated in a quasi-random fashion using a software package (Microsoft Excel), giving on average 6 of each trial type per block of 60 trials. (N.B. For the analysis of the behavioural preparation effect, such a strict criterion of trial type did not need to be employed. Instead, trials were classified as being either switch (BA) or repeat (AA) trials, where both the current and preceding trial required a ‘go’ response (GG)).
4.4.
EEG recording, processing and ERP formation
EEG was recorded throughout each block, using a 128-channel electrical geodesic net (Electrical Geodesics, Inc.; Tucker et al., 1994) and digitised at 250 Hz. The recording was performed with a hardware bandpass filter of 0.01 Hz to 100 Hz. Before recording impedance on each of the 128 electrodes was reduced to <50 kΩ. Precue-locked and target-locked epochs were created, with each epoch starting 100 ms before onset of the respective stimulus and ending 1200 ms (precue-locked) or 1000 ms (target-locked) afterwards. Segments were rejected if contaminated by eye-blinks/movements (indicated by EOG activity greater that 70 μv), or an error of response (incorrect response or omission of response), or if the trial followed an error on either of the two previous trials. Trials containing voltage amplitudes greater than 200 μv or a change greater than 100 μv were also removed. Channels that were bad for more than 20% of trials were rejected. With this rationale, approximately on average 20% of trials were removed within the precue-locked epoch, and 35% of trials from the targetlocked epoch (owing to excessive ocular artefacts). The data from each participant included in any ERP waveform had > 30 waveforms from which to form averages. Based upon this rationale all 18 participants produced sufficient waveforms to be included in the precue-locked comparisons, whereas only 15 participants produced sufficient to be submitted to the target-locked comparisons. In summary, ERPs were formed from long-CTI trials only, were free from behavioural errors, and had been preceded by two appropriate (see Fig. 6), accurate and long-CTI trials. The specific segments were average-referenced to a standard adult 128-electrode montage. Epochs were baselinecorrected for the first 100 ms, before the onset of the stimulus, and epochs lasted for a subsequent 1200 ms or 1000 ms. Switch and repeat waveforms were explicitly compared (all statistics conducted using MATLAB v5, scripts developed in-house), by means of a T-test for every 4 ms sample. Two waveforms were counted as significantly different if this significance persisted for more than 10 samples, and two temporal regions of consecutive significance were counted as a continuation of the same difference if the gap between them was fewer than 10 samples. This approach is based upon Rugg et al. (1993) and was used by Swainson et al. (2003, 2006); Jackson et al. (2001). All ERP comparisons that reached consecutive significance are included in the Results section.
Acknowledgment This study was supported by a BBSRC grant to GMJ.
REFERENCES
Allport, A., Wylie, G., 1999. Task-switching: positive and negative priming of task-set. In: Humphreys, G.W., Duncan, J., Treisman, A.M. (Eds.), Attention, Space and Action: Studies in Cognitive Neuroscience. Oxford Univ. Press, Oxford, UK, pp. 273–296. Allport, A., Wylie, G., 2000. ‘Task-switching’, stimulus-response bindings and negative priming. In: Monsell, S., Driver, J. (Eds.), Attention and Performance XV: Conscious and Nonconscious Information Processing. MIT Press, Cambridge, MA, pp. 421–452. Altmann, E.M., 2002. Functional decay of memory for tasks. Psychol. Res. 66 (4), 287–297. Altmann, E.M., 2004. The preparation effect in task switching: carryover of SOA. Mem. Cogn. 32 (1), 153–163. Barcelo, F., 2003. The Madrid card sorting test (MCST): a task switching paradigm to study executive attention with event-related potentials. Brain Res. Brain Res. Protoc. 11 (1), 27–37. Brass, M., Ullsperger, M., Knoesche, T.R., von Cramon, D.Y., Phillips, N.A., 2005. Who comes first? The role of the prefrontal and parietal cortex in cognitive control. J. Cogn. Neurosci. 17 (9), 1367–1375. De Jong, R., 2000. An intention-activation account of residual switch costs. In: Monsell, S., Driver, J. (Eds.), Control of Cognitive Processes: Attention and Performance XVIII. MIT Press, Cambridge MA, pp. 357–376. Duncan, J., Emslie, H., Williams, P., Johnson, R., Freer, C., 1996. Intelligence and the frontal lobe: the organization of goal-directed behavior. Cogn. Psychol. 30 (3), 257–303. Falkenstein, M., Hoormann, J., Hohnsbein, J., Kleinsorge, T., 2003. Short-term mobilization of processing resources is revealed in the event-related potential. Psychophysiology 40 (6), 914–923. Husain, M., Parton, A., Hodgson, T.L., Mort, D., Rees, G., 2003. Self-control during response conflict by human supplementary eye field. Nat. Neurosci. 6 (2), 117–118. Jackson, G.M., Swainson, R., Cunnington, R., Jackson, S.R., 2001. ERP correlates of executive control during repeated lanaguage switching. Biliguilism: Lang. Cogn. 4 (2), 169–178. Karayanidis, F., Coltheart, M., Michie, P.T., Murphy, K., 2003. Electrophysiological correlates of anticipatory and poststimulus components of task switching. Psychophysiology 40 (3), 329–348. Kieffaber, P.D., Hetrick, W.P., 2005. Event-related potential correlates of task switching and switch costs. Psychophysiology 42 (1), 56–71. Koch, I., Gade, M., Philipp, A.M., 2004. Inhibition of response mode in task switching. Exp. Psychol. 51 (1), 52–58. Kok, A., 2001. On the utility of P3 amplitude as a measure of processing capacity. Psychophysiology 38 (3), 557–577. Luria, A.R., 1966. Higher Cortical Functions in Man (B. Haigh, Trans), 2nd ed. Basic Books, Inc., New York. revised and expanded ed. Mayr, U., Keele, S.W., 2000. Changing internal constraints on action: the role of backwards inhibition. J. Exp. Psychol. Gen. 129, 4–26. Milner, B., 1963. Effects if different brain lesions on card sorting. Arch. Neurol. 9, 100–110. Nicholson, R., Karayanidis, F., Poboka, D., Heathcote, A., Michie, P.T., 2005. Electrophysiological correlates of
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anticipatory task-switching processes. Psychophysiology 42 (5), 540–554. Rogers, R.D., Monsell, S., 1995. The costs of a predictable switch between simple cognitive tasks. J. Exp. Psychol. Gen. 124, 207–231. Rosler, F., Heil, M., Roder, B., 1997. Slow negative brain potentials as reflections of specific modular resources of cognition. Biol. Psychol. 45 (1–3), 109–141. Rugg, M.D., Doyle, M.C., Melan, C., 1993. An event-related potential study of the effects of within-and across-modality word repetition. Lang. Cogn. Processes 8 (4), 357–377. Rushworth, M.F., Passingham, R.E., Nobre, A.C., 2002. Components of switching intentional set. J. Cogn. Neurosci. 14 (8), 1139–1150. Sandson, J., Albert, M.L., 1984. Varieties of perseveration. Neuropsychologia 22 (6), 715–732. Schuch, S., Koch, I., 2003. The role of response selection for inhibition of task sets in task shifting. J. Exp. Psychol. Hum. Percept. Perform. 29 (1), 92–105.
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Swainson, R., Cunnington, R., Jackson, G.M., Rorden, C., Peters, A.M., Morris, P.G., Jackson, S.R., 2003. Cognitive control mechanisms revealed by ERP and fMRI: evidence from repeated task-switching. J. Cogn. Neurosci. 15 (6), 785–799. Swainson, R., Jackson, S.R., Jackson, G.M., 2006. Using advance information in dynamic cognitive control: an ERP study of task-switching. Brain Res. 61–72. Tucker, D.M., Liotti, M., Potts, G.F., Russell, G.S., Posner, M.I., 1994. Spatiotemporal analysis of brain electrical fields. Hum. Brain Mapp. 1, 134–152. Walter, W.G., Cooper, R., Aldridge, V.J., McCallum, W.C., Winter, A.L., 1964. Contingent negative variation: an electric sign of sensorimotor association and expectancy in the human brain. Nature 203, 380–384. Wylie, G.R., Javitt, D.C., Foxe, J.J., 2003. Task switching: a high-density electrical mapping study. NeuroImage 20 (4), 2322–2342.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s
Research Report
Dissociating neural indices of dynamic cognitive control in advance task-set preparation: An ERP study of task switching D.E. Astle a,⁎, G.M. Jackson b , R. Swainson a a
School of Psychology, University of Nottingham, University Park, Nottingham, NG7 2RD, UK Division of Psychiatry, University of Nottingham, UK
b
A R T I C LE I N FO
AB S T R A C T
Article history:
Switching between different tasks is associated with performance deficits, or ‘switch costs’,
Accepted 27 September 2006
relative to repeating the same task. Recent evidence suggests that response rather than task
Available online 7 November 2006
selection processes may be a major cause of switch costs [Schuch, S., Koch, I., 2003. The role of response selection for inhibition of task sets in task shifting. J Exp Psychol Hum Percept
Keywords:
Perform, 29 (1) 92–105]. Thus, switch costs are not incurred if on the preceding trial a task has
Task switching
been prepared for but no response required (a ‘no-go’ trial). We investigated the relationship
ERPs
between response selection and the subsequent preparation of an alternative task set.
Task set
While switch costs were absent following ‘no-go’ trials, ERP differences during the precueing
Cognitive control
interval showed that response selection has implications for subsequent task preparation as
Executive function
well as for task performance per se. The results are discussed in relation to the dissociation of intention versus action in behavioural control and the role of inhibition in switching between task sets. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
Although we are able to exert a great degree of control over our behaviour, it seems clear that the pattern and efficiency of the behaviour we exhibit is driven not only by our intentions but also by our recent behaviour. That is, there can be a dissociation of intention and action. Such dissociations are particularly common in patients with lesions of the frontal lobe: such patients' behaviour tends to be ‘perseverative’, with repetition of an action despite its no longer being appropriate (Milner, 1963; Sandson and Albert, 1984); some patients even comment that they are aware that their behaviour does not match their intention (Luria, 1966). A recent example is provided by the patient JR with a lesion of the supplementary eye field, described by Husain et al. (2003). Following a change in the rule governing the appropriate eye movement response
to a visual stimulus, JR tended to respond according to the previous rule before self-correcting the response to reflect the current rule; thus, while he clearly understood the task and was able to execute the appropriate response, his performance, at least initially, was still more in line with his previous actions than with his current intentions. Such instances are not restricted to those with brain damage, however. Duncan et al. (1996) described the phenomenon of ‘goal neglect’ among healthy individuals, especially those with low general intelligence, whereby instructions which are fully understood are simply not acted upon. In that study, subjects failed to switch attention to an alternative stream of visual input in a letter monitoring task, and Duncan et al. pointed out that it is in leaving behind a line of action to switch to another that such failures of intention to control action tend to occur.
⁎ Corresponding author. E-mail address: [email protected] (D.E. Astle). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.09.092
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The experimental ‘task-switching’ paradigm has been much used in recent years to examine the nature and limits of our dynamic control over behaviour. One of the most intriguing findings from this research is the phenomenon of the ‘residual switch cost’ (Rogers and Monsell, 1995). This is a cost to performance in terms of speed and/or accuracy which is incurred on trials where the rule governing responding differs from that used on the previous trial. Crucially, this cost cannot be overcome by voluntary preparation undertaken in advance of the target. The existence of the residual switch cost seems to indicate that performance of a task at least once is necessary to enable subsequent task performance to be efficient, and it appears to demonstrate that even in healthy individuals of high intelligence, our recent actions have an effect upon the efficiency of current performance which is inescapable, despite the strength of our current intentions.1 In their work on cognitive control using the task-switching paradigm, Mayr and Keele (2000) put forward evidence that the residual switch cost is at least partly due to the presence of what they termed ‘backward inhibition’. By this, they referred to the apparent inhibition of a task set, which was abandoned in order that an alternative task set could be used instead. The existence and persisting nature of such inhibition can be inferred from the slowing of response times seen when a recently abandoned task set is switched back to. Mayr and Keele argued that backward inhibition is an executive control process, driven by our intentions to perform a specific task from among a number of competing alternatives. Interestingly, although the presence of backward inhibition appeared to depend upon a high-level intention, such as a verbal representation in working memory, apparently it could not be overcome by an analogous voluntary mechanism. Instead, it survived long preparatory intervals during which the switched-to task was being prepared. This led Mayr and Keele to highlight the apparent functional dissociation between two levels of representation of a task set: a highlevel representation which is necessary for eliciting inhibition but apparently powerless to remove it, and a lowerlevel, ‘effective’ representation, which is itself modulated by inhibition. Despite the existence of such dissociations, most of the research into cognitive control has not attempted to distinguish between the effects of an intention to perform a particular task and the effects of previous performance. In all of the examples above, for instance, performance was measured after a switch from an alternative task (or goal, or action) which was actually carried out, so it is not clear whether the same behavioural effects would occur when a switch was required from an alternative course of action which was only intended rather than actually executed. However, a recent study by Schuch and Koch (2003) cleverly
1
There is evidence that, given appropriate circumstances which maximise the motivation to exercise voluntary control, the cost of switching may not affect a proportion of individual trials (De Jong, 2000); nevertheless, we seem to be limited in the degree to which we can exercise such control trial after trial because overall the switch cost remains, providing evidence of a persisting effect of the previous task upon current performance.
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dissociated preparation (intention) from performance in a task-switching paradigm and showed that it is actually the performance (i.e., selection of a task-specific response in relation to a target stimulus), not simply the preparation, of an alternative task on the previous trial which is responsible for the existence of switch costs on the current trial. Using a combined task-precueing and ‘go/no-go’ paradigm, they induced participants to prepare for a particular task on every trial, but on 25% of trials, without warning, a target was presented which required no response (a ‘no-go’ trial). While robust residual switch costs were observed following ‘go’ trials, they were absent following ‘no-go’ trials; further, there was evidence that these residual costs were the result of overcoming inhibition of the current task, and that this inhibition had been triggered by performance – but not by preparation alone – of the alternative task on the previous trial. So far we have seen that there can be a dissociation between our intentions and actions, in that these can exert differential effects upon subsequent performance. But this only covers the effect of the dissociation upon observed performance—what about the potential effects upon preparation processes? Schuch and Koch (2003) found no costs to performance following ‘no-go’ trials, but it is still an open question as to whether preparation for a subsequent task switch is affected by prior response selection. This question is difficult to address using behavioural measures alone as these necessarily require performance, but it can be tackled using the event-related potential (ERP) technique. In this study, we adopted Schuch and Koch's ‘go/no-go’ task-switching procedure to enable us to dissociate task switches away from alternative tasks which were only prepared and those from tasks which were also performed, and we used ERPs to provide indices of neural activity during the preparatory period separately from that occurring during task performance.
2.
Results
2.1.
Behavioural data
2.1.1.
Preparation
In order to ascertain whether or not participants were using the precue actively to prepare in advance of the target, switch costs at the short-CTI were compared to those at the long-CTI using a two-way ANOVA with factors CTI and switching (including only accurate ‘go’ trials preceded by accurate ‘go’ trials). This revealed a significant interaction [F(1,16) = 4.60, p = 0.048]. Switch-trial RTs were reduced from 754 ms in the short-CTI condition to 683 ms in the long-CTI condition, whereas repeat-trial RTs were reduced from 658 ms to 621 ms. There was a reduction in switch costs with preparation, therefore, from 96 ms in the short-CTI condition to 62 ms in the long-CTI condition. It may be the case that by blocking CTIs and placing the short-CTI trials at consistent positions within a block, subjects may have learned to predict their occurrence and become ‘lazy’, not bothering to prepare rapidly on predicted long-CTI trials (Altmann, 2004). Nevertheless, the significant interaction of the switch and CTI effects shows that on average subjects were making use
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repeat waveforms were compared with each other, separately for each of the response sequences (GGG and GNG).
2.3.
Precue-locked
2.3.1.
Late parietal positivity (LPP) for switches
We observed an increased positivity for switch compared with repeat trials over parietal electrodes. Fig. 2 shows that this effect was present for both the GGG and the GNG response sequences. There was a similar topography for the effect in each response sequence condition, clustered around Pz (electrode 62), where it reached consecutive significance relatively late in the epoch (488–646 ms for GGG, and 472– 716 ms for GNG).
2.3.2.
Fig. 1 – Behavioural data for task switching following a ‘go’ (ggg) and following a ‘no-go’ (gng). (A) Mean RT. (B) Mean proportion error.
Late frontal negativity (LFN) for switches
A precue-locked late right-frontal negativity for switch versus repeat trials was seen for the GGG but not the GNG condition. The effect was most prominent over right frontotemporal electrodes; Fig. 3A (right) shows the effect at FC6 (electrode 117). (N.B. Because its time course overlaps with that of the late parietal positivity, this negativity can also partially be seen in the topographical plot in Fig. 2A.) This effect resembles a superimposed slow wave somewhat like a contingent negative variation (CNV) for GGG-switch trials, with a negativity over frontal electrodes towards the end of the epoch (i.e., towards the time of target onset); consecutive significance at
of the CTI to engage in preparatory switching, and so we would expect to see electrophysiological correlates of such processing in the precue-locked ERPs of the long-CTI trials. Error data were analysed using Wilcoxon matched-pairs tests. There was no difference in the error switch cost between CTI conditions (Z = − 0.43, p = 0.670) (proportion error scores, short-CTI: switch trials, 0.16; repeat trials, 0.12; switch cost 0.04; long-CTI: switch trials, 0.14; repeat trials, 0.08; switch cost, 0.06).
2.1.2. costs
Relationship between response sequence and switch
Long-CTI trials (from which the ERPs waveforms were produced offline) were used to assess the relationship between response sequence and task switching using a twoway ANOVA. This revealed a significant interaction [F(1,16) = 20.308, p < 0.001] (Fig. 1A), with significant switch costs following a ‘go’ [F(1,16) = 33.125, p < 0.001] but not following a ‘no-go’ [F(1,16) = 2.123, p = 0.164]. The difference in performance between the trials following a ‘no-go’ and those following a ‘go’ was apparent both on repeats [F(1,16) = 25.618, p < 0.001] and switches [F(1,16) = 6.508, p = 0.021]. There were no significant switch-related differences for the error data [p > 0.05] (Fig. 1B), with the difference in repeat trials and switch trials between the two response sequences being nonsignificant [p > 0.05]. The pattern of these behavioural data replicates that of Schuch and Koch (2003, Experiment 1b).
2.2.
ERP results
The analysis of the ERP waveforms was split into two separate epochs: one following the precue (‘precue-locked’), and one following the target (‘target-locked’). In each case, switch and
Fig. 2 – The late parietal positivity, time-locked to precue-onset at 0 ms. The effect is present both following a ‘go’ (A) and following a ‘no-go’ (B). Waveforms are shown from electrode Pz, and topographical plots apply to the time period of consecutive significance at Pz. In the topographical plots, dark circles indicate significantly greater positivity for switch than repeat trials, and vice versa for dark squares. Arrows indicate the electrode site at which the accompanying waveforms were recorded.
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Fig. 3 – The late frontal negativity, time-locked to precue-onset at 0 ms. The effect is present following a ‘go’ (A; negative pole at FC6 and positive pole at P3) but not following a ‘no-go’ (B). Other plotting conventions as for Fig. 2.
FC6 was reached from 752 to 1148 ms. There was an associated positivity with a similar time course (reaching consecutive significance from 788 to 904 ms and 952–1080 ms) over left parieto-occipital electrodes; Fig. 3A (left) shows P3 (electrode 53), located within this positive cluster. While the time period of this effect overlapped with the late parietal positivity, its topography and time course were clearly distinguishable. (Fig.
3A shows the topography of the late frontal negativity towards the end of the epoch, during which time it is not overlapped by the late parietal positivity.) More critically, these components dissociated according to response sequence; while the late parietal positivity was present for both response sequences, the late frontal negativity was present in the GGG response sequence but completely absent in the GNG response
Fig. 4 – The centro-parietal positivity, time-locked to target-onset at 0 ms. The effect is present both following a ‘go’ and following a ‘no-go’, with slightly differing topography. The positivity is shown at CPz following a ‘go’ (A, left) and at electrode 86 following a ‘no-go’ (B, left). Following a ‘no-go’, a corresponding negativity is also present with a similar time course, shown in the waveform from AFz (B, right); this was not present following a ‘go’ (A, right). Other plotting conventions as for Fig. 2.
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condition (Fig. 3B). The pattern of waveforms on switch and repeat-trial types across the response sequence conditions is interesting in that it appears (see Fig. 3) that, rather than there being an additional frontal negativity for GGG-switch trials, there may in fact be a reduced negativity (or additional positivity) for GGG-repeat trials relative to all other trial types (and vice versa for the left parieto-occipital positivity). However, additional analyses conducted at electrodes P3 and FC6 showed no significant effect of response sequence on either repeat trials or switch trials.
2.4.
Target-locked
2.4.1.
Centro-parietal positivity (C-PP) for repeats
A centro-parietal positivity for repeats was present in both GGG and GNG sequences. Fig. 4A (left) shows the centroparietal positivity over CPz (electrode 55) in the GGG condition (312–568 ms), and Fig. 4B (left) over electrode 86 in the GNG condition (448–580 ms); comparison of their respective topographies indicates that the GNG centropositivity was shifted in a right-posterior direction. Fig. 4B (right) also shows an associated negativity over left frontal sites in the GNG condition (344–504 ms at AFz, electrode 16), which was absent in the GGG condition (Fig. 4A, right). (N.B. In the topographical plot for 5A, the lateral positive shift (see below and Fig. 5A) is also observable over right hemisphere electrodes.)
2.4.2.
Lateral positive shift
This component was located over right-temporal sensors, starting relatively early in the epoch, with a gradually increa-
Fig. 5 – The lateral positive shift, time-locked to target-onset at 0 ms. The effect is present following a ‘go’ (A), but not following a ‘no-go’ (B). Other plotting conventions as for Fig. 2.
sing switch-versus-repeat difference (with switch more positive than repeat), reaching consecutive significance at 388– 496 ms, 572–640 ms, 716–756 ms and 932–996 ms at T8 (electrode 109). Fig. 5 demonstrates that this was present in the GGG condition but absent in the GNG condition. The lateral positive shift has been reported previously. Swainson et al. (2003) observed something very similar in the target-locked epoch of a predictable task-switching design, where the component was visible as a positivity at lateral electrodes over both hemispheres. Those authors concluded that while this might appear to be a distinct component in its own right, it is likely the result of re-baselining on presentation of the target. In the present data set this is also plausible, with electrode T8, where the target-locked lateral positive shift is clearly seen, also showing a clear precue-locked negativity for switch trials, part of the precue-locked late frontal negativity (shown in Fig. 3).
3.
Discussion
This study explored the effect of dissociating task performance from task preparation upon subsequent task switching. Using a precued design, with ‘no-go’ responses required on 25% of trials, residual switch costs were found to be absent following a trial on which a task was only prepared and not actually performed (i.e., following a ‘no-go’ response). Of more importance, preparatory activity evident in ERPs occurring before target onset was shown to be affected by previous task performance. While a switch-related late parietal positivity (LPP) was present in the precueing interval following both ‘go’ and ‘no-go’ responses, a switch-related late frontal negativity (LFN) in the same interval was only found following a ‘go’ response, with repeat and switch not significantly differing following a ‘no-go’. A target-locked centro-parietal positivity for repeats was present following both ‘go’ and ‘no-go’ responses. We replicated the results of Schuch and Koch (2003), showing that switch costs are absent when a ‘no-go’ response was made on the previous trial (i.e., the task was prepared, but not performed). Our data showed the same pattern as in that original study, with the absence of switch costs following a ‘no-go’ apparently being due both to an additional cost on repeat trials and a benefit on switch trials. A number of factors could explain the additional cost on repeat trials, for instance: elimination of task-set priming (Allport and Wylie, 1999); addition of a ‘restart cost’ (Allport and Wylie, 2000; Altmann, 2002); and lack of priming of the ‘go’ response (Schuch and Koch, 2003). We suggest a further alternative, which is that in this type of paradigm, switching may occur on two levels: the ‘task-set’ level (consonant/vowel versus upper/ lower case judgements) and the ‘response-set’ level (‘go’ versus ‘no-go’ responding). In order to produce the infrequent ‘no-go’ response, subjects probably have to inhibit the tendency to respond immediately upon target onset (the ‘go’ response set). When a subsequent ‘go’ response is required, therefore, such inhibition (if it persists) would have to be overcome, potentially producing a cost to performance (see Koch et al., 2004, for evidence of persisting inhibition of response mode). Whichever of these explanations is true, it
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ought to affect both task-switch and task-repeat trials in a GNG sequence. But critically its effect might be counteracted on switch trials by an additional relative benefit—i.e., absence of the need to overcome task-set inhibition when no response was selected on the previous trial. Clearly there was an additional benefit on switch trials in this study because there was a significant benefit to performance of a preceding ‘no-go’ on switch trials. In addition, the error data ought to be sensitive to effects of switching ‘task set’ rather than those of switching ‘response set’ (in this case, switching from not responding to responding), and these showed effects only on switch trials. Thus, being in the inappropriate ‘task set’ would lead to erroneous responses (responses being driven by the inappropriate task on incongruent trials), so eliminating the need to switch tasks should reduce error rates on switch trials. But inappropriately retaining a tendency to inhibit immediate responding (or remaining in a ‘no-go’ response set) ought to lead only to delayed responding rather than increased errors. While nonsignificant, this is the pattern (i.e., with a larger difference on switch trials than repeat) that we observe in the error data.
3.1. ERP switch effects modulated by prior task performance The important finding in the current study is that the neural correlates associated with preparing to switch versus repeat the current task on the next trial are different following a ‘nogo’ versus a ‘go’ response. The LFN ERP, seen during the preparatory interval for switch trials in comparison to repeat trials, was present only for the GGG sequence and not for the GNG sequence. Therefore, it is clear that when a prepared task is performed (used to produce a task-specific response to a target stimulus) as opposed to only being prepared, the implications of this are evident during subsequent preparation of an alternative task set on the next trial (as shown by the LFN effects) as well as at the time of performance (as shown by the behavioural effects). Although this issue was not explicitly addressed in the current study, there is reason to speculate that the specificity of the LFN effect to GGG sequences relates to the need to overcome inhibition of the switched-to task. Schuch and Koch (2003) showed that following a ‘no-go’ trial, not only were switch costs removed, but also that the specific cost of switching back to a task abandoned on the previous trial (i.e., the cost of performing an ABA relative to a CBA task sequence) was removed. They suggested that ‘backward inhibition’ (Mayr and Keele, 2000) of a previous task is triggered at the time of response selection for the new task and not during the preceding preparation period. The consequence of this would be that following a ‘no-go’ trial (on which a new task was only prepared and never actually performed), there would be no cost of switching back to the original task (which had been abandoned on the previous, ‘no-go’, trial) because there would be no residual inhibition of that task to be overcome. In terms of the dissociation of preparatory and performance-based processes upon task inhibition, then, our behavioural data support the view that inhibition is neither triggered by preparation (switch costs
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being absent following a ‘no-go’ response), nor can it be entirely overcome during the preparation interval (residual switch costs remaining robustly in the GGG condition despite a substantial precue-target interval). However, the ERP findings from our study – specifically, the presence of the precue-locked LFN for GGG but not GNG sequences – suggest that this residual inhibition may nevertheless be encountered during the time when one prepares to return to the previous task set, and not only at the time of preparing a specific response appropriate to that task. Although we found no significant differences between the two response sequences (GGG versus GNG) on either repeat or switch trials in terms of the LFN waveforms, Fig. 3 does suggest that the presence of a task-switch effect on GGG trials may be due to a reduced negativity on GGGrepeat trials at least as much as an increased negativity on GGG-switch trials. How can this be squared with the explanation offered above that the effect on GGG trials may be due to encountering inhibition of a task set on a GGG-switch trial? As discussed above, it may be that the same need to inhibit a contradictory tendency (or ‘set’) applies to both tasks and types of response. Therefore, performing both repeat and switch trials in the GNG condition may entail encountering response suppression (but neither task suppression), whereas – if the GNG sequence does indeed prevent the need for persisting inhibition of the switched-to task – only switch trials in the GGG condition would entail encountering task suppression (but not response suppression). Thus, it is consistent with our hypothesis (that the LFN reflects processes to do with encountering persisting inhibition) that a similar degree of negativity should be present in both GNG-trial types as in GGG-switch trials, and more than on GGG-repeat trials (when neither persisting response suppression nor task suppression are encountered). As yet, we cannot be specific as to a potential role of the LFN. For instance, if it is related to the presence of persisting ‘backward inhibition’ upon the (recently abandoned) switched-to task, it may index an active attempt to overcome inhibition, a passive result of encountering inhibition during preparation, or the recruitment of resources needed to overcome inhibition once the target is presented. The effect resembles a slow wave (negative over frontal scalp) superimposed upon the underlying waveforms on switch trials in the GGG condition. Such slow waves have been associated with the allocation of cognitive resources to a particular processing module in the cortex (Rosler et al., 1997). One such slow wave is the contingent negative variation (CNV)—i.e., a negativity which increased towards the time of onset of a predictable stimulus (Walter et al., 1964). An increased frontal CNV has been shown on trials where subjects made a particular ‘effort’, in anticipation of a target, to perform well and has been interpreted as a reassignment of resources (Falkenstein et al., 2003). The LFN seen here on switch trials relative to repeat, following a ‘go’, may similarly represent the recruitment of resources, ready to deal with generating a response to a difficult type of target—i.e., one which it is known will involve a switch in task and the overcoming of persisting backward inhibition. We have also seen this effect recently (Mueller, Swainson and Jackson, submitted) when
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participants switched between tasks which shared response meanings (i.e., where responses were bivalent with respect to tasks—a left-hand response, for example, being used to indicate either that a letter was a vowel or that a digit was an odd number) but not between tasks which used separate sets of (univalent) responses. If the LFN does relate to overcoming inhibition of a recently abandoned switched-to task, it may index the need specifically to overcome inhibition of the response meanings associated with that task.
3.2.
ERP effects independent of prior task performance
A commonly reported ERP effect associated with task switching is a late positivity over parietal scalp (Barcelo, 2003; Jackson et al., 2001; Karayanidis et al., 2003; Kieffaber and Hetrick, 2005; Nicholson et al., 2005; Rushworth et al., 2002; Swainson et al., 2003, 2006) and this effect was replicated in the present study. Like the LFN effect discussed above, this late parietal positivity (LPP) occurred prior to the target onset during the precue-target interval, but, unlike the LFN, it was evident following both ‘go’ and ‘no-go’ responses. Thus, it appears to index a process which is independent of switching between performing alternative tasks per se. This component appears to occur at the time within a trial when ‘endogenous’ reconfiguration would be expected to take place—i.e., that aspect of task switching which, it is postulated, can be performed prior to target-onset, given certain necessary conditions such as advance information and the expectation of sufficient time in which to use it, but which otherwise occurs after target onset (Rogers and Monsell, 1995). Thus, the component has been seen both prior to target onset (Nicholson et al., 2005; Rushworth et al., 2002) and subsequent to target onset (Jackson et al., 2001; Swainson et al., 2003, 2006). In fact, its timing seems to be relatively consistent in relation to the timing of the task cue in these studies. Therefore, we have suggested that it reflects processing driven by a cue signaling the need for a task switch (Swainson et al., 2006). (N.B. In the study by Karayanidis et al., 2003, tasks switched predictably and the positivity for task switching was observed to be timelocked to the preceding response, but a task-cueing grid was present throughout and the positivity may in fact have been time-locked to participants’ attending to the grid. The effect may prove eventually not to be specific to the processing of an external cueing stimulus, but none of the studies so far have shown this to be the case.) The component has a relatively long latency after precue-onset of around 500–700 ms and in our previous study, where response times were very fast (usually <500 ms), we showed that this component need not precede the response; therefore, we suggested that it may index a nonobligatory switch-related process, such as a shift from controlled to relatively automatic task processing rather than an obligatory reconfiguration allowing performance of the appropriate task (Swainson et al., 2006). The current data are consistent with the effect being related to processing of a task cue. Whether this is generated exclusively by parietal regions, or whether this component is composed of the activity of a number of generators (for instance, including the prefrontal cortex; Brass et al., 2005) remains unclear at this stage. The similarity of the effect between the GGG and the
GNG response sequence conditions indeed implies that the effect reflects switching at the level of the abstract task set, goal or intended task put in place by the task cue on the previous trial whether or not that task was actually performed, rather than at a level corresponding to the ‘effective task’ and which would therefore be dependent upon recent performance (Mayr and Keele, 2000). A second component observed regardless of whether there was a switch in the performed task was the target-locked centro-parietal positivity (C-PP) for task repetition. While this was observed in both the GGG and GNG response sequence conditions, there was nevertheless a difference in topography, although to what extent this difference may reflect different psychological process occurring following a ‘go’ and following a ‘no-go’ response is unclear. We have previously observed this component in the target-locked epoch of a predictable task-switching paradigm (Swainson et al., 2003, 2006) and have interpreted it as indexing the current ‘strength’ of taskspecific stimulus evaluation, such that when a task is performed with the knowledge that it will immediately be repeated, it may be actively consolidated, or maintained, thereby allowing improved performance on the subsequent trial. Wylie et al. (2003) also observed the effect in a predictable task sequence with three trials in each run (…AAABBBAAA…) where they saw its amplitude increase along with the number of repetitions. They attributed this increase to a decreasing conflict between the task sets with task repetition, consistent with a parallel improvement in performance. Similarly, Barcelo (2003) saw that the component increased with task repetition and with improved performance on repeat trials. We have previously found the effect to be specific to a predictable task sequence, with no C-PP evident for repeat versus switch trials in random task sequences (Swainson et al., 2006), and suggested that this was because a task set could potentially be more rapidly consolidated when subsequent repetitions were predictable. The data from the present study and those from Barcelo (2003) indicate, however, that after more than one repeat trial, the effect can be observed without repeat trials being predictable (in the current study, all repeat trials occurred after at least two previous trials with the same task, which was not the case in our previous study). This implies that the same mechanism of task-set consolidation which previous data suggests is available with predictable task repeats still occurs (only more gradually) even without predictability, or at least that the two mechanisms have the same effect in terms of increasing the C-PP. Unlike these previous studies, though, in the present data the component was not seen to have direct behavioural consequences. That is, in the previous studies an increased C-PP was seen to coincide with a behavioural repetition benefit, but in the GNG condition of the current study, despite there being a switch/ repeat modulation in the C-PP, there was no behavioural ‘repeat benefit’ (or in other words, no switch cost). We suggest that this may be because the component indexes consolidation at a perceptual level, producing speeded stimulus evaluation on repeat trials (Kok, 2001). This would not necessarily also lead to a speeding of response times, especially if slowing for another reason – such as the need to overcome a ‘no-go’ response tendency – renders speeding of a parallel process impotent to affect response times. It is unclear
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why the topography of this effect differed between the response sequence conditions. It is plausible that the same consolidation process occurs both following a ‘go’ and ‘no-go’ response yet reaches significance over different electrodes, perhaps due to there being additional processes occurring following a ‘no-go’ which affect its observed scalp distribution such that its negative pole becomes apparent over frontal sites. Conversely, the difference in topographies could indicate that genuinely different psychological processes occur in the two response sequence conditions, for instance that following a ‘no-go’ there is a qualitative difference in the task-consolidation mechanism. As yet either alternative is possible.
3.3. The relationship between response selection and task-set preparation While the LPP seems to be relatively independent from whether or not subjects switch between performing (as opposed to only preparing) alternative tasks on successive trials, this study provides clear evidence that the effects of previous action are encountered during preparation for the subsequent trial (in the form of the LFN). The combination of task-switching and ‘no-go’ paradigms demonstrates that it is possible to dissociate neural indices of cognitive control in task switching, between those that relate to changing performance per se and those involved in changing intended performance.
4.
Experimental procedures
4.1.
Subjects
Eighteen healthy right-handed participants completed a combined EEG and behavioural experiment. Fourteen were female, with an average age of 23.63 ± 3.55 (SD) years. Participants provided written informed consent. Eighteen subjects were included in the EEG data set, and 17 in the behavioural (due to a technical error meaning that some behavioural data were lost for one subject).
4.2.
4.3.
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Experimental design
After practice at performing each task separately and switching between the two (until each participant had achieved 90% accuracy on a ‘switching’ block), the participant performed 17 blocks of 60 trials. Each block contained both short (100 ms) and long (1200 ms) CTI (cue-target interval) trials, with the short-CTI trials being arranged in a short run of 7–9 trials at the beginning of each block, and again at the midpoint. (These trials were blocked rather than interleaved to avoid introducing an additional factor of switching between long and short trials.) Participants were not informed of this pattern, only that the time to prepare could be short or long on each trial. Each cue (in fact, a ‘precue’, as all cues were presented in advance of targets) was on the screen for the full duration of the CTI, and each target was displayed on the screen for 160 ms. Following each target there was a response window of 800 ms, which ended upon response execution. This was followed by a subsequent fixed-length response window of either 800 or 1900, depending upon CTI, such that each trial time lasted 2000 ms + RT regardless of the preparation interval. ERPs were generated only from long-CTI trials which were responded to accurately and which were also preceded by two other accurate long-CTI trials. The short-CTI trials were included to provide subjects with incentive to engage in preparation immediately on presentation of the precue and to provide reaction time information for this to be assessed. As in Schuch and Koch's (2003) study, there were no repeats of ‘nogo’ trials; thus, trials fell at the end of either a go/go/go (GGG) or a go/no-go/go (GNG) ‘response sequence’ triad. But in a modification to their original design, we imposed another constraint that the task pertaining to the third trial in these triads also pertained to the first (see Fig. 6). Thus, there was either a switch back to the task performed two trials ago (switch triads: ABA, where A and B are alternative task sets) or no switch at all within the triad (repeat triads: AAA). The reason for this was in the cases where trial n − 1 required a ‘no-
Behavioural task
Each trial was precued by a fixation cross which changed colour prior to the onset of the imperative stimulus. The colour of the target and fixation cross indicated the task set to be prepared (the assignment of colour to task being counterbalanced across participants). Each target stimulus was selected from a set of vowels and consonants in upper and lower case (A,E,I,U,a,e,i,u,G,B,T,D,g,b,t,d). On a ‘go’ trial a single letter was presented, upon which either of two task sets could be implemented, responses being made with a two-button serial mouse: discrimination of the letter as either a vowel (left button) or a consonant (right button) or discrimination of the letter as lower (left button) or upper (right button) case. On a quasi-randomly selected 25% of trials, a task-irrelevant stimulus (#) appeared instead of the usual target, to which no response could be made (‘no-go’ trials). These trials never appeared in direct succession.
Fig. 6 – The four specific trial sequences used in the main analyses: two repeat and two switch sequences, two with a ‘no-go’ on trial n − 1 and two with a ‘go’ on trial n − 1. White or black precues and stimuli indicate one or other of the two tasks ought to be performed on that trial.
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go’ response, neither the ‘switch’ nor the ‘nonswitch’ triads would entail a switch at the level of performance. The four types of trial included in the analysis were therefore GGGswitch, GGG-repeat, GNG-switch and GNG-repeat. In order to ensure that enough of the these specific sequences were produced, the trials were generated in a quasi-random fashion using a software package (Microsoft Excel), giving on average 6 of each trial type per block of 60 trials. (N.B. For the analysis of the behavioural preparation effect, such a strict criterion of trial type did not need to be employed. Instead, trials were classified as being either switch (BA) or repeat (AA) trials, where both the current and preceding trial required a ‘go’ response (GG)).
4.4.
EEG recording, processing and ERP formation
EEG was recorded throughout each block, using a 128-channel electrical geodesic net (Electrical Geodesics, Inc.; Tucker et al., 1994) and digitised at 250 Hz. The recording was performed with a hardware bandpass filter of 0.01 Hz to 100 Hz. Before recording impedance on each of the 128 electrodes was reduced to <50 kΩ. Precue-locked and target-locked epochs were created, with each epoch starting 100 ms before onset of the respective stimulus and ending 1200 ms (precue-locked) or 1000 ms (target-locked) afterwards. Segments were rejected if contaminated by eye-blinks/movements (indicated by EOG activity greater that 70 μv), or an error of response (incorrect response or omission of response), or if the trial followed an error on either of the two previous trials. Trials containing voltage amplitudes greater than 200 μv or a change greater than 100 μv were also removed. Channels that were bad for more than 20% of trials were rejected. With this rationale, approximately on average 20% of trials were removed within the precue-locked epoch, and 35% of trials from the targetlocked epoch (owing to excessive ocular artefacts). The data from each participant included in any ERP waveform had > 30 waveforms from which to form averages. Based upon this rationale all 18 participants produced sufficient waveforms to be included in the precue-locked comparisons, whereas only 15 participants produced sufficient to be submitted to the target-locked comparisons. In summary, ERPs were formed from long-CTI trials only, were free from behavioural errors, and had been preceded by two appropriate (see Fig. 6), accurate and long-CTI trials. The specific segments were average-referenced to a standard adult 128-electrode montage. Epochs were baselinecorrected for the first 100 ms, before the onset of the stimulus, and epochs lasted for a subsequent 1200 ms or 1000 ms. Switch and repeat waveforms were explicitly compared (all statistics conducted using MATLAB v5, scripts developed in-house), by means of a T-test for every 4 ms sample. Two waveforms were counted as significantly different if this significance persisted for more than 10 samples, and two temporal regions of consecutive significance were counted as a continuation of the same difference if the gap between them was fewer than 10 samples. This approach is based upon Rugg et al. (1993) and was used by Swainson et al. (2003, 2006); Jackson et al. (2001). All ERP comparisons that reached consecutive significance are included in the Results section.
Acknowledgment This study was supported by a BBSRC grant to GMJ.
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