Cerebral Cortex July 2005;15:899--912
doi:10.1093/cercor/bhh189
Advance Access publication September 30, 2004
Cognitive Control Involved in Overcoming
Prepotent Response Tendencies and
Switching Between Tasks
A dissociable set of regions was active for the executive processing
associated with overcoming a prepotent response tendency and task
switching. Regions associated with overcoming prepotency were
primarily frontal and may be part of a system involved in top-down
biasing for conflict reduction. Posterior regions were recruited for
switching between tasks and likely play a role in reconfiguring
stimulus--response mappings. Precuneus activity was common to
both manipulations and may reflect increased visual attention due to
more difficult task demands.
Keywords: frontal, parietal, task switching, conflict effect, preparation
Introduction
Cognitive control comprises several sets of processes involved
in coordinating and planning actions. The recruitment of such
processes occurs under novel, difficult or complex conditions;
for instance, when overcoming habitual responses, ignoring
irrelevant stimuli or transforming representations. Under such
demands, cognitive control serves to direct the processing of
goal-relevant information and to schedule actions in order to
minimize conflict between potential responses (Schneider and
Detwiler, 1987, 1988; Botvinick et al., 2001). Theoretical
formulations of component processes include maintenance of
goal representations, updating goals, top-down guidance of
information-processing, and monitoring performance. Although
the frontal lobes and regions of posterior parietal cortex have
been implicated in a range of such control processes, the
specific contributions of these regions is not clear.
Electrophysiology studies find that neurons in the dorsolateral prefrontal cortex (DLPFC) fire when relevant information must be maintained across a delay (Goldman-Rakic, 1987;
Levy and Goldman-Rakic, 1999). Similarly, functional neuroimaging studies show DLPFC activity increases parametrically
with working memory load during the preparatory period of the
task when information must be maintained (Cohen et al., 1997).
However, this activity is not a reflection of maintenance per se.
DLPFC activity also occurs during performance of tasks without
a delay period (Merriam et al., 2001; Bunge et al., 2002). This
suggests that activity in this region not only increases with
maintenance demands, but also as a function of other control
operations. For instance, other studies implicate DLPFC activity
in allocating attentional resources (MacDonald, et al., 2000),
selecting between competing responses (Rowe et al., 2000;
Bunge et al., 2002) and overcoming residual inhibition (Dreher
and Berman, 2002).
Another region of the prefrontal cortex, the anterior cingulate cortex (ACC), has also been implicated in cognitive control.
MacDonald et al. (2000) found that this region is active during
the target phase of a task when competing responses induce
Cerebral Cortex V 15 N 7 Oxford University Press 2004; all rights reserved
Anita D. Barber1,2 and Cameron S. Carter3
1
University of Pittsburgh, Psychology, Pittsburgh, PA 15213,
USA, 2Center for the Neural Basis of Cognition, Pittsburgh, PA
15213, USA and 3University of California at Davis, Psychiatry
and Psychology, Davis, CA 95817, USA
conflict. It was suggested that activity in the ACC might interact
with the DLPFC to adjust cognitive control based on the amount
of response conflict induced by the task. Models of a dorsolateral
prefrontal--anterior cingulate cortical loop (Botvinick et al.,
2001) propose that when response conflict occurs, the ACC
monitors response output and signals the DLPFC for increased
allocation of attentional resources. This model hypothesizes
that the monitoring and signaling function of the ACC allows for
modification of attention based on task demands.
While this DLPFC--ACC system has been implicated in tasks
requiring that a prepotent response tendency be overcome, its
role in task switching has not been established. Various task
switching paradigms employ manipulations, such as varying the
degree of overlap in stimulus and/or response attributes
(Allport et al., 1994; Meiran, 2000), which might induce conflict
and, therefore, call for the recruitment of this system. Some
studies have found higher DLPFC activity for switch trials
compared with repeat trials (Dove et al., 2000; Sohn et al.,
2000), while others do not find DLPFC activity for task switching manipulations (Konishi et al., 1998, 1999; Nakahara et al.,
2002). Task switching often activates the posterior parietal
cortex to a greater extent in switch than repeat trials (Dove
et al., 2000; Kimberg et al., 2000; Sohn et al., 2000) and this
region has been implicated in stimulus--response (S--R) associations (Bunge et al., 2002).
The behavioral switch cost incurred from task switching
paradigms is often used as a measure of cognitive control.
However, it is not clear whether control processes are
necessarily recruited during task switching. Allport and Wylie
(2000) hypothesized that the switch cost may be accounted for
by the carryover of previous task processing into the current
trial. This view holds that an extra control process is not
necessary to account for the switch cost. Meiran et al. (2000)
suggest that there are at least three component processes that
contribute to the switch cost. They found that increasing the
response--cue interval (RCI) reduced the switch cost, which
supports the Allport and Wylie (2000) task carryover effect.
Further, they confirmed the formulation of Rogers and Monsell
(1995) of two contributions to the switch cost, an anticipatory
component and a stimulus-triggered component. Meiran et al.
(2000) found that although increasing the cue--target interval
(CTI) decreased the switch cost by reducing the anticipatory
component, CTI increases beyond 600 ms did not lead to an
additional reduction in the switch cost. This residual switch
cost reflects a stimulus-driven component, which Meiran et al.
(2000) implicate in reconfiguring S--R mappings.
The current study employs the task-cueing paradigm used by
Meiran et al. (2000), which allows for the independent examination of the three task switching components. The current
task design will focus on the anticipatory and stimulus-triggered
component processes. During the CTI, the anticipatory or
preparatory effect of the switch cost will be examined. This
effect will be indexed in the functional magnetic resonance
imaging (fMRI) results as additional activity during the CTI
for switch trials as compared with repeat trials. The CTI used in
the current study (7.5 s) is sufficiently long for preparation for
task switching to complete before the target stimulus appears.
For this reason, the stimulus-triggered contribution to task
switching may be considered independent of any anticipatory
effects.
The third component of task switching discussed by Meiran
et al. (2000) was the dissipating processing of the previous
trial’s task set. According to their results, this component should
not affect current trial processing when a sufficiently long RCI is
used. Meiran et al. (2000) showed that for RCIs >1 s the switch
cost reduction becomes very slow (~1.7 s reduction per 100 ms
of RCI increase). While these results suggest that for the current
paradigm the previous trial’s task set [Rogers and Monsell
(1995) used the term ‘task set’ to refer to ‘an effective intention
to perform a particular task’. In their words, ‘to adopt a task-set
is to select, link, and configure the elements of a chain of
processes that will accomplish a task.’ For the purposes of this
paper, task set refers to the repertoire of stimuli, responses and
processing stages employed by a particular task condition]
processing should minimally affect the current task’s processing
(since the RCI of the current task is 12 s), Allport and Wylie
(2000) find that long-term, retrieved task bindings contribute to
the switch cost. This retrieval of the inappropriate task’s S--R
mappings should be present on all switch trials since the same
stimulus and response sets are relevant for both tasks.
For the current study, a cued S--R incompatibility task,
maximizing the S--R overlap between the two conditions, was
chosen in order to increase control demands. Subjects’ tendency to perform the prepotent task was set up in several ways.
First, the S--R mapping was more intuitive (i.e. the stimulus ‘l’
corresponded to a response with the left finger); second, trials
of this condition were more frequent (70% of trials); and third,
the first three trials of every block were prepotent trials.
The present study aimed at dissociating frontal and parietal
regions involved in overcoming a prepotent response tendency
from those involved in task switching. In particular, we expected
to replicate the MacDonald et al. (2000) results in an attempt to
validate the role of the DLPFC--ACC system in overcoming
a prepotent response tendency and conflict reduction. Further,
we aimed to determine whether this system is also recruited for
task switching. In order to distinguish control processes associated with these two cognitive constructs, two manipulations
were performed: (i) prepotency and (ii) task switching. The first
manipulation compared differences between prepotent trials
and nonprepotent trials for both behavioral and fMRI measures.
The second manipulation compared differences between repeat
and switch trials for both of these measures.
Each manipulation was performed twice for each trial phase,
once for the preparation phase and once for the target
(response) phase of the trial. This allowed for the temporal
distinction of processes associated with different stages of the
trial (e.g. the distinction of processes associated with attentional allocation from those recruited in the monitoring of
response conflict for the prepotency manipulation, and the
distinction of anticipatory and stimulus-triggered processing for
the task switching manipulation).
900 Overcoming Prepotency and Task Switching
d
Barber and Carter
Further, this task design enabled comparison of processes
associated with each phase of the task for the two task
manipulations to determine whether similar processing stages
are recruited for these two cognitive constructs. In this way,
preparatory processes associated with overcoming prepotency
could be compared with preparatory processes associated with
task switching, and likewise, response-phase regional activity for
overcoming prepotency and task switching could be compared.
Methods and Materials
Subjects and Task
Seven male and seven female right-handed subjects, between the ages of
20 and 35 years, participated in the study. One male subject was
excluded for excessive movement. All subjects performed a cued S--R
incompatibility task.
Stimuli were presented with PsyScope software on a Macintosh
computer and viewed through a mirror reflecting a back-projection
screen mounted inside the scanner bore. Subjects foveated a centrally
located fixation point throughout the task. Figure 1 depicts the S--R
incompatibility task. At the beginning of a trial, a colored cue (green or
red) appeared in place of the fixation for 500 ms. Seven seconds later
a target stimulus (either the letter ‘l’ or ‘r’) replaced the fixation cross,
also for 500 ms. The inter-trial interval was fixed at 12 s. Subjects
responded to the target by pressing with their index or middle finger on
a glove affixed to their right hand.
The colored cue instructed subjects to perform one of the two task
conditions. A green cue signaled the subject to perform the prepotent
condition, or to make the intuitive S--R mapping. For this condition,
subjects responded to the target with their right (middle) finger when it
was an ‘r’ and their left (index) finger when an ‘l’ appeared. A red block
instructed subjects that the current trial is of the nonprepotent condition, in which the S--R mapping is reversed. In this condition, subjects
were to respond with their right (middle) finger to an ‘l’ and their left
(index) finger to the appearance of an ‘r’. During the preparatory phase,
subjects could prepare the S--R mapping for the relevant task, but not
the particular motor response. The relevant response was unknown
until the presentation of the target stimulus.
For the switching analysis, a switch trial was defined as one in which
the previous trial type was different from the current one (i.e. a prepotent
trial preceded by a nonprepotent trial). A repeat trial is one in which the
previous and the current trial types are the same. The prepotency of the
conditions was further enforced by the frequency of trial type as well as
the inclusion of three prepotent trials at the beginning of each block.
Seventy percent of trials were of the prepotent condition and 30% were
of the nonprepotent condition. After presentation of the first three
Figure 1. Stimulus--response incompatibility task. The timeline across the bottom
represents the task timing for each trial. Each tick represents one scan (1500 ms). Cue
onset occurs at trial start (0 s). Target onset occurs 7.5 s after trial start.
prepotent trials of each block, trials appeared in a random task order.
Each subject performed four blocks of 20 trials.
For each of the analyses, a trial was broken into two phases: the
preparatory phase and the response phase. Each trial lasted for 13 scans
(see Image Acquisition and Analysis below). The preparatory phase
lasted for the first five scans (0--7.5 s after trial start), during which the
cue was either presented or maintained for further processing. The
target phase lasted from scan 5 through scan 13 (7.5--19.5 s after trial
start), during which the target was presented and a response was made
to the target.
Image Acquisition and Analysis
Functional and structural scanning was performed in a 3.0 T GE scanner
with standard head-coil. Twenty-eight 3.2 mm slices with anterior
commissure--posterior commissure (AC--PC) in-line resolution were
acquired. On each trial, thirteen 1.5 s functional scans were obtained,
using a one-shot spiral T2*-weighted sequence with TR = 1500 ms,
TE = 18 ms, flip angle = 70 and a field of view of 20 cm. Structural images
were obtained with a standard T1-weighted pulse sequence.
Movement correction was performed using an automated algorithm
(AIR) in which the first functional image volume is compared with each
subsequent image volume and each image is moved closer to the original
along six dimensions. A within-block linear detrend was performed with
subtractive mean normalization between blocks. Each subject’s structural volume was cross-registered to a reference volume using a 12parameter automated algorithm (AIR). Volumes were then normalized
for image intensity and smoothed (8 mm full width-half maximum
Gaussian filter). Talairach coordinates were obtained with AFNI.
Statistical analysis was performed using voxel-wise analysis of variance
(ANOVA). First, two planned contrasts were examined: prepotency-byscan and switch-by-scan ANOVAs. Prepotency-by-scan ANOVAs included subject as a random factor and prepotency and scan as within
subject factors. Switch-by-scan ANOVAs were performed with subject as
a random factor and switch and scan as within subject factors. For each
contrast, scans 1--5 (the preparatory phase) were analyzed separately
from scans 5--13 (the response phase). Incorrect trials were discarded
before the analyses were performed.
Once ROIs were identified for each manipulation, sensitivity analyses
were performed to determine that regional activity associated with one
of the manipulations (e.g. overcoming prepotency) did not also occur
for the other manipulation (e.g. task switching). Within each ROI
discovered for one of the two manipulation types (switch and prepotency), the other analysis was performed on a voxel-by-voxel basis to
ensure that no voxels were active for both analyses. This analysis was
performed separately for each trial phase. For example, for an ROI found
in the preparatory prepotency-by-scan ANOVA, a voxel-wise preparatory switch-by-scan ANOVA was performed. Second, switch-by-scan
ANOVAs were performed with prepotent-only trials (excluding nonprepotent trials). While the design and aims of the current study did not
allow for a full interaction analysis between switch and prepotency, this
sensitivity analyses ensured that regional activity for the switch analysis
was not driven by activity on nonprepotent, switch trials.
that is induced by the nonprepotent S--R mapping (mean
nonprepotent RT--mean prepotent RT), was 86 ms.
Subjects also performed significantly worse in the switch
trials as compared with the repeat trials (t = 5.00, df = 12, P <
0.0003). The mean RT for trials on which subjects switched
conditions was 796 ms (SD = 310) whereas on trials in which
subjects performed the same condition in the preceding trial,
the mean reaction time was 681 ms (SD = 246). The switch
effect, a reflection of extra processing involved in switching
between conditions, was 115 ms.
For the green trial only switch analysis, RTs were similar to
the overall switch RTs. Green-trial-only repeat RTs were 670 ms
(SD = 240) and switch RTs were 786 (SD = 314 ms, t = 4.46, df =
12, P < 0.0008). The switch effect was 116 ms.
For the RT bin analyses, mean RTs and behavioral costs are
displayed in Table 1. A 232 ANOVA, with subject as a random
factor and RT bin and task switch as fixed factors, was
performed to determine the effects of preparation on task
switching. Results find that the switch cost for the fast RT bin is
substantially reduced from that of the slow RT bin. It has been
hypothesized that preparation for switch is all-or-none and
occurs only on these fast RT trials (De Jong, 2000). The current
results support this hypothesis. Fast RT trials have smaller
switch costs. An interaction of RT bin and task switch (F = 33.87,
df = 1, 12, P < 0.0001) reveal that task switching is greater for
the slow RT bin than for the fast RT bin. Further, there were
main effects of both RT bin (F = 100.78, df = 1, 12, P < 0.0001)
and task switch (F = 33.61, df = 1, 12, P < 0.0001).
The interaction of task switch and response switch was
examined to determine whether the effects could be attributed
to the selection of a particular response. The mean RTs and the
behavioral costs are displayed in Table 2. A 2 3 2 ANOVA, with
subject as a random factor and task switch and response switch
as fixed factors reveals that there was a significant main effect of
task switch (F = 24.788, df = 1, 12, P < 0.0001). However, the
effect of response switch (F = 0.025, df = 1, 12, P = 0.877) and
the interaction of task switch by response switch (F = 0.251, df =
1, 12, P = 0.625) were insignificant.
Functional Imaging Analyses
Regions of interest (ROIs) were selected based on either
a prepotency-by-scan or a switch-by-scan ANOVA at P < 0.005
with a clustering threshold of eight contiguous voxels in order
Table 1
Behavioral data for RT bin and task switching
Results
Behavioral Analyses
On 2.45% of all trials subjects made incorrect responses and on
0.88% of trials no response was made. Error rates were between
two and three percent for each condition (prepotent, nonprepotent, switch, and repeat) and the no-response trial rates
did not exceed one percent for any condition. Because overall
error rates were low and error rates for each condition were
comparable, only correct trials were included for both the
behavioral and the functional imaging analyses.
The mean reponse time (RT) for the prepotent condition
(702, SD = 267) was significantly different from the mean RT for
the nonprepotent condition (787, SD = 300 ms, t = 2.79, df = 12,
P < 0.05). The conflict effect, a measure of the response conflict
Fast
Slow
Total
Repeat
Switch
Total
Switch effect
463 (135)
1031 (323)
681 (246)
524 (154)
1272 (340)
796 (310)
488 (146)
1132 (350)
62
241
Table 2
Behavioral data for task switching and response switching
Response switch
Task switch
Repeat
Switch
Total
Switch effect
Repeat
Switch
Total
Switch effect
684 (226)
802 (296)
736 (266)
118
678 (262)
788 (310)
720 (292)
110
680 (246)
796 (310)
ÿ6
ÿ14
Cerebral Cortex July 2005, V 15 N 7 901
to reduce the likelihood of type 1 error. This threshold ensured
0.05 probability protection for the full 28 slice volume, using the
correction method described by Forman et al. (1995). Only
ROIs showing greater activation in the prepotent or switch
conditions for their respective analyses are reported. Further,
only ROIs with time-series peaks within the hemodynamic
range (3--9 s) above baseline are reported. Table 3 displays all
ROIs that met these criteria for the prepotency-by-scan
analyses.
The preparatory (first five scans) prepotency-by-scan interaction identified mostly frontal ROIs. The bilateral DLPFC (25,
15, 32; –42, 16, 36; BA 9), left medial PFC (–3, 42, 28; BA 32), left
anterior frontal cortex (–28, 51, 9; BA 10) and left insula (–45, 15,
Figure 2. Regions active for preparing to overcome a prepotent response tendency.
902 Overcoming Prepotency and Task Switching
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Barber and Carter
7) showed higher activity for the nonprepotent condition than
the prepotent condition (see Fig. 2).
Figure 3 displays activity found for the response period
prepotency-by-scan manipulation. As expected, a medial frontal
region (–16, –1, 48; BA 6/32) showed greater activity for the
nonprepotent condition than for the prepotent condition
during the target period (last eight scans). This region extended
into the left caudal ACC and supplementary motor area, which
supports the role of the ACC in the monitoring of response
conflict. Regions of the superior frontal cortex (6, –29, 71; –27,
–13, 70; 10, –1, 66; BA 6) and a region of the superior parietal
lobule (–25, –76, 43; BA 7) were also differentially active for the
target period.
Figure 3. Regions showing activity for overcoming a prepotent response tendency for the target phase of the task.
For the switch-by-scan analyses, mostly posterior regions
were identified (see Table 4). A precuneus region (–6, –78, 38;
BA 7) was more active to switch than repeat trials during the
preparatory period (Fig. 4). For the response phase of the task,
several posterior ROIs were more active to switch than repeat
trials: a left inferior parietal lobule ROI (–45, –58, 33; BA 39/40)
and a left precuneus ROI (–23, –76, 43; BA 7) (see Fig. 5). This
precuneus region overlapped with the precuneus activity in the
prepotency-by-scan response phase analysis. An inferior frontal
region (33, 28, 19; BA 45/46) also showed greater responserelated switch activity.
Confirmatory and Exploratory Analyses
In order to dissociate control processes involved in task
switching from those involved in overcoming a prepotent
response tendency, a voxel-wise, switch-by-scan ANOVA was
performed within each ROI found in the prepotency-by-scan
ANOVA, and vice versa. This ensured that no voxels that were
differentially active for the prepotent and the nonprepotent
conditions were also involved in switch related processes. The
precuneus activation was the only ROI with voxels significant at
the P < 0.05 level in the dissociation analyses. Within this ROI,
there were eight voxels that were significant for both analyses.
Activity within a region of interest (ROI) that appears in one
analysis but not the other could be driven by a difference in the
relative difficulty for the two conditions of each manipulation.
However, a t-test of the behavioral difference between the
switch effect and the conflict effect, found no detectable
difference between these two measures (t = 1.14, df = 12, P >
0.27). In as far as the control process performed by a region is
evidenced by these behavioral measures, we can exclude the
possibility that the dissociation of regions involved in switching
and overcoming prepotency is due to differential recruitment of
the same process.
Three additional post-hoc analyses were performed to better
determine the nature of switch-related processing and to
address possible reasons for the lack of significant prefrontal
cortical activity for the task switching analyses. First, a sensitivity analysis was performed. This analysis was a switch-by-scan
ANOVA including prepotent trials only. All nonprepotent
trials were excluded from the analysis and only currenttrial-prepotent repeat trials (prepotent / prepotent) and
Cerebral Cortex July 2005, V 15 N 7 903
Figure 4. Preparation related task switch activity.
904 Overcoming Prepotency and Task Switching
d
Barber and Carter
lobule (–39 –54 42, BA 40). This ROI overlaps with the ROI
found in the all-trial switch analysis however, the center of this
region is superior to that of the original parietal region.
For the bin analysis, RT bin-by-switch-by-scan analyses were
performed and activity for the two trial phases was examined
separately. The superior frontal gyrus (–20, –6, 64; BA 6) showed
a significant preparatory interaction with the greatest activity
for the fast RT bin, switch condition. For the response phase,
only the superior frontal gyrus showed the greatest activity for
the fast RT bin, switch condition (17, –8, 60; BA 6). For the slow
RT bin, switch condition, regions of precuneus (16, –69, 54; BA
7), middle frontal sulcus (51, 17, 31; BA 9), the middle occipital
gyrus (–24, –85, 20; BA 18/19), and the anterior frontal cortex
(31, 58, 5; BA 10) showed the greatest activity.
Regions involved in switching between individual responses
were also determined. For the 3 way, task switch-by-response
switch-by-scan ANOVA, regions of superior frontal cortex (4 27
47; BA 8) and precuneus (–16, –70, 44; BA 7), and the postcentral gyrus (–44, –22, 43; BA 3 4) showed an interaction of
activity for task switching and response switching.
=
current-trial-prepotent switch trials (nonprepotent / prepotent trials) were compared. This analysis ensured that regional
activity due to overcoming prepotency (activity on nonprepotent trials) did not interact with regional activity due to task
switching to produce spurious or null results.
Second, switch-by-scan analyses were performed separately
for high-performance and low-performance trials. According to
De Jong (2000), switch-related preparation is engaged only on
a proportion of trials and switch costs are the average of highperformance, prepared trials and low-performance, unprepared
trials. The bin analyses were performed in order to detect
switch-related processes that occur earlier or show greater
activity on those trials in which participants enter a prepared
state (Braver et al., 2003). For this analysis, a three-way RT binby-switch-by-scan ANOVA was performed. Activity for trials with
the fastest reaction times (the top decile of the reaction time
distribution for switch trials and for repeat trials) was compared
with activity for the slowest reaction time trials (the bottom
decile of the reaction time distributions for switch and for repeat
trials).
Third, in order to further examine switching mechanisms,
a task switch-by-response switch-by-scan analysis was performed. This response--switch analysis addressed the possibility
that prefrontal regions may be involved in the selection of
particular responses, and therefore, may show switch-related
activity for particular responses rather than for task sets. This
analysis was only performed for the response phase of a trial.
The particular response is not known during the preparatory
period and therefore, should not differentially influence processing for the two conditions.
For the green-trial-only switch analysis, only one ROI was
more active in the switch than repeat condition. This ROI was
active in the response phase of the task in the inferior parietal
Discussion
The current study aimed at dissociating processes recruited
during overcoming a prepotent response tendency and task
switching. Both manipulations have been associated with
competitive processing and, therefore, may require processes
devoted to conflict reduction. For this reason, the role of the
DLPFC--ACC system, which has been implicated in conflict
reduction, was examined for both of these manipulations.
Planned contrasts for prepotency-by-scan and switch-by-scan
ANOVAs were analyzed separately. The current study was
not designed and did not provide power to examine the interaction of prepotency and scan. Two sensitivity analyses were
Figure 5. Target related task switch activity.
performed post-hoc to ensure that task switching activity was
not merely a reflection of nonprepotent-trial activity.
Task cueing paradigms have proven useful in temporally
isolating processes contributing to both prepotency effects
(MacDonald et al., 2000) and task switching effects (Meiran
et al., 2000). The current study employed this type of task
design in order to temporally separate control processes in each
of these manipulations. On each trial, the preparatory phase of
the task occurred after the appearance of the cue and lasted
until the appearance of the target. The response phase began
with the appearance of the target and lasted until the end of the
trial. In this manner, preparatory processes and responserelated processes were examined independently for both the
prepotency and task switching manipulations.
The current study found regional activation differences
related to component cognitive control processes for overcoming a prepotent response tendency and task switching. The
processes recruited during performance of the nonprepotent
task set reflect the need for strong top-down biasing and
conflict reduction when the tendency to perform the more
automatic task set must be overcome. Processes necessary for
switching between tasks include changing task representations
and reconfiguring S--R mappings for performance of the new,
now relevant task condition. The current study also revealed
regions active in both manipulations, reflecting general processes recruited commonly while overcoming a prepotent
response tendency and task switching.
Overcoming a Prepotent Response Tendency: Top-down
Biasing and Conflict Reduction
In the current study, greater DLPFC activation occurred in
preparation for the nonprepotent condition. This activity is
analogous to the DLPFC activity reported by MacDonald et al.
(2000) in that activity occurs for overcoming the more habitual,
prepotent task set. However, this activity differed from that of
the MacDonald et al. (2000) study in two ways. First, the
activation was bilateral, whereas MacDonald et al. (2000) found
unilateral left DLPFC activity. Second, the time course of this
activity peaked at 3--4.5 s after stimulus presentation and
returned toward baseline by the end of the preparatory period.
MacDonald et al. (2000) found DLPFC activity maintained
throughout the preparatory period. Because DLPFC activity is
not sustained throughout the inter-stimulus interval, we exclude the interpretation that this activation is simply a reflection
of greater maintenance demands during performance of the
nonprepotent condition. This account is consistent with other
Cerebral Cortex July 2005, V 15 N 7 905
studies implicating the DLPFC in cognitive control processing
other than maintenance (Bunge et al., 2002; Dreher and
Berman, 2002; Rowe et al., 2000).
One proposed role for the DLPFC in cognitive control is the
selection of a target response (Rowe et al., 2000; Bunge et al.,
2002). Bunge et al. (2002) suggested that a repertoire of
potential S--R associations is activated within posterior parietal
cortex and the DLPFC selects from amongst competing
responses. In the current task, DLPFC activity occurred during
the preparatory phase of the task. Since the particular response
was unknown during this period, the target response could not
be prepared at this time. Miller and Cohen propose that the
DLPFC performs top-down biasing necessary for overcoming
the activation of the dominant task set. Since the dominant task
set is primed by more frequent use and since the stimuli in this
task set also inherently prime the responses, strong attentional
biasing is necessary to activate the nonprepotent task set.
Similarly, MacDonald et al. (2000) suggest that the DLPFC is
more active in preparation to overcome a prepotent response
tendency, when the demands for attentional allocation are
increased.
Other regions that displayed greater preparatory activation
for the nonprepotent task set were the rostral cingulate,
anterior frontal cortex and left inferior frontal cortex. The
rostral cingulate activation may be due to the violation of
stimulus expectation. The P3b event-related potential component shows amplitude variation as a function of stimulus
expectancy and has been implicated in evaluation of infrequent
target events in determination of behavioral relevance. In the
oddball paradigm, the less frequent the target stimulus, the
greater the amplitude of the P300. Studies employing fMRI and
event-related potential have implicated regions of the cingulate
cortex in a system involved in rare target/novel stimulus
detection (Friedman et al., 2001).
Anterior frontal activation has been implicated in the monitoring of internally generated information (Christoff and
Gabrieli, 2000; Christoff et al., 2001), cognitive branching
(Koechlin et al., 1999) and subgoal activation during working
memory processing (Braver and Bongiolatti, 2002). In the nonprepotent condition, this region may be involved in integrating
the contents of the subgoal with the contents of working
memory in preparation for the upcoming S--R reversal.
Two regions showed greater activity for the nonprepotent
condition during the response phase of the trial, the left ACC
and the precuneus. The ACC region shows similar activity to the
ACC region found in the MacDonald et al (2000) study. The
activity was implicated in response conflict detection and the
current results support the role of the ACC in monitoring
competing response tendencies in order to gauge the amount of
control necessary for task performance (Botvinick et al., 2001).
The response-period precuneus activity may be a reflection of
greater visual attention to stimulus features during more
difficult task demands. Activity in this region was also found in
the task switching analyses suggesting that the precuneus
performs a task general function.
Task Switching: Cognitive Flexibility
Prefrontal Cortex Activity and Task Switching
A dissociable set of regions was active for the switching analyses
from those active for overcoming prepotency. No prefrontal
regions were active for the a priori switch preparation analysis.
906 Overcoming Prepotency and Task Switching
d
Barber and Carter
A region of the right inferior frontal cortex did show targetrelated switch activity, however, activation was overlapping for
switch and repeat trials until the last scan (scan 13) of the trial.
This suggests that target and response-related switch activity
increases for both switch and repeat trials.
Of particular interest was the lack of DLPFC activity for
preparation to switch. In the literature, DLPFC recruitment
during an endogenously cued task switch is variable, with some
studies finding DLPFC activity (Sohn et al., 2000; Konishi et al.,
2002; Braver et al., 2003) and others finding no DLPFC activity
for such manipulations (Konishi et al., 1999; Nakahara et al.,
2002). Many studies use exogenous cueing or predictable task
order and therefore, do not distinguish activity due to preparation for a task switch and activity for stimulus-triggered task
switching (Dove et al. 2000; Kimberg et al., 2000; Rogers et al.
2000; Dreher et al., 2002; Dreher and Grafman, 2003).
This discrepancy of findings may be due to the recruitment of
a variable set of component processes associated with task
switching. For example, the Wisconsin Card Sorting Task
(WCST), a paradigm commonly employed for set shifting
manipulations, involves processes that may not be related to
set shifting per se, such as feedback and trial-and-error guessing,
but which are recruited during dimensional set shifting manipulations (Rogers et al., 2000; O’Reilly et al., 2002). In order to
isolate set shifting processes, Konishi et al. (1999) had subjects
perform a cued version of the WCST, which emulated task
switching paradigms. They found no DLPFC activity for the set
shifting manipulation.
The variability in findings in the task switching literature may
also be because the umbrella terms ‘task switching’ and ‘set
shifting’ include a variety of shift types (e.g. decision, dimensional shifts, reversal learning, visuomotor transformations).
Tables 5 and 6 illustrate the variability in the literature in both
the types of task switching paradigms used and in the PFC
activity that is elicited during performance of these paradigms.
Table 5 is organized separately by frontal and parietal coordinates to facilitate comparison of regions activated in the
current study with those activated in prior task switching
manipulations.
The current study employs a low-level S--R reversal, like that
of Dove et al. (2000). However, the current study, failed to
produce DLPFC activity for preparation to switch. The absence
of higher order dimensional or decisional shifts in the current
paradigm may account for the lack of such dorsal PFC activity.
The current study did find switch-related inferior frontal activity
for the response phase of the task. Sohn et al. (2000) also found
inferior frontal activity for task switching; however, this activity
was greater for the preparation phase of the task and was
related to endogenous preparation for task switching. The
discrepancy in results between this study and the current study
may be due to the type of shift required. Sohn et al. (2000) used
a similar paradigm to that of Rogers and Monsell (1995), which
requires a different decision to be made upon a task switch trial.
DLPFC activity has been implicated in response selection
(Bunge et al., 2000; Rowe et al., 2000) and may play a role in
overcoming prepotency at the level of specific responses rather
than at the level of task set. This suggests that DLPFC activity
should be reflected in the switching of individual responses
rather than the switching of task sets. Three pieces of evidence
suggest that this is not the case. First, the current results find
DLPFC activity for preparing to overcome a prepotent response
tendency. The target response was unknown during this phase
Table 3
Regions involved in overcoming a prepotent response tendency
Trial phase
Brain region
Preparation (scans 1--5)
Precuneus
Left dorsolateral PFC
Right dorsolateral PFC
Left medial PFC
Left caudate
Left anterior frontal cortex
Insula
Superior frontal gyrus
Precentral gyrus
Paracentral lobule
Medial frontal gyrus
Superior parietal lobule
Postcentral gyrus
Superior temporal gyrus
Lingual gyrus
Target (scans 5--13)
X
Y
Z
Size (mm3)
Size (voxels)
Condition F (1,12)
Scan F (4,48)
Condition 3 Scan F (4,48)
10
44/45
ÿ3
ÿ42
25
ÿ3
ÿ13
ÿ28
ÿ45
ÿ78
16
15
42
1
51
15
40
36
32
28
19
9
7
374.52
686.62
436.94
1154.77
312.10
842.67
1061.14
12
22
14
37
10
27
34
4.889*
10.429**
2.354
25.901***
11.425**
6.803*
6.242*
10.338***
3.382*
1.797
2.894*
3.170*
1.416
1.288
4.841**
7.239***
8.151***
6.032***
5.425**
7.884***
7.833***
6
6
6
6/32
7
43/40
42
18
10
ÿ27
6
ÿ16
ÿ25
ÿ45
55
ÿ13
ÿ1
ÿ13
ÿ29
ÿ1
ÿ76
ÿ11
ÿ19
ÿ64
66
70
71
48
43
18
12
5
686.62
280.89
1092.35
1279.61
624.20
436.94
2278.33
780.25
22
9
35
41
20
14
73
25
Condition F (1,12)
3.605
0.049
0.305
0.414
6.107*
1.660
3.902
3.869
Scan F (8,96)
2.693*
12.032*
1.459
5.238***
3.526**
3.465**
0.986
2.305*
Condition 3 Scan F (8,96)
4.299***
3.707***
4.284***
5.576***
4.875***
3.856***
5.946***
4.701***
Scan F (4,48)
Condition 3 Scan F (4,48)
BA
7
9
9
9
BA 5 Brodmann’s area.
*Statistically significant at P \ 0.05.
**Statistically significant at P \ 0.01.
***Statistically significant at P \ 0.005.
Table 4
Regions involved in switching between tasks
Trial phase
Brain region
Preparation (scans 1--5)
Precuneus
Target (scans 5--13)
Precentral sulcus
Precuneus
Supramarginal gyrus
Inferior frontal sulcus
BA
7
4
7
40
45/46
X
ÿ6
ÿ39
ÿ23
ÿ45
33
Y
Z
Size (mm3)
Size (voxels)
ÿ78
38
592.99
19
ÿ9
ÿ76
ÿ58
28
51
43
33
19
249.68
499.36
280.89
436.94
9
16
8
14
Condition F (1,12)
6.060*
Condition F (1,12)
3.098
2.792
10.533**
0.161
8.995***
Scan F (8,96)
11.875***
3.159**
0.632
1.24
7.131***
Condition 3 Scan F (8,96)
3.918***
3.790***
3.915***
4.318***
BA 5 Brodmann’s area.
*Statistically significant at P \ 0.05.
**Statistically significant at P \ 0.01.
***Statistically significant at P \ 0.005.
of the task. Second, as evidenced by the current response switch
analysis, response switching does not elicit DLPFC activity.
Third, Dreher and Berman (2002) found that the DLPFC activity,
which increased for overcoming residual inhibition, did not
interact with motor priming. This led the authors to suggest that
lateral PFC is involved with overcoming cognitive-level rather
than motor-level prepotency.
Another possible reason for the lack of DLPFC activity in the
task switching manipulations is that DLPFC is recruited for both
switch and repeat trials, and therefore, the current manipulation
is not sensitive to the difference in activity between these two
trials types. Studies have suggested that associative retrieval of
previous tasks utilizing the current task-stimuli may increase the
switch cost for several min after performance of the previous
task set (for review, see Allport and Wylie, 2000; Monsell, 2003).
Since these long-term priming effects last over several trials,
they are likely affecting both the switch and repeat trials in the
current study. Allport and Wylie (2000) found that processing
associated with the competing task set affected overall RTs as
well as switch costs. As with these behavioral results, DLPFC
activity may increase for both switch and repeat trials, and while
activity may not be differentially greater for switch compared
with repeat trials, such activity could be detected relative to
a baseline condition of pure task performance. Because the aim
of the current study was to detect activity related to task
switching per se, and not dual-tasking, the current paradigm was
not sensitive to such differential activity.
While dorso-lateral regions of the PFC did not show switch
activity for the overall preparatory switch analyses, regions of
PFC did show greater switch activity for the response phase of
the overall switch analysis and the post-hoc fast and slow bin
overall analyses. For the overall switch phase analysis, a region of
the inferior frontal cortex (33, 28, 19; BA 44/45), the middle
frontal sulcus (51, 17, 31; BA 9), and the anterior frontal cortex
(31, 58, 5; BA9) were more active for task switching in the slow
bin, response phase analysis. These results differ from Braver
et al. (2003). They found inferior PFC activity, which peaked
earlier for prepared, switch trials. However, this activity was
similar in magnitude for switch and repeat trials in the slow bin
analysis, which suggests that this activity may increase for dualtask conditions, and not specifically for switch trials. The
current study found prefrontal activity that was superior to
that found by Braver et al. (2003). Activity was greater for the
slow bin analysis in the response period of the task, which
suggests that this activity may be compensating for lack of
switch preparation.
The discrepancy in results between the current and Braver
et al. (2003) study’s inferior frontal activation may be due to the
different task demands. The Braver et al. (2003) results required
the maintenance of a verbal cue over the cue period of the task,
Cerebral Cortex July 2005, V 15 N 7 907
Table 5
Frontal and parietal regions implicated in previous task switching studies
Study
Task
Manipulation
Braver et al. (2003)
Dreher and Grafman (2003)
semantic classification task
2 letter discrimination tasks
switch/repeat (transient)
switch/baseline
7
7/40
switch/dual-task
7/40
Cools et al. (2002)
Dreher et al. (2002)
probabilistic reversal learning
2 letter discrimination tasks
Dual-task[switch
final reversal err/correct baseline
unpredictable task order
switch/baseline
Konishi et al. (2002)
WCST
negative feedback (A--B)
cognitive set shifting (B--C)
Nakahara et al. (2002)
Rushworth et al. (2001)
visuomotor intentional shift task (RS)
set-shift activation
SR switch vs stay
visual attentional shift task (VS)
stim switch/repeat
BA reported
7
7
40
7/40
7/40
7
40
7
7/40
40
40
7
7
RS--VS
40
VS--RS
Konishi et al. (2001)
intentional encoding (words/faces)
block transition effect (transient)
Dove et al. (2000)
SR mapping switch
switch/repeat
Kimberg et al. (2000)
Rogers and Monsell (1995) task
switch/repeat
Rogers et al. (2000)
ID/ED shift learning task
discrimination performance
7/40
31/7
40/19
19/7
7
40
7
7
40
7
Sohn et al. (2000)
Rogers and Monsell (1995) task
WCST
reversal--ID shift
foreknowledge/no foreknowledge
switch/repeat (no foreknow)
switch/repeat
39
40
39/40
40
semantic classification task
switch/repeat (transient)
6
44/9
45/47
9/10
46/10
6
6
8/9/44
8/9/46
Konishi et al. (1998)
Braver et al. (2003)
a
mixed/single task (sustained)
Dreher ans Grafman (2003)
2 letter discrimination tasks
switch/baseline
switch/dual-task
8
8/9/44
44/45
10
Talairach
ÿ28
ÿ36
36
ÿ36
48
ÿ12
ÿ4
42
ÿ32
40
ÿ36
36
4
ÿ56
12
ÿ10
ÿ52
ÿ54
10
ÿ29
ÿ33
17
3
ÿ11
ÿ23
ÿ6
9
ÿ48
29
ÿ4
ÿ38
ÿ33
50
ÿ10
ÿ23
16
ÿ33
ÿ19
ÿ45
ÿ34
15
ÿ17
ÿ26
ÿ29
ÿ53
ÿ30
ÿ29
33
7
ÿ31
11
ÿ8
ÿ32
0
ÿ30
26
ÿ32
34
ÿ54
ÿ36
ÿ37
ÿ45
38
ÿ16
ÿ46
ÿ40
22
34
ÿ28
36
ÿ52
48
48
4
ÿ60
60
ÿ28
48
ÿ66
ÿ56
ÿ60
ÿ60
ÿ48
ÿ56
ÿ68
ÿ42
ÿ56
ÿ48
ÿ56
ÿ60
ÿ74
ÿ48
ÿ58
ÿ70
ÿ36
ÿ34
ÿ73
ÿ69
ÿ60
ÿ64
ÿ61
ÿ63
ÿ71
ÿ71
ÿ57
ÿ41
ÿ40
ÿ77
ÿ44
ÿ72
ÿ35
62
ÿ70
ÿ66
ÿ54
ÿ72
ÿ34
ÿ59
ÿ66
ÿ78
ÿ79
ÿ75
ÿ38
ÿ45
ÿ48
ÿ57
ÿ43
ÿ53
ÿ71
ÿ68
ÿ50
ÿ75
ÿ52
ÿ64
ÿ64
ÿ66
ÿ70
ÿ51
ÿ72
ÿ36
ÿ33
3
15
30
39
48
4
8
12
16
44
28
12
12
60
52
Terminology
45
52
48
56
52
68
24
40
48
48
52
44
40
42
62
60
42
46
55
55
54
52
52
51
50
48
48
48
45
44
39
38
30
50
49
48
46
42
38
37
34
53
47
46
25
54
44
48
40
40
30
57
45
42
50
45
44
44
34
41
30
48
39
63
21
0
18
18
60
60
36
40
28
52
32
8
ÿ4
0
task switch
task switch
reversal learning
order predictability
task switch
feedback
set shift
set shift
SR map switch
attentional
task switch
restart cost
task switch
task switch
general learning
reversal learning
preparation
task switch
set shift
task switch
task switch
(Continued)
908 Overcoming Prepotency and Task Switching
d
Barber and Carter
Table 5
Continued
Study
Task
Manipulation
BA reported
Talairach
Dual-task[switch
32/24
8
47
32
6
8
ÿ8
4
0
8
38
4
ÿ49
42
27
39
ÿ28
ÿ60
ÿ36
0
ÿ44
36
ÿ28
ÿ40
48
60
ÿ4
ÿ44
40
ÿ36
ÿ28
4
4
38
6
0
38
ÿ40
36
0
46
36
ÿ38
ÿ48
ÿ40
46
40
38
4
ÿ10
ÿ48
8
34
46
ÿ52
ÿ32
30
42
5
41
7
ÿ9
ÿ29
29
ÿ8
ÿ44
40
ÿ36
28
ÿ16
ÿ56
ÿ30
ÿ10
4
42
ÿ48
52
ÿ42
38
ÿ24
8
ÿ6
Cools et al. (2002)
probabilistic reversal learning
final reversal err/correct baseline
Dreher and Berman (2002)
3 letter discrimination tasks
restart cost
overcoming residual inhibition
Dreher et al. (2002)
2 letter discrimination tasks
regular timing
predictable task order
unpredictable task order
switch/baseline
task order 3 timing
Konishi et al. (2002)
WCST
negative feedback (A--B)
cognitive set shifting (B--C)
Hemisphere 3 effect
L hem dominance for (B--C)
R Hem Dominance for (A--B)
Set-shift activation
Nakahara et al. (2002)
9/46
45
6
8/9/44
10/46
10
8/9/44
44/45
6
8/9/44
8/9/46
6
4/6
9/46
10
24
6/8
6
8/6
8
6/44
9
9/8
45/44
47/12
6/44
45/44
46
45/44
6/44
46
6
8
6/44
45/44
47/12
Konishi et al. (2001)
intentional encoding
block transition effect (transient)
Dove et al. (2000)
SR mapping switch
switch/repeat
Kimberg et al. (2000)
Rogers and Monsell (1995) task
switch/repeat
Rogers et al. (2000)
ID/ED shift learning task
discrimination-performance
reversal--ID shift
6/32
6/44
32
32/24
9
9/46
6/32
9/6/44
9/6
44/45
6
44/45
6
9
45
10/46
10
10/11
9
24/32
56
ÿ48
28
36
32
24
15
4
4
27
19
0
12
60
64
16
24
4
20
16
28
12
ÿ12
28
28
48
28
30
10
18
36
2
8
18
48
10
24
2
12
42
10
4
38
ÿ6
12
ÿ4
28
2
16
14
20
26
12
9
3
23
23
39
45
11
5
8
20
23
ÿ10
ÿ7
26
14
20
10
14
20
50
54
52
56
34
Terminology
28
24
20
12
52
ÿ2
42
30
23
27
11
72
32
ÿ4
8
32
20
56
32
40
32
56
32
28
24
8
16
58
58
50
40
40
38
36
34
24
ÿ6
36
20
14
20
36
14
66
50
50
52
38
24
20
6
0
ÿ2
48
38
32
28
36
26
47
37
36
13
8
51
33
ÿ5
56
46
44
38
22
10
4
ÿ10
20
0
reversal learning
restart cost
overcoming inh
fixed task timing
order predictability
task switch
order and timing
predictability
feedback
set shift
set shift
restart cost
task switch
task switch
general learning
reversal learning
(Continued)
Cerebral Cortex July 2005, V 15 N 7 909
Table 5
Continued
Study
Task
Manipulation
BA reported
Talairach
ED--ID shift
8
9/46
10
6
9/46
47
8
46/45
44/45
24/32
44/45
ÿ54 8
16 46
ÿ8 60
28 8
18 44
36 40
26 23
53 27
inf frontal
ÿ3 25
ÿ40 18
39 15
ED shift-reversal
Sohn et al. (2000)
Rogers and Monsell (1995) task
Konishi et al. (1999)
Konishi et al. (1998)
WCST
WCST
switch/repeat (no foreknow)
foreknowledge/no foreknowledge
switch/repeat (w/cue)
switch/repeat
Terminology
44
26
8
62
24
ÿ8
43
6
35
22
22
set shift
set shift
task switch
preparation
task switch
set shift
Table 6
Type of shift for various task switching paradigms
Study
Task
Manipulation
Type of shift
Dimension
Dreher and Grafman (2003)
Cools et al. (2002)
Dreher and Berman (2002)
2 letter discrimination task
probabilistic reversal learning task
3 letter discrimination task
switch/baseline, switch/dual-task
final reversal/baseline
switch/repeat, ABA/ABC
x
Dreher et al. (2002)
2 letter discrimination task
switch/repeat, fixed/random timing,
pred/unpred order
x
Gurd et al. (2002)
O’Reilly et al. (2002)
verbal fluency categorical switching
ID/ED shift learning task
Ravizza et al. (2002)
Rogers et al. (2002)
Rushworth et al. (2002)
Cools et al. (2001)
Allport and Wylie (2000)
Goschke (2000)
Meiran (2000)
Sohn et al. (2000)
variant of Rogers and Monsell (1995)
task
ID/ED shift learning task
visuomotor intentional set shift task
(RS)
visual attentional set shift task (VS)
variant of Rogers and Monsell (1995)
task
standard Stroop
variant of Rogers and Monsell (1995)
task
Meiran (2000) task
Rogers and Monsell (1995) task
(w/cue manip)
910 Overcoming Prepotency and Task Switching
d
Barber and Carter
task switch
reversal learning
task switch, overcoming residual
inhibition
task switch, timing and task
order predictability
task switch
set shift/switch
set shift/switch
x
x
x
x
x
ED shift
ID shift
ED or ID reversal shift
SR switch
x
selective attention switch
switch/repeat, crosstalk/no crosstalk
x
Stroop task
x
x
mixed/pure task
switch/repeat
x
Activation of Posterior Regions for Task Switching
While no regions of the frontal cortex were more active for
preparation to switch in the a priori analyses, posterior regions
produced greater activity for these analyses. The precuneus
region displayed greater activity for preparation to switch and
this ROI overlaps with precuneus activity for the switch analysis
in the response period of the task. Activity was greater during
the switch than the repeat trials for both phases of the task.
Likewise, this activity overlaps with an ROI found in the cue-byscan response-related manipulation and showed greater activity
during performance of the nonprepotent than the prepotent
task trials.
Two points of interest may be made about the precuneus
activations. First, though activation did not reach threshold in
S--R
x
x
ED shift
ID shift
reversal shift
which may have required subvocal rehearsal in both the switch
and the repeat conditions. The current study however, required
the maintenance and reconfiguration of S--R mappings, which
may result in frontal activation only during the retrieval of the
S--R mapping. This interpretation is consistent with the pattern
of current results.
Terminology
Stimulus
x
task switch/set shift
x
x
x
set shift
set shift
reversal learning
reversal learning
x
visual attention switch
set shift/switch
x
task switch
task switch
task switch
the cue-by-scan preparatory analysis, this region showed subthreshold activation in the dissociation analysis (F > 2.56, df =
1,4, P > 0.05). This suggests that subthreshold increased activity
was present in preparation for the nonprepotent condition
as well as in the other manipulations. Second, the time course
of activity for the response-related switch and overcomingprepotency manipulations are very similar, suggesting that this
region is performing a similar process in both the switch and the
nonprepotent task conditions.
These results suggest that precuneus activity increases with
the attentional demands for detection of stimuli or stimulus
features. Astafiev et al. (2003) found that the precuneus was
more active when subjects were required to make a saccade to
a region of space than when they were required to simply
attend to a target location, and the activation was even greater
when subjects were required to point to the target. As in the
present study, the authors found that, for both the preparatory
and the target-related phases of the task and activity, this pattern
of activity was greater during the target-phase. Further, Astafiev
et al. (2003) found a small area within this precuneus region
that was more active during attention than eye movement
preparation. The authors suggest that this may be a region
recruited exclusively for shifting attention. This region is near
a superior posterior parietal area (Gurd et al., 2002) implicated
in the supramodal control of attentional switching, and switching attention between visual attributes (Le et al., 1998).
Although these studies suggest that the precuneus may be
specific to attentional shifting, the current results and those of
other studies suggest that this activity is not exclusively related
to attentional shifting.
Several other studies employing S--R incompatibility paradigms have also found superior parietal and precuneus activations for compatibility manipulations (Iacoboni et al., 1996;
Dassonville et al., 2001; Merriam et al., 2001; Schumacher and
D’Esposito, 2002), many of which overlap with the regions
implicated in attentional switching. These results, as well as the
current finding of general precuneus activation for the task
switching and incompatibility manipulations, suggest that this
region is active under increased attentional demands, perhaps
for the detection of stimulus features necessary for S--R
associations.
Rushworth et al. (2001) found several precuneus and
superior parietal activations for two different switching paradigms: visual attentional set shifts (VS), in which the rule for
attending to one of two stimuli switched, and visuomotor
intentional set shifts (RS), in which the rules for the S--R
mapping switched. This activity was greater and more extensive
in the RS than the VS paradigm, especially for a region that falls
very close to the precuneus activity found in the current study.
These results suggest that the processing demands on some
regions of the precuneus may be greater during visuomotor
transformations, such as in the current study, than simply for
the selective attention to stimulus features.
The left inferior parietal lobule activation also showed greater
switch-trial activity for the response portion of the task. This
activity is consistent with posterior parietal activity in other
studies for task switching (Dove et al., 2000; Kimberg et al.,
2000; Sohn et al., 2000) as well as parietal activation that has
been implicated in S--R associations (Bunge et al., 2002). These
results suggest that this region reflects facilitation of S--R
reversals during task switching.
The inferior parietal activation found in the current task
switching manipulation falls near a posterior region of the
lateral intraparietal area (LIP) active for attending, looking and
pointing to a target location (Astafiev et al., 2003). Regions of
posterior LIP are also activated for switch manipulations
(Rogers et al., 2000; Sohn et al., 2000; Rushworth et al., 2001)
and have been implicated in visuomotor transformations
(Rushworth et al., 2001).
The current findings support the role of the posterior parietal
lobe in both the anticipatory and stimulus-triggered components of task switching. The precuneus may contribute to the
anticipatory component of task switching by playing a general
role in readying the cognitive system for task performance
under high attentional demands, while the inferior parietal
activation found during the target-phase of the task may reflect
the stimulus-triggered component of task switching associated
with the reconfiguration of task set.
Notes
Address correspondence to A.D. Barber, University of Pittsburgh,
Psychology, Pittsburgh, PA, USA. Email: [email protected].
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