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Task switching is not a unitary phenomenon: Behavioral and neuroimaging evidence
Depts. of
1Psychology

M
E
T
H
O
D
S
t
t
t
b
b
b
s
x
s
n
n
v
n
Switch
s
t
Procedure
x
o
The task in both conditions is to find the odd target.
In the perceptual condition, participants responded to
the target’s location whereas, in the contextual
condition, participants responded to the key
memorized to the target identity in a previous practice
block. Perceptual and contextual switches were
blocked and there were 256 trials in each condition.
Stimuli stayed on the screen until a response was made
and the response-to-stimulus interval was 500 ms.
0.6
shift
1000
repeat
800
600
400
R
E
S
U
L
T
S
200
0
Contextual
Perceptual
Shift cost is greater for perceptual switches
(shift x task: F (1,9) = 7.6, p < .05)
Stimulus Repetitions
2000
1800
1600
1400
1200
repeat
1000
reverse
800
600
400
200
0
Contextual
I
M
A
G
I
N
G
M
E
T
H
O
D
S
Perceptual
 Shift cost was greater in the perceptual
D
I
S
C
U
S
S
I
O
N
condition
 One difference between the two
conditions is that the irrelevant dimension is
present in the perceptual condition
 However, shift cost is absent without the
presentation of the irrelevant dimension in
the perceptual condition (-4 ms vs 384 ms).
 Therefore, shift costs in the perceptual
condition seem to be driven by
interference from the irrelevant dimension
at the time of presentation
 Stimulus repetition benefits were greater
in the contextual condition
 This may be because targets were
always associated with the same response
in the contextual but not the perceptual
condition
S
Percent Change
RT
1200
S X S
+
2000ms
T T T X
500ms
+
2000ms
13500ms
Procedure & Analyses
7 – 9 runs of 24 trials each were obtained for
each participant.
One block of each condition was presented in
each run, and the starting condition was random
between runs.
Only the 2nd trial of each pair was analyzed
ROIs were obtained using a shift (2) x task (2) x
time (7) ANOVA.
 Voxel-wise tests used a threshold of p < .001
and a cluster size of 4 voxels
Type
EPI
Scanner
Voxel
size
Flip angle
Slices 36
3T Siemens Trio
3.438 x 3.438 x
3.4mm
90
TR
TE
2000 ms
25 ms
FOV 220 mm
DLPFC is greater for shifts of context
than perceptual shifts
0.3
R
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S
U
L
T
S
0.2
0.1
0
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3
4
5
6
7
-0.1
8
9
Context repeat
Context shift
Spatial repeat
Spatial shift
-0.2
-0.3
Left DLPFC (-42,40,28)
This is not due to greater difficulty with switching
as shift cost in the contextual condition was less
than the perceptual (65 ms vs 212)
R
E
S
U
L
T
S
0.4
0.2
0
2
3
4
5
6
7
8
9
Context repeat
Context shift
Spatial repeat
Spatial shift
-0.2
-0.4
Left Angular Gyrus
(-43,-72,7)
1
Percent Change
1400
Stimulus repetition priming is greater for
perceptual switches (F (1,9) = 8.52, p < .05)
Repeat
s
Stimuli
1600
Contextual switches
n
Parietal regions are more active for
perceptual switches than contextual
Six undergraduates participated (avg. age = 28)
Percent Change
s
B118
Subjects
1800
Ten undergraduates participated
Perceptual switches
C.S.
1,2
Carter
University of California, Davis, Imaging Research Center
2000
Subjects
Stimuli
&
2Psychiatry,
Shift Cost
RT
I
N
T
R
O
D
U
C
T
I
O
N
Task switching has been
operationalized in a variety of ways
across studies
 No studies have been run assessing
whether behavioral effects vary
according to the type of switch required
 Only recently have attempts been
made to dissociate neural activity in
regard to different types of shifts (Wager,
et al., 2005).
 We focus on two types of shifts:
 Perceptual shifts – shifts between the
processing of stimulus features such as
color and shape
 Contextual shifts – shifts in the rules or
relevant information needed to perform
tasks
Question 1: Will behavioral effects differ
depending on the type of shift
performed?
Question 2: Will neural regions be more
important for one type of shift than the
other?
S.M.
1
Ravizza &
0.8
0.6
Context repeat
Context shift
Spatial repeat
Spatial shift
0.4
0.2
0
2
3
4
5
6
7
8
9
-0.2
Right Superior Parietal
(-43,-72,7)
Other regions showing this pattern are the right
cuneus (BA 18) and cerebellar vermis
Contextual and perceptual shifts of
attention invoke differences in behavior
and
neural
engagement.
D
Stimulus repetitions are a bigger
I component of the shift cost in the
S contextual condition whereas stimulus
interference primarily drives shift cost in the
C perceptual condition.

The
DLPFC,
a
region
associated
with
the
U
maintenance and updating of context,
S was important when the relevance of S-R
mappings
switched
S
Parietal, occipital, and cerebellar
I regions were most important for shifts
O between relevant features of a stimulus
These results indicate that task switching
N is a complex form of cognitive control that
is instantiated by different neural regions
depending on the type of switch required