#682.16
RR89
Detecting Conflict-Related Changes in the ACC
Judy Savitskaya1, Jack Grinband1,3, Tor Wager2, Vincent P. Ferrera3, Joy Hirsch1,3
1. Program for Imaging and Cognitive Sciences (PICS), 2. Dept. of Psychology, 3. Dept. of Neurobiology & Behavior, Columbia University
Effect of Response Time
Incongruent RTs > Congruent RTs
(All Trials)
Incongruent
Congruent
Time (s)
Fixation
incongruent trials > congruent trials produced robust activity in medial
prefrontal cortex (mPFC). This activity was used to create a mask to
perform event-triggered averaging (Figs 4 & 5).
Trial 2 - Congruent
Blue
Fixation
Time
Yellow
Trial 3 - Incongruent
Long Trials > Short Trials
Fixation
Green
% BOLD signal change (x 10-2)
Red
Trial 1 - Congruent
Incongruent
Congruent
B
Trial 4 - Incongruent
Time (s)
ITI=0.5 - 7.0 s
Z = 4.0
RT Performance
Z = 1.6
Frequency
Incongruent Trials
Congruent Trials
RT
Figure 2. Incongruent trials have longer mean RTs than
congruent trials.
Figure 4. To simulate the effects of RT on a standard GLM
contrast, we compared long duration trials vs short duration trials
on a data set in which there were no differences in condition
type. A simple visual stimulus (flashing checkerboard) was
presented to subjects for variable durations (mean = 840 ms,
gamma distributed). There were no differences in condition type,
only differences in stimulus duration. A contrast of long trials (>
840 ms) vs short trials (< 840 ms) produced a result in visual
cortex similar to the activation map in mPFC for the incongruent
vs congruent contrast.
% BOLD signal change (x 10-2)
(Short incongruent & Long congruent trials)
Four colors (red, green, blue, yellow) were used. To eliminate repetition
priming effects, color combinations were randomized such that the
same color was not presented on two consecutive trials.
C
High Control Trials (preceded by incongruent)
Incongruent RTs = Congruent RTs
Incongruent
Congruent
Time (s)
Incongruent RT < Congruent RT
Figure 1. Subjects performed a modified version of the Stroop task.
Congruent
Time (s)
(Trials within 100 ms of median)
Z = 1.6
Figure 3. A standard GLM analysis was performed. A contrast of
Incongruent
A
Incongruent RTs = Congruent RTs
Stroop Task
Low Control Trials (preceded by congruent)
Incongruent RTs = Congruent RTs
% BOLD signal change (x 10-2)
Scanning was performed on a 1.5T GE Twinspeed Scanner. Sixteen
Functional imaging studies have suggested that the anterior
subjects were scanned for 5 runs (8 min each). TR = 2.0 s, 25 slices,
cingulate cortex (ACC) is an integral component of the decision-making
system. Using the Stroop task, it has been shown that BOLD activity in the 4.0 mm slice thickness, FOV = 200, 64 x 64.
ACC is increased for high conflict (incongruent stimuli), compared to low
conflict (congruent stimuli), decisions. This result has been interpreted to
Incongruent > Congruent
mean that the ACC is involved in conflict detection and/or resolution and
that the ACC may represent a conflict-related module in the brain.
Recent work (Grinband et al, 2008) has shown that stimulus
processing duration can modulate the BOLD response independent of
condition-type. In some decision-making tasks, response time (RT) may be
correlated to the condition type. This creates ambiguity in brain activation
results: neural activity can be due to differences in condition or differences
in RT.
We used a model-free (non-GLM) analysis of fMRI data.
determine whether ACC activity stems from the processing of cognitive
Z = 4.0
conflict or is related to conflict-independent effects of response time.
Effect of Cognitive Control
% BOLD signal change (x 10-2)
Imaging
% BOLD signal change (x 10-2)
Introduction
Incongruent
Congruent
Figure 6. To determine the effect of cognitive control on activity in
mPFC, congruent and incongruent trials with equal RTs were sorted
by previous trial type. When the previous trial was congruent (low
control), mPFC showed more activity on incongruent trials. When
the previous trial was incongruent, the result was reversed.
Discussion
• Differences in RT between conditions can obscure differences
in neural activity due to condition type.
Time (s)
Figure 5. A model free analysis. Hemodynamic responses
were measured in the mPFC for congruent and incongruent
trials of varying RTs. Fig. 5A shows the event-triggered average
for all the trials; this result is consistent with the GLM result in
Fig 3. Fig. 5B shows HDRs for trials with equal RTs (i.e. RTs
within a 200ms window). Fig. 5C compares fast incongruent
trials (> mean RT) with slow congruent trials (< mean RT). The
activity in mPFC is completely determined by RT rather than the
presence of response conflict.
• The response in the mPFC does not depend on the presence
of response conflict. Instead, it is determined by mean RT of
each condition and the level of cognitive control.
• After eliminating differences in RT, mPFC activity was
determined by the level of cognitive control.
• GLM analyses should be validated by model-free analyses.
Grinband, Wager, Lindquist, Ferrera, Hirsch (2008) Detection of
time-varying signals in event-related fMRI designs. Neuroimage 43,
509-520.
For further information contact:
Jack Grinband, jg2269@columbia.edu