Neurale mechanismen betrokken bij conflict- en beloningsverwerking

The experience of cognitive conflict is known to trigger control improvements
(Botvinick et al., 2001; Gratton et al., 1992). Executive control can be
measured by classical paradigms like the Stroop and the Flanker task. In the
Flanker task, for example, subjects respond to target stimuli while ignoring
irrelevant distractor stimuli. Given that distractors can often not be filtered
out completely, they typically create conflict, especially if they suggest a
different response than the actual target. For instance, if participants
respond to a central visual target surrounded by visual distractors, they
perform better if target and distractor call for the same response (congruent
trials) than if they call for different responses (incongruent trials). It has
been shown that conflict produced during demanding incongruent trials recruit
neural conflict monitor mechanisms which drive adaptations in top-down
informational control (for a review, see Egner, 2007). This effect has been
demonstrated both by improved behavioral performance and neural activation
reflecting increased goal representations and task-relevant information (Egner
& Hirsch, 2005; Kerns et al., 2004). A so-called conflict-control network
involving the anterior cingulate cortex (ACC) and dorsolateral prefrontal
cortex (dl-PFC) is thought to underlie conflict-driven control adaptations
(Egner & Hirsch, 2005).
A recent series of studies aims to identify the affective component involved in
cognitive conflict (e.g. Botvinick, 2007; van Steenbergen et al., 2009). It has
been suggested that conflict is processed like an aversive event and may drive
phasic dips in mesolimbic dopamine levels (Jocham & Ullsperger, 2009). Indeed,
recent studies by the authors have shown that conflict can be counteracted by
rewarding events (van Steenbergen et al., 2009; van Steenbergen et al., 2010a;
van Steenbergen et al., 2010b). These studies thus suggest that rewarding
stimuli may counteract conflict processing by neural interactions between the
mesolimbic reward system and the conflict-control network. The current study
aims to test this prediction.

The primary objective of this study is to understand the neurocognitive basis
of conflict and reward processing and their interactions. To this end, we will
acquire fMRI data and behavioral responses of 25 healthy female adults (aged
18-30 years).

Stimuli
To measure conflict processing we will use a variant of the classical Eriksen
Flanker task (Eriksen & Eriksen, 1974). Subjects have to respond to a central
arrow target while ignoring flanking distracters. Conflict is manipulated by
using flankers that are either congruent (C) or incongruent (I) with the
target. Following previous studies (Egner, 2007), neural and behavioral data
will be analyses as a function of conflict on the current trial and adaptations
following conflict on the subsequent trial.
To provide rewarding events subjects will view emotionally positive stimuli,
after which they perform a short sequence of flanker trials. Humor cartoons
will be used that have been shown to recruit reward-related brain areas. As a
control condition we use neutral cartoons which are created by removing the
funny cues of the original humor cartoons. Stimuli are matched for visual
characteristics (cf. Mobbs et al., 2003; van Steenbergen et al., 2010a).
Each cartoon is presented for 6 s and is - after a jittered interval of 4 s -
followed by a series of 5 flanker stimuli each presented for 1 s and followed
by a jittered interval of 4 s.
Trial types will be presented intermixed, in a pseudorandom order.
Procedure
The proposed study will consist of one experimental session within the MRI
scanner. It is followed by humor ratings of the cartoons and validated
questionnaires measuring individual differences in reward sensitivity (e.g.
BIS-BAS scale; Franken et al., 2005) outside the scanner. Total scanning time
will constitute maximal 60 minutes. The total duration of the experiment,
including the questionnaires, will be approximately 90 minutes.
While fMRI data is acquired, participants perform the tasks as described above
in blocks of about 5 minutes followed by short breaks. Subjects are instructed
to perform the flanker task as fast and accurate as possible. After the
experiment, they will be fully debriefed about the purpose of the experiment
and will be given monetary compensation.
Data acquisition
MRI scanning will be performed at the Leiden imaging center, located in the
radiology department of the LUMC. Data will be acquired on the 3T Philips
scanner. Standard fMRI procedures will be adopted. Whole-brain structural
images will include high-resolution Fast Spin Echo scans that will be used for
fMRI localization. Functional images will be acquired using standard LUMC
scanning parameters
fMRI analysis
Primary analyses investigate BOLD responses to conflicting Flanker stimuli in
interaction with cartoon processing.
1. In keeping with previous research (cf. Egner, 2007), neural conflict
processing is analyzed using a 2 x 2 design including the factors Current Trial
Conflict (I vs C) and Previous Trial Conflict (i vs c).
2. In keeping with previous research (Mobbs et al., 2003), individual humor
ratings of cartoons are used as regressor.
3. Contrasts also compare predicted interactions between conflict (1) and
reward (2)
Expectations for behavior
We expect that our behavioral findings will replicate our earlier pilot study
(van Steenbergen et al., 2010a). Accordingly, conflict-driven increases in
executive control (measured by reduction in interference effects in response
times) are expected to be reduced by rewarding stimuli in comparison to the
neutral stimuli.
Expectations for brain activity
1. In keeping with previous research (Egner & Hirsch, 2005; Kerns et al.,
2004), we expect conflict monitoring activity during demanding flanker trials,
which is reduced if the preceding trial evokes conflict. Conflict-driven goal
activation is thought to involve dorsolateral prefrontal areas.
2. In keeping with previous research (Mobbs et al., 2003), rewarding stimuli
are thought to activate brain areas that play a central role in reward
processing (i.e., the mesolimbic dopamine system).
3. Interactions between (1) and (2) are predicted to show that reward
processing counteracts conflict processing.

Risks:
There are no risks associated with behavioral testing except the occasional
possibility of some boredom or fatigue. Testing will stop if a subject displays
frustration or appears tired.
There are no known risks associated with participating in an fMRI study. This
is a noninvasive technique involving no catheterizations or introduction of
exogenous tracers. Numerous human subjects have undergone magnetic resonance
studies without apparent harmful consequences. Radiofrequency power levels and
gradient switching times used in these studies are within the FDA approved
ranges. Some people become claustrophobic while inside the scanner and in these
cases the study will be terminated immediately at the subject's request. The
only absolute contraindications to MRI studies are metal implants, intraocular
metal and heart arrhythmia. Relative contraindications include pregnancy and
claustrophobia. Subjects who may be pregnant, who may have metallic foreign
bodies in the eyes or head, or who have cardiac pacemakers will be excluded
because of potential contraindications of MRI in such subjects.
Benefits:
Although there is no direct benefit to the participants, the proposed research
is expected to make a significant contribution to our understanding of the
neural mechanisms underlying conflict and reward processing. Ultimately, this
can be beneficial for various practical purposes, including the treatment of
mood disorders which are associated with dysfunctional conflict and reward
processing. In terms of scientific contribution, the study will be the first
study to investigate the neural basis of interactions between reward and
conflict processing.
The importance of the benefits gained from this research far outweighs the
minimal risks involved.