Elsevier

NeuroImage

Volume 44, Issue 3, 1 February 2009, Pages 1201-1209
NeuroImage

How the brain resolves high conflict situations: Double conflict involvement of dorsolateral prefrontal cortex

https://doi.org/10.1016/j.neuroimage.2008.09.026Get rights and content

Abstract

Executive control is a human ability that allows to overcome automatic stimulus-response mappings and to act appropriate in the context of a task where the selection of relevant stimuli and the suppression of interfering information are crucial. In order to address the question which brain areas are involved in the detection and processing of two simultaneously operating sources of interference derived from a spatial incompatibility task, we used functional MRI to contrast neural activity related to a double conflict situation to single incompatibility conditions. Results show signal increase of left dorsolateral prefrontal cortex when monitoring simultaneously presented conflict. There was no additional activity in the medial prefrontal cortex or anterior cingulate cortex although these regions are expected to play an important role in all types of conflict monitoring. Further analyses of conflict resolution and post-error adaptation pointed to different underlying functional mechanisms. While the resolution of high conflict was associated with rostral ACC activation, the post-error adaptation reflecting activity during post-error trials suggests a specific medial and lateral prefrontal network which was functionally distinct from conflict-related activity. Our results also suggest a major role for the basal ganglia during error detection and resolution.

Introduction

Common conflict tasks require subjects to respond to one stimulus dimension while ignoring another conflicting dimension. The most commonly used conflict tasks are the Stroop task (Stroop, 1935, MacLeod, 1991), the Eriksen Flanker task (Eriksen and Eriksen, 1974), and the Simon task (Simon, 1969, Simon and Berbaum, 1990). In the case of the Simon task, the costs in reaction time and accuracy are due to an incompatibility between stimulus location and response side. Although stimulus location is irrelevant for the task, subjects are faster and make fewer errors when stimulus and response location correspond (compatible condition) than when the stimulus is presented opposite to the response side (incompatible condition). The most common interpretation of the Simon effect refers to a conflict between an indirect and a direct route of response selection (Kornblum et al., 1990, Kornblum and Stevens, 2002). The indirect route provides the correct response on the basis of the relevant stimulus feature; and in parallel the direct route triggers an automatic response tendency towards the spatial position of the stimulus.

In an earlier study, variants of the Simon task based on form-from-motion induced strong conflict effects (Wittfoth et al., 2006). Two different but comparable types of conflicts were introduced. In a location-based Simon task stimulus position served as conflicting information, while in a motion-based Simon task the direction of dot motion had to be ignored. In the latter variant, stimuli were always presented at the centre of the screen and a Simon effect was induced by dot movement. Brain areas associated with conflict monitoring were located in the medial frontal gyrus and superior parietal lobule (SPL) which have been suggested to play major roles monitoring different types of conflict (Liston et al., 2006).

Based on these findings, we investigated possible influences of two simultaneous and conflicting information streams. While several functional MRI studies of cognitive conflict resolution pointed to an extensive network of brain areas (particularly the medial prefrontal, anterior cingulate cortex (ACC), lateral prefrontal and parietal regions (Peterson et al., 2002, Liu et al., 2004)), there has been no investigation of double conflict processing based on the same source of interference. So far, there have been attempts to investigate how a combination of different conflict tasks influences interference processing. Fan et al. (2003) provided behavioral data of a task in which incongruent flankers were merged with a spatial incompatibility task (see also Hommel, 1997, Wendt et al., 2006). Moreover, Stroop and Simon tasks were arranged together in the same experimental designs (Simon and Berbaum, 1990, Kornblum, 1994, Hommel, 1997, Wendt et al., 2006, Egner et al., 2007). This was done in order to find out whether there are independent or additive mechanisms processing two simultaneously presented conflicts.

Concerning the underlying neural networks these studies provided evidence of differing neural strategies for resolving conflict. While in the Stroop task incompatible stimulus features generate conflict that arises from task-relevant versus task-irrelevant stimulus processing (MacLeod, 1991), it is the inhibition by task-irrelevant information on motor output that serves as a mechanism to overcome conflict in the Simon task (Stuermer et al., 2002, Stuermer and Leuthold, 2003, Nieuwenhuis and Yeung, 2005). Other recent data suggest that these control mechanisms are executed in an independent and parallel fashion (Egner et al., 2007). Nevertheless, it is still an open question whether these differences ground on the fact that both conflict tasks were based on qualitatively different sources of interfering information.

The advantage of the present experimental design is that both conflicting information are based on spatial incompatibility. If it is assumed that the same type of conflict is processed on the same route, the simultaneous presence of two interferences of the same type should draw on the same neural resources, which would result in equal reaction times in single and double conflict trials. Alternatively, one could expect a double conflict condition with two interfering information streams to require additional cognitive resources in order to maintain appropriate response behavior when compared to single conflict conditions (e.g. location-based and motion-based Simon tasks).

Based on previous studies on cognitive conflict activity in stimulus-response compatibility tasks (Dassonville et al., 2001, Iacoboni et al., 1996, Merriam et al., 2001, Peterson et al., 2002, Schumacher and D'Esposito, 2002) we would expect lateral prefrontal brain regions to be additionally recruited during double conflict processing. Lateral prefrontal areas are known to implement performance adjustments in a variety of tasks while the ACC monitors ongoing performance and signals the need for higher control in conflicting trials. Higher ACC activation following double conflict trials may indicate that this area also reflects additional response conflict (Ridderinkhof et al., 2004a, Ridderinkhof et al., 2004b, Botvinick et al., 2004).

Moreover, recent findings regarding the conflict adaption effect may be replicated in the present investigation. The conflict adaption effect reflects the phenomenon that prior context situations influence the interference effects in subsequent trials. Especially the Simon effect is reduced or even absent when a high-conflict incompatible trial is followed by another incompatible trial compared to a task situation where the preceding trial is compatible (Stuermer et al., 2002, Wuhr and Ansorge, 2005). The conflict monitoring hypothesis (Botvinick et al., 2001) interprets these performance adjustments as an adaption of conflict control areas after conflict has been detected in the preceding trial. In essence, we compared brain activity induced by incompatible trials which were preceded by a compatible trial providing a high amount of cognitive conflict with incompatible trials preceded by another incompatible trial resulting in low cognitive conflict. However, conflict resolution processes are also likely in trials following incorrect responses. Subjects tend to react slower after error commission (Kleiter and Schwarzenbacher, 1989, Kerns et al., 2004) indicating a high degree of response conflict on post-error trials.

Thus, we investigated brain activation patterns associated with different mechanisms of cognitive control by separating brain activation in conflict and error detection on one side and high-conflict resolution processes (incompatible trial after incompatible trial; e.g. Etkin et al., 2006) and post-error adaptation after error commitment (post-error trials) on the other side.

Section snippets

Participants and task

Fourteen right handed, 22- to-30-year-old volunteers (mean age 25 (SD 2.6); 2 males) with no history of neurologic or psychiatric disease participated in the study. All subjects had normal or corrected-to-normal vision. Written informed consent was obtained from all participants prior to scanning and the study was approved by the local ethics committee. Subjects were paid € 10 for participation.

One of two different shapes was presented during each trial: a triangle or a square. Subjects' task

Behavioral data

The ANOVA for reaction times yielded significant main effects of ‘compatibility level’ (F(1, 13) = 80.4, p < 0.001) and ‘repetition’ (F(1, 13) = 6.2, p = 0.02). The interaction of ‘compatibility level’ × ‘repetition’ was also significant (F(1, 13) = 5.6, p = 0.03). Post-hoc Fisher's Least Significant Difference (LSD) tests showed that reaction times were significantly longer for incompatible compared with compatible trials (p < 0.001). Reaction times were also longer for unequal compared with equal trial

Discussion

The present study was aimed at analyzing the neural mechanisms involved in response conflict based on two simultaneously interfering information streams. Both Simon tasks clearly showed significant interference effects thus replicating the data of a previous study (Wittfoth et al., 2006). Subjects committed more errors in the double conflict condition than in the single incompatible conditions, while no significant differences in reaction times between double conflict and location-based Simon

Acknowledgments

MW was supported by the BMBF Neuroimaging programme (01GO0202) from the Center for Advanced Imaging (CAI)–Magdeburg/Bremen. The authors wish to thank Ekkehard Küstermann for the technical support; two anonymous reviewers for their helpful comments to improve the manuscript, and all the participants of this study for their motivation and patience.

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    Present address: Department of Neurology, Hanover Medical School, Hanover, Germany.

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