Transcranial direct current stimulation (tDCS) over the right dorsolateral prefrontal cortex affects stimulus conflict but not response conflict
Introduction
A robust finding from experimental psychology is that when the human brain encounters a conflict, the efficiency of its performance suffers noticeably. Various experimental conflict paradigms have provided ample evidence demonstrating that irrelevant, incongruent information affects individuals’ response time and accuracy. This is evident in the flanker task introduced by Eriksen and Eriksen (1974), which shows slow and less accurate response to central visual target stimuli when these are flanked by stimuli that are incongruent with the target. Systematic experimentation has revealed two sources of conflict in this task, one related to the incongruence between the flankers and the target and one related to the incongruence between the response signaled by the flankers and the response signaled by the target (Wendt et al., 2007). Hence, the flanker effect reflects stimulus conflict and response conflict. Another extensively studied paradigm is the Simon task (Simon and Small, 1969), where responses to a non-spatial stimulus feature are slower and more error-prone when the location of the response is spatially incongruent to the location of the stimulus. Given the non-spatial nature of the relevant stimulus feature, this effect does not rely on stimulus conflict but on response conflict only (Hommel, 2011, Kornblum, 1992).
It has been suggested that when conflict (in incongruent trials) is detected, a cognitive control mechanism is engaged so to reduce and deal with the conflict according to the task’s requirements (Botvinick et al., 2001). While the flanker task and the Simon task have often been used to explore conflict-related cognitive control mechanisms, the fact that they show comparable behavioral outcomes does not necessarily imply the same neural mechanisms. Previous imaging studies have associated conflict resolution with the dorsolateral prefrontal cortex (DLPFC; Durston et al., 2003) and specifically in the right hemisphere (Egner, 2008, Egner, 2011, Egner and Hirsch, 2005, Kerns et al., 2004). However, imaging studies provide only correlational evidence for associations between cognitive functions and brain regions, which calls for additional evidence from studies using methods that allow for causal inferences. A non-invasive, safe method that allows for such inferences is transcranial direct current stimulation (tDCS). By inducing either positive (anodal) or negative (cathodal) intracranial current flow on a specific brain region, and thus affecting its excitability, brain functions can be temporarily and reversibly modulated (Nitsche and Paulus, 2001). A number of tDCS studies have provided evidence for a role of the right DLPFC in cognitive control mechanisms; for instance, tDCS stimulation over the right rather than the left DLPFC reduced cognitive control of stimulus–response binding (Zmigrod et al., 2014). In addition, modulation of performances in a Go/NoGo task after stimulation over the right DLPFC was reported by Beeli et al. (2008). These observations suggest an involvement of the right DLPFC in cognitive control functions.
The aim of the present study was to examine the role of the right prefrontal cortex in the cognitive control of conflict by means of tDCS. We were particularly interested in testing whether the flanker task and the Simon task would be equally affected. Comparable effects on both tasks would indicate a role of the right DLPFC in dealing with response conflict while a selective effect on the flanker task would indicate a role in dealing with stimulus conflict.
Section snippets
Experimental design
A randomized sham-controlled within-subject design experiment was conducted on healthy volunteers. The experiment comprised of three sessions of tDCS (anodal, cathodal, and sham) over the right DLPFC with the order of the sessions being counterbalanced across participants. The interval between the different sessions was at least 48 h, in order to minimize carryover effects. The study conformed to the ethical standards of the declaration of Helsinki and was approved by the Ethics Committee of
Results
All participants completed the three sessions without major complaints or discomfort as measured by the tDCS Adverse Effects Questionnaire (Brunoni et al., 2011). To compare the effect of the stimulation over the right DLPFC across the two tasks, mean reaction times (RTs) of correct responses and percentage of accuracy were analyzed per participant for congruent and incongruent trials in each task for each stimulation session. Repeated measures ANOVAs were performed on flanker trials and Simon
Discussion
The aim of this study was to examine the involvement of the right DLPFC in conflict situations, either in the case of combined stimulus and response conflict (Eriksen flanker task) or in the case of response conflict only (Simon task). The results are clear: while the flanker effect was mediated by cathodal stimulation over the right DLPFC (reflected in a larger flanker effect), there was no stimulation effect on performance in the Simon task (Fig. 3), which was further confirmed by a
Financial disclousures
We have no relevant financial or non-financial relationship or potential conflicts of interest to disclose.
Acknowledgments
We thank Lindsey van der Lans, Aafke Ruiter, and Patrick Fortier-Brown for their enthusiasm and invaluable assistance in recruiting, testing the participants of this study.
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