Examining a supramodal network for conflict processing: a systematic review and novel fMRI data for related visual and auditory Stroop tasks

Cognitive control over conflicting information has been studied extensively using tasks such as the colour-word Stroop, flanker, and spatial conflict task. Neuroimaging studies typically identify a fronto-parietal network engaged in conflict processing, but numerous additional regions are also reported. Ascribing putative functional roles to these regions is problematic, since some may have less to do with conflict processing per se, but could be engaged in specific processes related to the chosen stimulus modality, stimulus feature, or type of conflict task. In addition, some studies contrast activation on incongruent and congruent trials, even though a neutral baseline is needed to separate the effect of inhibition from that of facilitation. In the first part of this paper, we report a systematic review of 34 neuroimaging publications, which reveals that conflict-related activity is reliably reported in anterior cingulate cortex and bilaterally in lateral prefrontal cortex, anterior insula, and the parietal lobe. In the second part, we further explore these candidate ‘conflict’ regions through a novel fMRI experiment in which the same group of subjects perform related visual and auditory Stroop tasks. By carefully controlling for the same task (Stroop), the same to-be-ignored stimulus dimension (word meaning), and by separating out inhibitory processes from those of facilitation, we attempt to minimise the potential differences between the two tasks. The results provide converging evidence that the regions identified by the systematic review are reliably engaged in conflict processing. Despite carefully matching the Stroop tasks, some regions of differential activity remained, particularly in the parietal cortex. We discuss some of the taskspecific processes which might account for this finding