- The purpose of this experiment was to directly examine the neural mechanisms of attentional control involved in the Simon task as compared to a spatial Stroop task using event-related fMRI. The Simon effect typically refers to the interference people experience when there is a stimulus-response conflict. The Stroop effect refers to the interference people experience when two attributes of the same stimulus conflict with each other. Although previous imaging studies have compared the brain activation for each of these tasks performed separately, none had done so in an integrated task that incorporates both types of interference, as was done in the current experiment. Both tasks activated brain regions that serve as a source of attentional control (dorsolateral prefrontal cortex) and posterior regions that are sites of attentional control (the visual processing stream-middle occipital and inferior temporal cortices). In addition, there were also specific brain regions activated to a significantly greater degree by one task and/or only by a single task. The brain regions significantly more activated by the Simon task were those sensitive to detection of response conflict, response selection, and planning (anterior cingulate cortex, supplementary motor areas, and precuneus), and visuospatial-motor association areas. In contrast, the regions significantly more activated by the Stroop task were those involved in biasing the processing toward the task-relevant attribute (inferior parietal cortex). These findings suggest that the interference effects of these two tasks are caused by different types of conflict (stimulus-response conflict for the Simon effect and stimulus-stimulus conflict for the Stroop effect) but both invoke similar sources of top-down modulation.