Stimulus and response conflict in the color-word Stroop task: a combined electro-myography and event-related potential study.

We aimed to separate stimulus and response conflict in a manual color-word Stroop task by combining electro-myography (EMG) and event-related brain potentials (ERPs). EMG served as a measure of response conflict by detecting incorrect response hand activation in correctly responded trials. Response conflict was enhanced in both the incongruent and congruent conditions relative to neutral conditions. This suggests that the Stroop conflict (incongruent vs. congruent condition) could be explained by stimulus conflict in our study and that response conflict was not restricted to the incongruent condition of the Stroop task. Incorrect response activity was detected in a neutral condition using words but not in a neutral condition with non-words. This suggests that the mere presence of task-irrelevant stimuli semantically related to task-relevant stimuli can prime incorrect response activity. The above factors are important to consider in studies which do not have any explicit measure of response conflict. We have detected a robust marker of Stroop conflict, the N450 ERP effect, which has been related to ACC activity. The N450 was probably affected by stimulus conflict in our study and it may be a marker of general conflict processing.