When in doubt, do it both ways: Brain evidence of the simultaneous activation of conflicting motor responses in a spatial Stroop task

Response competition is often considered an important contributor to the de layed reaction to stimuli for which physical and semantic information are i n conflict (''Stroop" effect). Response competition implies that brain area s associated with correct and incorrect responses (e.g., left and right mot or cortices) should be simultaneously activated in conflict conditions. How ever, there is at present little direct evidence of this phenomenon, in par t because of the paucity of brain imaging techniques that can independently monitor the time course of activation of adjacent brain areas, such as the motor areas. In the present study, we show that the event-related optical signal (EROS) can provide these types of data. The results confirm the pred iction that conflict trials elicit simultaneous activation of both motor co rtices, whereas nonconflict trials elicit brain activity only in the contra lateral motor cortex. These data support a parallel view of the human infor mation processing system.