* Response competition is often considered an important contributor to the delayed reaction to stimuli for which physical and semantic information are in conflict ("Stroop" effect). Response competition implies that brain areas associated with correct and incorrect responses (e.g., left and right motor cortices) should be simultaneously activated in conflict conditions. However, there is at present little direct evidence of this phenomenon, in part 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 prediction that conflict trials elicit simultaneous activation of both motor cortices, whereas nonconflict trials elicit brain activity only in the contralateral motor cortex. These data support a parallel view of the human information processing system.
The current study demonstrates the utility of a new imaging technique for visualizing brain activity with good temporal and spatial resolution--the event-related optical signal (EROS)--for examining the activation of conflicting motor responses in a spatial Stroop task. Stroop (1935) described a paradigm in which subjects were presented with stimuli whose semantic (color name) and physical (ink color) dimensions were associated with conflicting response requirements. A large number of subsequent studies has demonstrated that this type of conflict is associated with increased response latency and errors (for a review, see MacLeod, 1991). A number of brain imaging studies have identified structures (in particular, the anterior cingulate) whose activity is correlated with the presence of conflicting information (e.g., Peterson et al., 1999; Carter, Mintun, Nichols, & Cohen, 1997; Carter et al., 1998). Current theoretical views consider that the anterior cingulate is probably involved in the monitoring of conflic t conditions (see Gehring & Knight, 2000; Carter et al., 1998) or in other attention-control processes (Mesulam, 1990). The conflict itself, however, is presumed to occur somewhere else, presumably through the concurrent activation of motor cortex structures associated with competing responses. Surprisingly, there is little available evidence to support or refute the view that concurrent activation of competing responses contributes to the Stroop effect (but see Dehaene et al., 1998 for evidence of response competition in a different paradigm).
This scarcity of direct data demonstrating the occurrence of response competition stems, at least in part, from the difficulty of obtaining independent central measures of the time course of activation of different motor responses, and, therefore, to demonstrate the simultaneous activation of response structures associated with alternative responses. For instance, functional magnetic resonance imaging (fMRI) can, in principle, detect bilateral activation of the motor cortex areas associated with correct and incorrect responses on trials in which subjects are confronted with conflicting stimulus information. However, the limited temporal resolution of the hemodynamic signal makes it difficult to conclude that the activation of the two areas actually occurs at the same time. A technique combining temporal and spatial resolution may provide more definitive information on the concurrent activation of...