Neural Activation During Response Competition Eliot Hazeltine NASA Ames Research Center Russell Poldrack Stanford University, MGH-NMR Center, and Harvard Medical School John D. E. Gabrieli Stanford University Abstract & The flanker task, introduced by Eriksen and Eriksen [Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143±149], provides a means to selectively manipulate the presence or absence of response competition while keeping other task demands constant. We measured brain activity using functional magnetic resonance imaging (fMRI) during performance of the flanker task. In accordance with previous behavioral studies, trials in which the flanking stimuli indicated a different response than the central stimulus were performed significantly more slowly than trials in which all the stimuli indicated the same response. This reaction time effect was accompanied by increases in activity in four regions: the right ventrolateral prefrontal cortex, the supplementary motor area, the left superior parietal lobe, and the left anterior parietal cortex. The increases were not due to changes in stimulus complexity or the need to overcome previously learned associations between stimuli and responses. Correspondences between this study and other experiments manipulating response interference suggest that the frontal foci may be related to response inhibition processes whereas the posterior foci may be related to the activation of representations of the inappropriate responses. & INTRODUCTION The successful performance of any task requires filtering out inappropriate actions and selecting actions that are consistent with current goals. The flanker task, intro- duced by Eriksen and Eriksen (1974), provides a well- controlled method for examining the neural systems that resolve the conflict among response options. In the task, a central target stimulus is presented simulta- neously with two distractor stimuli (flankers) and parti- cipants are instructed to respond according to the target and ignore the flankers. These task demands require participants to select the relevant information in a dis- play and inhibit the surrounding irrelevant information in order to make the correct response. The effect of the irrelevant information can be assessed by comparing reaction times on trials in which the flankers indicate the same response as the target (congruent trials) to trials in which they indicate a conflicting response (incongruent trials). The flanker task has some distinguishing properties in relation to other tasks that bring about conflict between competing responses. The Wisconsin Card Sorting Task (WCST), which requires individuals to change the sti- mulus dimension (e.g., shape or color) to which they respond, and the Stroop task, which requires that they name the color of a printed word while ignoring its meaning, induce response interference by creating un- certainty about the relevant stimulus information. In both tasks, participants must inhibit responses that were previously linked to the stimuli. In contrast, the flanker task uses a stimulus±response (S±R) mapping that is constant throughout the experiment. This property relates to an important difference between the flanker task and both the Stroop and the WCST: In the flanker task, participants must choose the relevant object in the display rather than the relevant rule to apply to generate the response. Thus, visuospatial attention may be used to gate the appro- priate information on to the response selection pro- cesses. Such a process is not applicable for the Stroop and WCST tasks because the relevant and irrelevant information are typically integral parts of the same stimulus. Spatial attention, however, is not the only means by which interference is resolved in the flanker D 2000 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 12:Supplement 2, pp. 118±129