Intuitive interference in quantitative reasoning.

It is well known that surface features of a task can queue multiple reasoning strategies. Interference or conflict among the strategies is signaled by increasing reaction times and error rates. To determine the neural basis of this interference, we studied 14 volunteers using event-related fMRI, as they compared the perimeters of geometrical shapes in congruent (where the shapes' perimeter changed in the same direction as its area) and incongruent (where the shapes' perimeter was conserved but its area changed) conditions. We found evidence for the engagement of bilateral parietal lobe systems during congruent trials and incorrect incongruent trials. Activation of bilateral orbital frontal cortex was evident when subjects inhibited interference associated with processing the salient feature 'area' and correctly completed the perimeters in the incongruent condition. Varying the level of interference exerted by the unattended feature 'area' (filled vs. unfilled shapes) affected the relative level of activation of right parietal regions but not orbital frontal cortex suggesting that the former is responding to the degree of facilitation while the latter is responsive to the presence of conflict (rather than the degree of conflict).