The effect of medial frontal and posterior parietal demyelinating lesions on stroop interference

Functional imaging has consistently shown that attention-related areas of medial frontal and posterior parietal cortices are active during the attentional conflict induced by color naming in the presence of distracting words (Stroop task). Such studies, however, have provided few details of the correlational nature between observed regional brain activations and reaction time delay occurring in this situation. We analyzed the effect of medial frontal and posterior parietal lesions on the Stroop response in a group of patients with multiple sclerosis, a neurological disorder in which Stroop response speed is affected to varying degrees. Forty-five patients were assessed using a computer-presented verbal version of the Stroop task and specific MRI protocol. Demyelination areas were measured on five anatomical divisions of the medial frontal white matter and on white matter of the posterior parietal lobe. We found that a combination of frontal and parietal lesion measurements accounted for 45% of the Stroop interference time variance. Patients with more right frontal than left parietal demyelination showed slowed Stroop responses, whereas the predominance of lesions in the left posterior parietal region was associated with a reduced Stroop interference. These results may contribute to defining the specific participation of these attention-related brain areas in the conflict of attention represented by the Stroop paradigm. They also help to explain the variability of the Stroop effect in multiple sclerosis patients and suggest that the Stroop test does not assess just a single cognitive operation, but rather the combined effect of anatomically segregated neural processes.