The electrophysiological dynamics of interference during the Stroop task.

If subjects are required to name the color of the word red printed in blue ink, interference between word meaning and ink color occurs, which slows down reaction time. This effect is well known as the Stroop effect. It is still an unresolved issue how the brain deals with interference in this type of task. To explore this question, an electroencephalogram (EEG) study was carried out. By analyzing several measures of EEG activity, two main findings emerged. First, the event-related potential (ERP) showed increased fronto-central negativity in a time window around 400 msec for incongruent items in contrast to congruent and neutral items. Source localization analysis revealed that a source in the anterior cingulate cortex (ACC) contributed most to the difference. Second, time-frequency analysis showed that theta oscillations (4-7 Hz) in the ACC increased linearly with increasing interference and that phase coupling between the ACC and the left prefrontal cortex was longer persistent for incongruent items compared to congruent and neutral items. These effects occurred at a time window around 600 msec. We conclude that interference between color naming and word meaning in the Stroop task manifests itself at around 400 msec and mainly activates the ACC. Thereafter, sustained phase coupling between the ACC and the prefrontal cortex occurs, which most likely reflects the engagement of cognitive control mechanisms.