Changes in event related potentials during processing of Stroop stimuli.

In a simplified Stroop test, subjects responded with a button press to either the color or word dimension of Stroop stimuli. Response times were analyzed as a function of condition (color or word), stimulus type (congruent or noncongruent color-word pairing) and visual field of presentation (left or right). Changes in the stimulus evoked potential (Experiment I) and the preresponse (premotor) potential (Experiment II) were measured as a function of the independent variables and hemisphere of recording site. Stimulus type had the expected Stroop effect on response time, with noncongruent stimuli associated with longer RTs. The pattern of changes in the event related potentials suggested that the typical Stroop interference effect (color condition) is localized in the response selection stage of processing. However, the less common reverse Stroop effect (word condition) probably results from more complex processing interactions during encoding of the color and word dimensions.