Stroop interference effect in schizophrenic patients: an electrophysiological approach.

Schizophrenic patients present deficits in executive control functions. The Stroop test requires executive control functions, in particular response inhibition. So far only one study has employed the high temporal resolution of electrophysiological methods to investigate the neural correlates of the Stroop effect in schizophrenia. This study investigated medicated patients with schizophrenia or schizoaffective disorder (n=15) and healthy controls (n=15) using event-related potentials. The analyses of the P1 and N1 components revealed no differences between the groups indicating intact sensory processing in schizophrenia during the Stroop test. We found greater negativity in the incongruent as compared to the congruent and neutral conditions between 350 and 450 ms over prefrontal scalp areas in healthy subjects but not in schizophrenic patients. Later on, a sustained positivity was observed over parietal scalp regions in healthy subjects. This later sustained potential was attenuated in patients but only in the first block. This suggests that following practice patients show similar parietal effects as healthy subjects. The total errors in the incongruent condition in patients correlated negatively with the difference in mean activity between incongruent and congruent conditions over the left parietal area (time window 600-1000 ms). In other words the more errors were made by patients, the more attenuated was the Stroop related electrophysiological effect. This suggests that the parietal activity is related to successful resolution of the Stroop conflict in schizophrenic patients. Furthermore, the absence of the frontal deflection in patients reflects dysfunctional neural processes associated with executive control.