What is the nature of increased Stroop interference in schizophrenia?

In comparison to controls, patients with schizophrenia classically display (1) an overall slowing in response times (RTs) and (2) a disproportionate slowing in RTs in the conflict condition of the Stroop color/word interference task. These two effects appear repeatedly in the card version of the Stroop task but were not replicated in a number of studies using a computer item-by-item version of the task. The present study was aimed at understanding the exact nature of the increased interference classically found in the performance of patients with schizophrenia in the card version of the Stroop task. We used a computer trial-by-trial version in which we investigated the effects of two major methodological differences between the two versions: (1) blocked (card version) versus mixed (computer version) presentation of the neutral, congruent and conflict conditions and (2) presence (card version) versus absence (computer version) of distractors in the spatial surrounding of the target. We found an overall slowing in performance and a disproportionate slowing in the conflict condition for patients with schizophrenia but only when the target was surrounded by distractors (in Experiments 2 and 3). The data are discussed in terms of a deficit in selective attention and inhibitory processes in schizophrenia.