Abnormal processing of irrelevant information in schizophrenia: the role of illness subtype

In a study using a trial by trial version of the Stroop color naming task, we previously found that unmedicated patients with schizophrenia show a pattern of abnormal performance characterized by increased facilitation (speeding) of color-naming, color-congruent words but normal amounts of interference (slowing) of color-naming, color-incongruent words (Carter et al., 1992). Since a similar finding had recently been reported in patients with Parkinson's disease, we suggested that this finding was consistent with hypotheses about the neurobiological substrates of cognitive impairment that draw upon parallel patterns of cognitive performance in the two illnesses. We now report results from an enlarged group of unmedicated patients with schizophrenia that extend our original finding by allowing us to evaluate the role of illness subtype in abnormal performance on the Stroop task. We found that patients with the undifferentiated subtype of the disorder account for the increased Stroop facilitation effect. Patients with the paranoid subtype show their own pattern of abnormal performance, with normal amounts of facilitation and increased interference. These findings are consistent with the results of other studies which suggest that illness subtype is an important source of variability in studies of cognitive functioning in schizophrenia.