Most patients suffering from schizophrenia achieve worse results than healthy controls in tests measuring attention. The studies show that among the patients suffering from schizophrenia, about 50% abused psychoactive substances during their lives. The data concerning the impact of substance abuse on attention in schizophrenia are inconsistent.

The objective of this study was to examine continuous attention differences between subjects with and without a dual diagnosis. A group of 80 patients with schizophrenia were examined. 40 of them never used illicit drugs, the other 40 also received a diagnose of addiction to psychoactive substances. The group with a comorbid addiction was examined after 6 weeks of detoxification and treatment in a therapeutic community. Continuous Performance Test was applied to for the neuropsychological assessment. The CPT-IP version of this test was used. The patients were presented 450 stimuli in three groups.

No statistically significant differences were found between two groups when they had to omit the identical pair stimuli (finger-up). The same happened in case of false alarms stimuli. However statistical significance appeared when the patients had to react to random stimuli. This part of the test was performed better by the group of schizophrenic patients without addiction.

The above inconsistence of the results may be due to the complexity of attention deficits. It is possible that the impacts of psychoactive substances may be different on the mechanism responsible for reaction to the sequence of experimentally important stimuli than to for ignoring those stimuli, which originally were defined as unimportant.