Impulsivity in bipolar disorder and schizophrenia: a review on functional imaging studies
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Pathological impulsivity is one of the most often reported characteristics of bipolar and schizophrenic patients. Besides impulsivity, symptoms of both diseases are seen in schizoaffective disorder and there are many genetic, structural brain and neurocognitive communalities between the two disorders. Therefore, we conducted a PUBMED search on studies investigating impulsivity by means of functional neuroimaging (fMRI and PET) during the Stroop and Continuous Performance Paradigm in schizophrenia and bipolar disorder. A total of 48 articles matching our criteria were identified and used as the basis for this review. Bipolar patients far less often showed decreased performance (1 out of 16 articles) relative to schizophrenia patients (18 out of 32 articles) when compared to controls. Moreover, in schizophrenia, accuracy deficits were found to be both the result of inattention as well as impulsivity related problems. We argue that schizophrenia patients are more hampered on task performance and related brain activation than bipolar patients, since the right middle frontal cortex, right ventrolateral prefrontal cortex and anterior cingulate cortex are reliably found to be hypoactivated in schizophrenia while hypoactivation is often restricted to the right ventrolateral prefrontal cortex in bipolar disorder. However, we suggest that the results of this study merely reflect a general finding on executive functioning in bipolar disorder and schizophrenia, which is supported by correspondence to recent literature on executive functioning and morphological alterations in schizophrenia and bipolar disorder. Besides, this review elaborates on methodological problems such as IQ differences between groups, medication intake and lack of statistical power. We argue these characteristics to be crucial confounders and recommend the application of novel paradigms to further investigate impulsivity in bipolar disorder and schizophrenia.