Backward masking task performance in stable, euthymic out-patients with bipolar disorder

Background. Several studies have suggested that visual backward masking (VBM) impairment is present in patients with bipolar disorder, but the clinical features, such as current symptoms, treatment status and past burden of illness that may contribute to the impairment have not been well described. This study examined well-characterized enthymic patients on two VBM tasks to ascertain the extent of VBM impairment in this group and the clinical correlates of this impairment. Method. Twenty-eight euthymic patients with a DSM-IV diagnosis of bipolar disorder were matched by age, sex and IQ with 28 non-psychiatric control subjects. Both groups completed two VBM tasks; one required subjects to locate the target stimulus, one required identification of the target stimulus. Reaction times and error rates across a range of target-mask inter-stimulus intervals were assessed. Results. Patients were significantly slower and had more errors on both VBM tasks. There was a significant relation between reaction times on the identification task and past burden of illness, particularly past number of depressions. There was no discernible impact of treatment status on reaction time or performance, including no difference in lithium-treated versus not treated subjects. Conclusions. These results are consistent with previous reports of neuropsychological deficits in euthymic bipolar disorder patients. The potential benefit to employing tasks such as VBM is that it may provide a method for relating clinical variables such as illness burden with known neural pathways in order to elucidate better the pathophysiology leading to impaired cognitive performance in patients with bipolar disorder.