Unipolar and bipolar depression are associated with neurocognitive impairment. However, both the specific pattern of deficits and whether unipolar and bipolar individuals differ on neurocognition remain unclear. Furthermore, neurocognition is related to current depression severity, but it is unknown whether this association stems from between-individual trait differences in disorder severity or from within-individual state variation in symptom severity. We addressed these questions in 43 unipolar and 51 bipolar participants drawn from a 25-year longitudinal study of severe mental illness. We assessed (1) differences among unipolar and bipolar individuals and the general population in attention/processing speed, verbal fluency, cognitive flexibility, and verbal memory; (2) the extent to which current symptom severity, overall trait disorder severity, and within-participant changes in symptom severity predicted neurocognition; and (3) the stability of neurocognitive measures over six years. Both groups showed generalized impairment relative to population norms. Bipolar participants performed more poorly than unipolar participants on measures of attention/processing speed; this may be attributable to differences in medication. Trait disorder severity predicted performance on attention/processing speed, cognitive flexibility, and long-term verbal memory. In contrast, within-participant state changes in depressive symptoms predicted change in only one non-specific cognitive measure. Most measures were stable over six years. Findings are consistent with previous evidence of generalized cognitive impairment in severe mood disorders. Associations of attention/processing speed, cognitive flexibility, and long-term memory with trait depression severity suggest these measures may be dimensional risk markers for severity of depressive illness
