Assessing working memory via N-back task in euthymic bipolar I disorder patients: a review of functional magnetic resonance imaging studies

Bipolar disorder (BD) is a chronic and highly disabling mood disorder, associated with the highest suicide rate among psychiatric disorders. Even though neurobiological bases of BD have still to be further elucidated, recent neuroimaging studies provided compelling evidence about functional correlates of cognitive deficits in BD patients, with working memory (WM) impairment being one of the most commonly reported findings. Such dysfunctions are likely to persist beyond acute phases of the illness, so they qualify as endophenotypic markers for the disorder. This review sought to synthesize, through a MEDLINE search up to December 2012, published functional magnetic resonance imaging (fMRI) studies on WM networks, conducted through N-back task in euthymic BD I patients and including a control comparison group. Eight studies meeting the search criteria were identified. Despite heterogeneity across findings, particularly in relation to task performance (i.e. accuracy and reaction time), most studies reported a loss of connectivity in BD patients' prefrontal networks, traditionally involved in WM, as well as patterns of abnormal activation in the dorso/ventrolateral prefrontal cortex, other prefrontal areas and the parietal and temporal cortex. These findings suggest the involvement of intact secondary systems in order to overcome lack of integrity across WM circuits in BD patients. Further investigation in the field is warranted.