Altered prefrontal cortex activity during working memory task in Bipolar Disorder : a Functional Magnetic Resonance Imaging study in euthymic bipolar I and II patients

Background: Working memory (WM) deficits are among the most frequently impaired cognitive domains in patients with Bipolar Disorder (BD), being considered promising cognitive endophenotype of the disorder. However, the related neurobiological correlates still deserve further investigation. The present study was aimed to explore whether dorsolateral prefrontal cortex (DLPFC) activity during WM processing was abnormal in euthymic bipolar patients and may represent a potential trait related phenotype associated with the disorder. Methods: Using 3 Testa functional Magnetic Resonance Imaging (3T fMRI), we studied 28 euthymic bipolar patients (15 BDI and 13 BDII), and 27 healthy controls (HCs), matched for a series of socio-demographic variables, while performing the N-back task for WM assessment. Results: We found that euthymic bipolar patients showed increased right middle frontal gyrus engagement compared with HCs (FWE-corrected p= 1 x 10(-3)), regardless of WM load, and in spite of similar WM behavioral performance between groups. In particular, BDl patients had greater BOLD signal change compared to HCs (post hoc Tukey HSD, p=1 x 10(-3)), while BDI patients expressed an intermediate pattern of activation between BDI patients and HCs. No other significant effects were detected in the corrected whole brain analysis. Limitations: Sample size, cross-sectional assessment and potential influence of some clinical variables. Conclusions: Results provide direct evidence of a primary physiological abnormality in DLPFC function in BDI and II, even in the absence of behavioral differences with HCs. Such exaggerated tMRI response suggests inefficient WM processing in prefrontal circuitry, and further studies are warranted to investigate whether the dysfunction is related to the genetic risk for the disorder.