Alterations of Intrinsic Brain Connectivity Patterns in Depression and Bipolar Disorders: A Critical Assessment of Magnetoencephalography-Based Evidence

Abstract

Despite being the object of a thriving field of clinical research, the investigation of intrinsic brain network alterations in psychiatric illnesses is still in its early days. Because the pathological alterations are predominantly probed using functional magnetic resonance imaging (fMRI), many questions about the electrophysiological bases of resting-state alterations in psychiatric disorders, particularly among mood disorder patients, remain unanswered. Alongside important research using electroencephalography (EEG), the specific recent contributions and future promise of magnetoencephalography (MEG) in this field are not fully recognized and valued. Here, we provide a critical review of recent findings from MEG resting-state connectivity within major depressive disorder (MDD) and bipolar disorder (BD). The clinical MEG resting-state results are compared with those previously reported with fMRI and EEG. Taken together, MEG appears to be a promising but still critically underexploited technique to unravel the neurophysiological mechanisms that mediate abnormal (both hyper- and hypo-) connectivity patterns involved in MDD and BD. In particular, a major strength of MEG is its ability to provide source-space estimations of neuromagnetic long-range rhythmic synchronization at various frequencies (i.e., oscillatory coupling). The reviewed literature highlights the relevance of probing local and interregional rhythmic synchronization to explore the pathophysiological underpinnings of each disorder. However, before we can fully take advantage of MEG connectivity analyses in psychiatry, several limitations inherent to MEG connectivity analyses need to be understood and taken into account. Thus, we also discuss current methodological challenges and outline paths for future research. MEG resting-state studies provide an important window onto perturbed spontaneous oscillatory brain networks and hence supply an important complement to fMRI-based resting-state measurements in psychiatric populations.

Keywords: bipolar disorder; connectivity; depression; magnetoencephalography; mental illness; oscillations; psychiatry; resting-state.

Figure 1

Schematic overview of the key brain regions that show abnormal long-range connectivity patterns in subjects with MDD. Here, we only show areas for which evidence has been confirmed across at least MEG and fMRI modalities. Orange arrows represent altered connection between two brain regions that has been confirmed using both MEG and fMRI resting-state paradigms. Red arrow represents altered resting-state connectivity between two regions that has been confirmed across MEG, EEG, and fMRI. Abbreviations: MEG, magnetoencephalography; EEG, electroencephalography; fMRI, functional magnetic resonance imaging; dlPFC, dorsolateral prefrontal cortex; mPFC, medial prefrontal cortex; sgACC, subgenual anterior cingulate cortex; PCC, posterior cingulate cortex; MDD, major depressive disorder. (Green–blue striped area represents dlPFC shown here from a medial view perspective for convenience.)