Altered functional resting-state connectivity in schizophrenia and bipolar disorder patients

Introduction:

Schizophrenia (SZ) and bipolar disorder (BP) are two separate psychiatric conditions with distinct courses and outcomes but nevertheless sharing a significant overlap in clinical symptoms, genetic risk and brain structure. Both are characterized by gray and white matter abnormalities and disrupted structural as well as functional connectivity. Resting state functional magnetic resonance imaging (rsfMRI) is a powerful tool for examining potential functional connectivity disturbances across large-scale brain networks. Several distinct, functionally connected resting state networks (RSNs) have been identified across different study populations and methodologies. Altered connectivity in RSNs has been previously reported in schizophrenia (e.g. Woodward et al. 2011) and bipolar disorder (e.g. Mamah et al. 2013), including the default mode (DMN), dorsal attention (DAN), executive control (ECN) and salience (SN) networks.

Methods:

In this study, we utilized group ICA (gICA) on rsfMRI data with the purpose to explore abnormalities in the previously mentioned key resting-state networks and to investigate their relationship among schizophrenia and bipolar disorder. Participants comprised 55 patients with schizophrenia (n = 36) and schizoaffective disorder (n = 19), 51 with bipolar disorder and 594 healthy subjects, who underwent 8-min resting state fMRI scans with their eyes closed. Low-model-order gICA was then applied to extract subject-specific brain networks of both patient groups and healthy subjects which were later compared at the group level.

Results:

Both patient groups were characterized by aberrant connectivity within the default mode and executive control networks. Patients suffering from bipolar disorder showed decreased connectivity in the frontal DMN and increased connectivity within frontal and parietal parts of the ECN. Schizophrenia patients, on the other hand, showed decreased (cingulate cortex) as well as increased (precuneus, right angular gyrus) connectivity in the posterior DMN. Moreover, connectivity of the supramarginal gyrus was bilaterally decreased within the ECN and DAN.

Conclusion:

In sum, these results are consistent with previously reported abnormalities in different resting state networks in schizophrenia and bipolar disorder (e.g. Meda et al. 2012, Du et al. 2015). Nonetheless, further studies with larger patient cohorts are required to investigate relationships of resting state networks to different symptoms underlying psychiatric disorders. However, compared to expensive and time-consuming task-dependent measurements, fMRI at rest is short and easy to perform even for patients with severe cognitive disturbances. Moreover, the RSNs were reported to be stable, even on single subject levels, making the investigation of resting state networks an interesting, feasible and generally reproducible method for improving future diagnostics.