Shared and specific patterns of interhemispheric functional connectivity in bipolar disorder and schizophrenia revealed by BOLD fMRI.

Schizophrenia and bipolar disorder are two kinds of serious psychiatric disorders. Despite different diagnostic criteria, the patients have significant biologic and clinical overlaps, challenging for the early identification, diagnosis, intervention, and management of these two diseases. Given the limited research on interhemispheric functional connectivity (FC) differences between schizophrenia and bipolar disorder, we included 38 schizophrenia patients, 34 bipolar disorder patients and 57 HCs from a publicly available fMRI dataset to investigate it across the whole brain by using voxel-mirrored homotopic connectivity (VMHC). Correlation analyses were also performed to examine the associations between VMHC values, clinical symptoms, and neuropsychological tests. Our findings revealed that schizophrenia and bipolar disorder shared the functional alteration in the insula, exhibiting decreased VMHC compared to HCs, but the specific regions differed: bipolar disorder demonstrated alternations in the inferior occipital gyrus, while schizophrenia showed changes in the postcentral gyrus. Both schizophrenia and bipolar disorder showed decreased VMHC relative to HCs. These alterations correlated with clinical symptoms, underscoring the importance of these brain regions in mental diseases. Our research provides new insights into the similarities and differences between the two diseases, suggesting that interhemispheric functional disconnection might critically contribute to the development and maintenance of these psychiatric disorders.