Is Schneiderian phenomenology an interface between bipolar disorder and schizophrenia? Neurocognitive and neuroimaging evidence from a cross-sectional comparative study

Bipolar disorder (BD) and schizophrenia (SZ) show clinical and neurobiological overlap, but the position of psychotic BD within this spectrum remains uncertain. We tested whether BD with Schneiderian first-rank symptoms (P-BD) more closely resembled SZ than BD without lifetime psychosis (NP-BD). Ninety clinically stable participants (30 P-BD, 30 NP-BD and 30 SZ) completed a neurocognitive battery and T1-weighted structural MRI analyzed with vol2Brain. Group differences were tested using omnibus comparisons with corrected post hoc tests. Benjamini-Hochberg false-discovery-rate (FDR) correction was applied within cognitive and anatomical families. Secondary covariate-adjusted multinomial models were exploratory. SZ showed slower Trail Making Test performance and poorer Auditory Verbal Learning Test total learning and immediate post-interference recall. The verbal-memory findings survived FDR correction; Trail Making Test findings were at the correction threshold. SZ also showed smaller cerebellar vermis measures and reduced occipital-pole thickness relative to P-BD, all of which survived FDR correction. Occipital-pole volume, anterior-insula and opercular findings did not survive correction and were considered exploratory. Secondary multinomial models identified cognitive and regional structural variables associated with diagnostic group, but no validated classifier was developed. The FRS-defined P-BD group did not converge with SZ across cognitive or structural measures. The findings are compatible with partial overlap between BD and SZ alongside quantitative differences, but the small sample, multiple comparisons and absence of a healthy comparison group require cautious interpretation and independent replication.