Is Schneiderian Phenomenology an Interface Between Bipolar Disorder and Schizophrenia? Neurocognitive and Neuroimaging Evidence from a Controlled Comparative Study

Background Bipolar disorder (BD) and schizophrenia (SZ) share overlapping psychotic, cognitive and neuroanatomical features, yet their precise positioning within the psychosis spectrum remains debated. Schneiderian first-rank symptoms (FRS), which may appear in both disorders, have been proposed as a potential marker of proximity to SZ along this continuum. This study aimed to investigate neurocognitive and structural brain differences among psychotic bipolar disorder (P-BD), non-psychotic bipolar disorder (NP-BD) and SZ, with a focus on FRS as a diagnostic reference point. Ninety remitted patients (30 per group) meeting DSM-5 criteria were recruited. All underwent a standardized neurocognitive battery assessing processing speed, executive functions, attention, working memory, verbal learning/memory and social cognition (including DST, Stroop, WCST, TMT, AVLT, CPT, FEIT/FEDT). High-resolution T1-weighted MRI scans were analyzed via vol2Brain for automated segmentation of global, lobar, subcortical, insular, opercular and cerebellar structures. Group differences were evaluated using ANOVA with post hoc Tukey tests; significant variables entered multinomial logistic regression controlling for clinical covariates. Results SZ patients had significantly slower TMT-A times (p < 0.001) and poorer AVLT total learning (p < 0.001) and short-term recall (p < 0.001) versus BD groups. Volumetric analysis revealed reduced total vermis (p = 0.040), lobules VIII–X (p = 0.049), occipital pole cortical thickness (p = 0.003) and anterior insula volumes (p = 0.005) in SZ, while NP-BD showed larger right parietal operculum (p = 0.004). No significant differences were found in total gray/white matter volumes. Regression identified a combination of TMT-A time, AVLT scores and select regional volumes as robust discriminators between groups. Conclusions While P-BD and SZ share some cognitive and structural features, SZ demonstrates more widespread neuroanatomical alterations, supporting partially overlapping yet distinct pathophysiological pathways. Integrating neurocognitive and neuroimaging data may enhance diagnostic precision and inform targeted interventions.