Functional dysconnectivity of the long-axis hippocampal parcels at rest and during memory tasks in schizophrenia and bipolar disorder.

Schizophrenia (SZ) and bipolar disorder (BD) are major psychiatric conditions that show both overlapping and distinct clinical features. The hippocampus, a structure central to multiple cognitive functions including episodic memory, is altered in both disorders. Rather than functioning as a single unit, the hippocampus contains subregions with distinct connectivity patterns that may help clarify similarities and differences between these disorders. The present study directly compared hippocampal functional connectivity in SZ and BD using six well-validated hippocampal seeds (bilateral anterior, mid, and posterior segments) across two experimental contexts: resting state and memory tasks and examined associations with clinical and cognitive measures. Relative to healthy controls, both clinical groups showed significant connectivity alterations. The right posterior hippocampal parcel exhibited hyperconnectivity with the right inferior temporal gyrus in SZ only. The right mid hippocampal parcel showed hypoconnectivity with the left middle temporal gyrus and right superior temporal gyrus, a pattern shared by both clinical groups. Finally, the right anterior hippocampal parcel demonstrated hypoconnectivity with the left middle temporal gyrus that was specific to SZ at rest but shared by both disorders during the memory condition. Overall, these findings reveal both shared and disorder-specific patterns of hippocampal dysconnectivity, with two connectivity clusters showing significant associations with mnemonic performance.