The human dorsal hippocampal commissure: Delineating connections across the midline using multi-modal neuroimaging in major depressive disorder.

Reclining on the lyra of the fornix, the dorsal hippocampal commissure (DHC) facilitates communication between the left and right hippocampi. Despite being a key commissural fiber bundle of the hippocampus, the DHC remains relatively underexplored in humans. Using high-resolution T1 and T2 magnetic resonance imaging in conjunction with High Angular Resolution Diffusion Imaging and Constrained Spherical Deconvolution tractography, we investigated if there are differences between the DHC of a sample of 54 healthy controls and 38 patients diagnosed with Major Depressive Disorder (MDD). After isolating the DHC using an anatomically derived protocol, segmented volumes of the left and right temporal pole, amygdala, hippocampus and parahippocampal gyrus extracted. In both controls and MDD patients, DHC fibers were spread across each of these four regions, with the parahippocampal gyri contributing the most to the volume of this white matter tract. Furthermore, diffusion metrics showed no effect of aging or sex on the integrity of the DHC in either group. This study is the first to investigate the DHC in MDD and the first to demonstrate that the DHC connects with the amygdala and temporal pole on both sides of the brain. Our data shows that the human DHC, rather than simply connecting the left and right hippocampi, allows connections between each hippocampus and the contralateral temporal pole, amygdala and parahippocampal gyrus. This suggests that the human temporal lobe, and the parahippocampal gyrus, may exert more influence on the contralateral limbic system via the DHC than previously thought.