A diffusion MRI-based structural assessment of the striatal compartments, striosome and matrix, in Obsessive...

The striatum is central to habit formation and behavioral control, and striatal dysfunction has been implicated in compulsive actions and loss of inhibition. Dysregulation of inhibitory control circuitry may contribute to the repetitive, disabling behaviors observed in obsessive compulsive disorder (OCD). Striatal medium spiny projection neurons are organized within two distinct tissue compartments: the striosome forms a web-like tubular structure which spans the whole striatum and is embedded within the surrounding matrix. The compartments serve discrete functions in behavior regulation and have selective vulnerabilities in neurological diseases, motivating the investigation of the compartments in OCD. We performed a connectivity-based parcellation of the human striatum in vivo using diffusion MRI, identifying voxels whose relative abundance, spatial distribution, and structural connectivity profiles matched those of striosome and matrix in striatal tissue. Cortico-striate projections were organized into somatotopic zones that were distributed throughout the striatum. In OCD we found a shift from matrix-like to striosome-like volume, most abundantly in the bilateral rostral caudate and putamen, where striosome-like volume was increased by 24.3%. This shift toward striosome-like connectivity was 2.5-fold larger in caudate than in putamen. Cortico-striate projections were organized into somatotopic zones distributed throughout the striatum. Compartment bias was shifted toward striosome-like volume in 80% of these somatotopic zones. Volume did not shift uniformly throughout the compartment-like probability distribution, suggesting that increased striosome branch diameter may be a neuroanatomic underpinning of OCD. Assessing the organization and connectivity of striosome and matrix in living humans is essential for understanding the pathophysiology of OCD.