Asymmetric scaling of cerebellar cortex and deep nuclei reshapes input-output architecture across primates

The cerebellum shapes distributed motor and association networks through precisely organized pathways linking the cerebellar cortex to the deep cerebellar nuclei (DCN), its principal output structures. Whether these cortical and nuclear compartments scale proportionally across primates, and how their relative expansion relates to cerebellar organization, remains unresolved. Here, we generate multimodal, high-resolution cross-species 3D cerebellar atlases in marmoset, macaque, and human by integrating iron-sensitive and diffusion MRI (MAP-MRI) with histological markers, enabling direct delineation of cerebellar lobules and DCN subdivisions within a unified framework. Comparative volumetric analyses reveal strikingly nonuniform scaling of cerebellar cortical-nuclear architecture: cortical expansion markedly outpaces DCN enlargement, indicating disproportionate growth of input relative to output systems. This divergence is accompanied by progressive reorganization of the DCN, with increasing dominance of the dentate nucleus. In contrast, cortical expansion is driven by posterior hemispheric territories, with lobules VI-IX and Crus I/II, linked to higher-order functions, showing the strongest scaling and largest absolute gains, whereas DCN subdivisions show selective rather than uniform scaling. Together, these findings establish nonuniform cortical-nuclear scaling as a systems-level organizational principle that reshapes cerebellar input-output architecture across primates.