Beyond homogeneity: intrinsic connectivity of 10 primary motor cortex subregions

The primary motor cortex [Brodmann area 4 (BA4)] is traditionally treated as a single functional region or divided into anterior and posterior components based on cytoarchitecture. However, converging evidence suggests a more complex internal functional organization.

Resting-state functional MRI data were obtained from 198 healthy young adults (75 males; age range: 18-30 years) drawn from the Cambridge-Buckner dataset of the 1000 Functional Connectomes Project. Functional connectivity of Brainnetome-defined BA4 subregions was examined. Five subregions per hemisphere (A4hf, A4ul, A4ll, A4t, A4tl) were analyzed to assess (a) connectivity among BA4 subregions and (b) their integration within major large-scale functional brain networks.

All BA4 subregions exhibited strong and consistent coupling with sensorimotor networks, forming a conserved motor backbone. Beyond this shared architecture, subregions demonstrated distinct connectivity profiles with dorsal attention, salience, frontoparietal, visual, language, and default-mode networks, alongside structured intra-area coupling. Hemispheric asymmetries were most evident in nonmotor network interactions, particularly those related to language.

BA4 comprises multiple functionally distinct subregions that are differentially embedded within distributed brain networks. These findings extend classical cytoarchitectonic models and provide a refined normative framework for understanding motor and nonmotor network organization within the primary motor cortex.