Whole-brain precision functional mapping of a proximal upper-extremity motor task

Upper-extremity motor control has been widely studied with hand-task functional magnetic resonance imaging (fMRI) in healthy and disease states. However, proximal-arm motor tasks, such as at the shoulder, are uncommon during fMRI due to practical limitations, though they are critical to a complete understanding of upper-extremity motor control. Previous fMRI studies of proximal upper-extremity control have generally used unconstrained movements or a single low force or torque condition, leaving unknown how distributed supraspinal brain activity scales with increasing proximal motor output. Here, we implemented a custom, MR-safe device to test supraspinal brain activity during three levels of isometric shoulder abduction in healthy individuals using a whole-brain fMRI acquisition. We conducted subject-specific analysis with repeated sampling of the controlled shoulder abduction task within each individual (i.e., "precision mapping"), as the exact localization of shoulder motor activity is relatively understudied. Our analysis showed spatial variability in subject-specific motor activation compared to group-level localization. We found that shoulder abduction had a significant effect on motor activity in the primary motor cortex, supplementary motor area, dorsal premotor area, thalamus, putamen, and cerebellum. Across repeated fMRI runs, amplitude of motor activity was consistent across all tested regions. Overall, these findings provide an individualized, whole-brain torque-response characterization of the proximal upper-extremity motor system and establish an approach for examining how distributed supraspinal motor networks are recruited as mechanical demand increases. These whole-brain, subject-specific methods can be a critical tool for probing motor dysfunction in clinical populations.