From Hands to Feet: Experience-Driven Plasticity in Secondary Somatosensory Cortex in People Born without Hands

In congenital handlessness, where individuals rely on their feet for compensatory manual functions, reorganization in the primary somatosensory cortex (SI) appears limited to local shifts by adjacent body parts. It remains unclear how congenital handlessness and lifelong compensatory foot use shape the sensorimotor system beyond SI, and whether compensatory abilities can drive broader functional reorganization. Here, we used task-based and resting-state functional magnetic resonance imaging (fMRI) to investigate how compensatory foot use changes body representations in the secondary somatosensory cortex (SII) in people born without hands (individuals with upper-limb dysplasia, IDs). We found that IDs showed foot selectivity in the left hand-selective SII, with stronger foot activity than typically-developed (TD) control participants during action execution. Notably, this reorganization in the left SII hand area cannot simply be explained by this areas intrinsic organization - the typical secondary preference of this area in the TDs when hand responses are excluded, as this area in TDs shows shoulder - but not foot - selectivity when the hand activity is excluded. This indicates a functional reorganization dependent on the IDs sensorimotor experience. Resting-state results further revealed altered functional connectivity between SII and primary sensorimotor regions in IDs. Our findings suggest that SII exhibits flexible, experience-dependent plasticity, revealing a hierarchical principle of cortical plasticity, whereby reorganization in the SI is constrained by somatotopic principles while SII reflects functionally or experience-dependent plasticity.