Postnatal sensorimotor experience causally shapes hemispheric specialization across motor and language networks

The functional organization of the human brain is shaped by the interplay between innate constraints and postnatal experience. Hemispheric specialization, as a fundamental organizing principle of brain function, raises unresolved questions about how postnatal experience shapes lateralization and importantly, whether different lateralized cognitive systems develop through shared or independent mechanisms. We leveraged a natural experiment in individuals with obstetric brachial plexus injury (OBPI) in the right upper limb, resulting in lifelong left-hand dominance dissociated from the typical predisposition, to address these questions. Using functional MRI experiments during manual motor tasks, we found that altered hand-use experience selectively reshaped hemispheric organization in higher-order frontoparietal motor regions. Specifically, individuals with right OBPI showed reduced left-hemisphere dominance and rightward reorganization of abstract, hand-independent action representations relative to controls and individuals with left-side OBPI, whereas primary sensorimotor and subcortical regions preserved their canonical lateralizedl organization. Strikingly, experience-dependent motor reorganization extended beyond the motor system:individuals with right OBPI exhibited reduced left lateralization of the language network in Broca's area (BA44) during sentence listening. Crucially, language lateralization in BA44 covaried with the left lateralization of frontal motor areas, particularly for the compensatory, functional left hand. These findings demonstrate that postnatal bodily experience shapes the developmental emergence of hemispheric specialization, specifically in higher-order motor and language networks, and revealed coordinated organization between action and language systems.