Basal but not complex motor control relies on interhemispheric structural connectivity after stroke

Abstract Objective While ample evidence highlights that the ipsilesional corticospinal tract (CST) plays a crucial role in motor recovery after stroke, very few studies have assessed cortico-cortical motor connections with inconclusive results. Given their unique potential to serve as structural reserve enabling motor network reorganization, the question arises whether cortico-cortical connections may facilitate motor control depending on CST damage. Methods Diffusion spectrum imaging (DSI) and a novel compartmentwise analysis approach were used to quantify structural connectivity between bilateral cortical core motor regions in chronic stroke patients. Basal and complex motor control were differentially assessed. Results Both basal and complex motor performance were correlated with structural connectivity between bilateral premotor areas and ipsilesional primary motor cortex (M1) as well as interhemispheric M1-M1 connectivity. While complex motor skills depended on CST integrity, a strong association between M1-M1 connectivity and basal motor control was observed independent of CST integrity especially in patients who underwent substantial motor recovery. Harnessing the informational wealth of cortico-cortical connectivity facilitated the explanation of both basal and complex motor control. Interpretation We demonstrate for the first time that distinct aspects of cortical structural reserve enable basal and complex motor control after stroke. In particular, recovery of basal motor control is supported via an alternative route through contralesional M1 and non-crossing fibers of the contralesional CST. Our findings help to explain previous conflicting interpretations regarding a vicarious or maladaptive role of the contralesional M1 and highlight the potential of structural connectivity of the cortical motor network as a biomarker post-stroke.