Contralesional activity reflects compensation, while brainstem detour pathways support skilled motor recovery after stroke

Large strokes frequently result in lasting motor deficits and trigger extensive reorganization within the brain and spinal cord. Altered neuronal activity in the contralesional hemisphere has been documented in both humans and rodent models, yet its role in functional recovery versus maladaptation remains unresolved. Here, we used chronic wide-field calcium imaging to monitor bilateral cortical activity in mice performing a skilled reach-to-grasp task before and days to weeks after stroke. Strokes produced persistent fine motor impairments, which were only partly alleviated by intensive rehabilitative training. While cortical activity was in particular suppressed in the ipsilesional cortex after stroke, training promoted sustained increases in contralesional sensorimotor activity. However, ridge regression analysis of neural and behavioral data indicated that this activity largely reflected compensatory use of the intact paw rather than recovery of the impaired forelimb. Axonal tracing nevertheless revealed enhanced projections from contralesional motor cortex to ipsilesional brainstem nuclei - including the midbrain and pontine reticular nucleus - specifically in trained animals. These findings identify a rehabilitation-induced corticofugal pathway supporting motor recovery, highlighting a target for neuromodulation strategies in chronic post-stroke impairment.