Mapping Corticoreticulospinal Motor Control Using Brainstem and Spinal Cord fMRI in Chronic Hemiparetic Stroke

Nearly 85% of stroke survivors experience significant unilateral impairment in upper extremity motor control, typically caused by damage to the corticospinal (CST) and corticoreticular (CRT) tracts (i.e., the corticofugal tracts). Alternative neural pathways, such as the contralesional cortico-reticulospinal tract (CRST), can be recruited to achieve movement of the affected arm and hand, but may have undesirable consequences. For example, the diffuse, bilateral branching of reticulospinal neurons can produce abnormal muscle co-activations (synergies) in the paretic limb, and involuntary mirror movements (associated reactions) between limbs. Together, these effects create stereotypical movement patterns post-stroke, and there is growing interest in novel "anti-synergy" interventions to enhance usage of residual CST systems rather than strengthening the CRST. Imaging has the potential to become an invaluable tool for evaluating whether rehabilitative strategies can preferentially access CST versus CRST pathways. However, current functional imaging research has focused on cortical activity, and must theoretically infer what pathway is used. Structural MRI directly assesses changes in white matter pathways, but it is limited to detecting long-term plasticity. To guide new interventions, there is a critical need to directly evaluate what descending motor pathways are active during movement. Thus, the overall objective of this study is to generate a novel fMRI dataset in participants with post-stroke hemiparesis, capturing neural activity during an innovative isometric shoulder abduction task, evaluating differences when abducting the paretic versus non-paretic arm. The investigators will acquire multi-echo fMRI data in individuals with post-stroke hemiparesis and age-matched controls, hypothesizing that increased reliance on the CRST will cause distinct activation patterns during shoulder abduction with the paretic limb, and that this will correlate with individual upper-extremity impairment (Upper-Extremity Fugl-Meyer Assessment). This work is significant because it will provide direct evidence of descending contralesional motor pathway involvement in post-stroke hemiparesis, and demonstrate the utility of neuroimaging for optimizing movements to preferentially engage specific systems and promote desired neural plasticity following injury.