Optimizing Cortical Priming to Enhance Bimanual Arm Coordination in Individuals with Chronic Stroke

Abstract

Background. Current brain stimulation approaches are focused on unimanual rather than bimanual movements. The standard approach of inhibiting contralesional primary motor cortex (cM1) has been shown to disrupt bimanual movements while facilitating ipsilesional M1 (iM1) failed to improve paretic arm function in severely impaired individuals. Here, we proposed an alternative neuromodulation target, the contralesional dorsal premotor cortex (cPMd), to augment bimanual coordination for individuals with stroke. In the first study, we examined the effects of facilitation of cPMd compared to iM1, to determine the more responsive neuromodulation target for each participant. Then, in the second study, we facilitated the more responsive region (cPMd vs. iM1) for each participant and coupled it with a short-term bilateral arm training, to determine whether this individualized cortical priming approach could augment bimanual performance, more than that of sham stimulation. Methods. A repeated measures crossover design was employed. Fourteen individuals participated in the first study, and a subset of ten individuals completed the second study. 5 Hz repetitive transcranial magnetic stimulation (rTMS) was used to facilitate cPMd and iM1. Bimanual force/neuromuscular coordination was examined using isometric force tasks in the first study. Kinematics of both arms were examined using bimanual simple and complex robotic tasks in the second study. Cortical excitability outcomes including motor evoked potentials (MEP), cortical silent periods (CSP) and ipsilateral silent periods (ISP) were assessed. Results. Two groups of responders were identified in the first study. Participants with greater arm/hand impairment and weaker baseline interhemispheric inhibition (IHI) improved bimanual force/neuromuscular coordination, ipsilesional MEP as well as reduced IHI after cPMd-rTMS. In contrast, those whom had fewer arm/hand impairment and stronger baseline IHI improved only after iM1-rTMS. In the second study, the control of both arms in complex bimanual tasks, as wells as the symmetry of both arms in simple and complex bimanual tasks, were improved only in the individualized cortical priming condition. In contrast, for the sham stimulation condition, improvements were seen only in control of the non-paretic arm in simple bimanual tasks. Conclusion. Cortical stimulation/priming protocols should be designed according to demands of bimanual tasks and individualized based on participants' impairments.