Role of the Mirror-Neuron System in Cross-Education

The present review proposes the untested hypothesis that cross-education performed with a mirror increases the transfer of motor function to the resting limb compared with standard cross-education interventions without a mirror. The hypothesis is based on neuroanatomical evidence suggesting an overlap in activated brain areas when a unilateral motor task is performed with and without a mirror in the context of cross-education of the upper extremities. The review shows that the mirror-neuron system (MNS), connecting sensory neurons responding to visual properties of an observed action and motor neurons that discharge action potentials during the execution of a similar action, has the potential to enhance cross-education. After a literature search we narrowed the review to studies that examined healthy young adults who performed unilateral strength training and unilateral motor tasks with or without a mirror and assessed outcome measures in relation to the changes in brain activity, motor cortical excitability, and corticospinal excitability. We identified six chronic studies that examined the effects of unilateral strength training on neural adaptations and 15 cross-sectional studies that examined acute changes in brain activation, motor cortical and corticospinal excitability using imaging, electroencephalographic, magnetoencephalographic, and magnetic brain stimulation. There were two chronic and nine cross-sectional studies in which participants performed unilateral motor tasks while viewing the image of the active hand superimposed on the resting hand’s image. Collectively, the data suggest that the MNS is involved in cross-education and the hypothesis is tenable. However, future studies are needed to elucidate the precise mechanism of how the use of a mirror in a cross-education study augments transfer to the non-exercised limb. Recent studies show a strength-sparing effect in the immobilized arm after strength training of the free arm in healthy individuals, and improved bilateral function after unilateral exercise therapy in stroke patients. It is thus conceptually justified to conduct randomized clinical trials that supplement cross-education protocols with a mirror. Such a treatment could reduce muscle weakness caused by limb fractures, anterior-cruciate ligament reconstruction surgery, stroke, and other unilateral motor dysfunctions.