In this experiment, both healthy subjects and stroke patients exhibited corticospinal facilitation in conditions of a virtual mirror task and a virtual mirror task with intermittent visual feedback condition.
One explanation for the enhanced facilitation of corticospinal activity by the virtual mirror paradigm compared with the real mirror could be that the virtual mirror paradigm is task oriented (to catch the cup), more interactive and interesting, thus increasing attention and evoking the visual illusion that might activates the putative mirror neuron system and the ipsilesional motor cortex.
It is still a matter of debate whether there is a difference in brain activation between the real and virtual action. Perani et al. found that only real actions in a natural environment activated a visuospatial network in fMRI study. Tai et al. found that the mirror neuronal system was activated in observation of real human grasping task but not in an artificial arm task in PET study. However, Gazzola et al. suggested that a mirror system was strongly activated by the sight of both human and virtual (robotic) actions in fMRI study, with no significant differences. They concluded that the goal might be more important for mirror neuronal activation than the way in which the action is performed. Furthermore, recent TMS research showed that goal-directed movement increases cortical excitability by enhancing the concentration of the patient[22, 23].
However, we only recorded the amplitude and latency of MEP (corticospinal excitability) in this study; we assumed that the large amount of facilitation MEP during this study might involve cortical level rather than spinal level. According to a previous study, the large amount of facilitation of MEP during a voluntary contraction of the ipsilateral hand muscle may involve the cortical level rather than spinal level. Furthermore, the facilitation of MEP induced by action observation was attributable to cortico-cortical facilitating connections.
The results showing more significant activation of the motor cortex in the intermittent feedback paradigm as compared to the continuous feedback paradigm is intriguing. In the intermittent feedback condition, subjects should continuously predict and move their upper extremity when there is an absence of the virtual upper extremity, which may require an estimation of the target point and induce proprioceptive integrated brain network control. These task-oriented and proprioceptive integrated exercises might activate the extended motor cortex strongly (including the parietal cortex, premotor cortex, and primary motor cortex), which can result in cerebral motor cortical facilitation[26–29].
Although VR has been applied to train and treat stroke patients in order to enhance their abilities (e.g., gait and cognitive ability)[30, 31], robust scientific evidence in support of more effective enhancement of motor relearning after stroke or brain damage by VR compared with conventional physical therapy is scarce. In this context, our results of stronger activation of MEPs by the virtual mirror paradigm developed in this study as compared to the real mirror paradigm could be regarded as meaningful. Moreover, the fact that the intermittent visual feedback paradigm evoked stronger MEPs may support evidence that attention to the task, problem-solving ability, and level of difficulty specific to individuals have a considerable effect on motor learning and the contention that the virtual environment may be more beneficial than the real training environment.
An insufficient number of subjects and the heterogeneity of the lesions, onset time, manual dexterity, stage of motor recovery and functional level may represent the limitations of our study. Therefore, we did try to minimize the large variability due to individual difference by being transformed MEP values into natural logarithm, as suggested by Nielsen and the influence of motor recovery on MEP values in the experimental condition by performing the experiment in short period (less than one hour for each subject).
Furthermore, it is hard to generalize this result to the total group of stroke patients, because subjects with better functional ability were recruited. In addition, the lack of assessment of the level of muscle (flexor carpi radialis) contraction during exercise could be another limitation of his study. However, we minimized muscle contraction by placing a roller under the movement plate and by maintaining a constant pace and angle during exercise in each task.
In future investigations, clinical studies examining the effectiveness of the virtual mirror program according to various stroke subgroups, training method, and training duration should be considered, and a method that enhances the effect via integration with other treatment methods, such as robot therapy and the various feedback paradigms, should be developed.