Upper extremity rehabilitation of stroke : facilitation of corticospinal excitability using mirror therapy and virtual environment technology

Abstract

ObjectivesSeveral experimental studies in stroke patients suggest that mental therapy using motor imagery, exercises with a mirror (mirror therapy), and various virtual environment technology approaches facilitate motor rehabilitation. However, the mechanisms underlying these therapy-related effects have not been described. In this study, we attempted to delineate the changes in corticospinal excitability when individuals are asked to exercise their hand using motor imagery, voluntary exercise, exercise with a mirror, bilateral exercise with a mirror, and exercise with a virtual mirror. Moreover, we attempted to delineate the role of visual modulation within the virtual environment that affected corticospinal excitability in healthy subjects and stroke patients.MethodExperiment I: The participants consisted of 30 healthy right-handed subjects and 15 stroke patients. Motor evoked potentials (MEPs) were recorded in the left-side abductor pollicis brevis (APB) of healthy subjects and in the affected-side APBof stroke patients. In healthy subjects, the peak-to-peak amplitudes and latencies of MEP were obtained using seven conditions: (A) rest; (B) imagery; (C) observation and imagery of the activity without a mirror; (D) observation and imagery of self ipsilateral hand activity without a mirror; (E) observation and imagery of the activity of the hand of another individual with a mirror; (F) observation and imagery of symmetric self ipsilateral hand activity (thumb abduction) with a mirror; and (G) observation and imagery of asymmetric selfipsilateral hand activity (little finger abduction) with a mirror. In stroke patients, MEPs were obtained in the A, C, D, E, and F conditions.Experiment II: The participants consisted of 15 healthy right-handed subjects and seven stroke patients. MEPs were recorded in the left-side first dorsal interossei (FDI) of healthy subjects and in the affected-side FDI of stroke patients. Transcranial magnetic stimulation (TMS) was applied over the left/affected hemisphere during the performance of grasping at different force levels (0%, 10%, or 50% of maximal voluntary contraction) using the contralateral hand. The ipsilateral hand performed grasping simultaneously, at various force levels (0%, 10%, 50%, or 100% of maximal voluntary contraction). We repeated the same experiment using a mirror. The peak-to-peak amplitudes of MEPs were compared, to assess differences in motor cortex excitability.Experiment III: The participants consisted of 18 healthy subjects and 18 hemiplegic stroke patients. MEPs from TMS were recorded in the flexor carpi radialis of the left or affected side using three different conditions: (A) rest; (B) real mirror; and (C) virtual mirror. Moreover, we compared the MEPs from the virtual mirror paradigm using continuous visual feedback or intermittent visual feedback.ResultsExperiment I: In the healthy group, the rates of amplitude increment and latency decrement were significantly higher during the observation and imagery of the activity of the hand of another individual and of self ipsilateral hand than during imagery only.In both groups, the rates of amplitude increment and latency decrement of MEPs were significantly higher during the observation of the activity of the hand of another individual with a mirror than without a mirror. Furthermore, the rates of amplitude increment and latency decrement were significantly higher during the observation of the activity of the symmetric self ipsilateral hand with a mirror than without a mirror. In healthy subjects, amplitudes were significantly increased during the observation of the activity of the symmetric self ipsilateral hand with a mirror compared with the observation of the activity of the asymmetric self ipsilateral hand with a mirror.Experiment II: In both two groups, bilateral activation of both hands led to a facilitation of the nondominant (left)/affected hemisphere. In both groups, the amplitude of MEPs increased by viewing the mirror image of gradual ipsilateral hand contraction; however, these effects subsided during contralateral hand contraction.Experiment III: The rates of amplitude increment and latency decrement of MEPs in healthy subjects were higher during the virtual mirror exercise than during the real mirror exercise. In healthy subjects and stroke patients, the virtual mirror paradigm with intermittent visual feedback significantly facilitated corticospinal excitability of MEPs compared with continuous visual feedback.ConclusionIn both groups, corticospinal excitability was facilitated by viewing the mirror image of the activity of the ipsilateral hand. This facilitation provided by mirror therapy was highly selective for the muscles seen through the mirror. Corticospinal excitability was facilitated by viewing the mirror image of gradual ipsilateral hand contraction; however, these effects subsided for contralateral hand contraction. Corticospinal excitability was facilitated to a greater extent in the virtual mirror paradigm than in the real mirror paradigm, in both groups. The virtual mirror paradigm with intermittent visual feedback facilitated corticospinal excitability more than the virtual mirror paradigm with continuous visual feedback, in both groups. This provides neurophysiological evidence supporting the application of mirror therapy programs and the virtual mirror paradigm using various visual modulation technologies to upper extremity rehabilitation in stroke patients.