Perceptual changes in illusory wrist flexion angles resulting from motor imagery of the same wrist movements

Recent neuroimaging studies have suggested that similar cortical motor areas are recruited both by kinesthetic sensations elicited by tendon vibration and by voluntarily imaging one's own movements of the same joints. Little is known, however, as to whether kinesthetic motor imagery interacts with kinesthetic illusion. We examined such interaction by behavioral analysis in which 19 subjects imagined wrist flexion or extension, with or without illusory flexion induced by tendon vibration. Electromyograms were also recorded to monitor the peripheral modulations caused by the interaction. The kinesthetic motor imagery had a psychophysical effect on kinesthetic illusion in the absence of overt movement. It was confirmed that the subjects could imagine wrist movements without facilitating muscle activities in the absence of vibration stimuli. The electromyogram activity of the vibrated extensor muscles was significantly higher than that of non-vibrated flexor muscles. Motor imagery of wrist extension, when illusory flexion was experienced, reduced the angle of illusory flexion while enhancing extensor muscle activities in comparison with the control. On the other hand, flexion motor imagery increased the angle of illusory flexion with or without enhancement of flexor muscle activities. Our results indicate that motor imagery interacts with kinesthetic illusion with or without enhancement of activities of the related muscles. This suggests (1) that common neural substrates shared by imagery and by illusion exist and (2) that different physiological mechanisms contribute to the enhancement of muscle activities of vibrated muscles and their antagonists.