Kinesthetic illusory feeling induced by a finger movement movie effects on corticomotor excitability

The present study aimed to clarify whether a kinesthetic illusion arises in our experimental condition (visual stimulus) and whether corticomotor excitability changes in parallel with the kinesthetic illusion. The visual stimulus was a movie in which someone else's limb was being moved. The computer screen showing the movie was installed at an appropriate portion of the subject's forearm, so that the performer's hand appeared as if it were the subject's hand (illusion). The experience of kinesthetic illusion under this condition was verified by interview using a visual analog scale. Healthy male subjects participated in this experiment. Transcranial magnetic stimulation was applied to induce motor-evoked potential (MEP) from the first dorsal interosseous and abductor digiti minimi muscle. Each subject was instructed to watch the same computer display shown as in the illusion, with his own stationary hand in full view (non-illusion) and to watch a display of non-biological movement (moving text) (sham) as the control conditions. The present results showed significant facilitation of MEP under the illusion compared with the control conditions for the index finger abducting in the movie, although not for adducting. MEP in the abductor digiti minimi showed no change during either abduction or adduction of the little finger. The present study demonstrated that an illusion of self-motion can be created by a video of a moving abstract index finger, and inputs to the corticomotor pathways during the self-motion illusion facilitated the corticomotor excitability. The excitatory effect of the illusion depended on the movement direction of the index finger.