Sensory processing during kinesthetic aftereffect following illusory hand movement elicited by tendon vibration

We investigated how the human sensory–motor system elicits a somatosensory aftereffect. Tendon vibration of a limb excites the muscle spindle afferents that contribute to eliciting illusory movements of the limb. After the cessation of vibration, a transient sensation in which the vibrated limb returns towards its original position (kinesthetic aftereffect) is often experienced, even in the absence of the afferent inputs recruited by the vibration. We vibrated the tendon of either the right wrist extensor or flexor muscle that elicited an illusory flexion or extension movement, which was followed by its corresponding extension or flexion aftereffect. First, we psychophysically investigated how the preceding illusory movement affects the aftereffect. Second, we examined the cortico-spinal excitability during the aftereffect to evaluate its changes from the time during the illusion. We measured the amplitude of the motor-evoked potential that is evoked by a single-pulse transcranial magnetic stimulation to the hand section of the contralateral motor cortex (M1). All 19 subjects experienced the aftereffect, and the amount of aftereffect was approximately 70% of the preceding illusion. During the illusion, the cortico-spinal excitability increased more in non-vibrated than in vibrated muscle, so as to reflect the illusory directions. During the aftereffect, the excitability was significantly reduced only in the non-vibrated muscle, with no change in the vibrated muscle, which, in turn, caused an opposite pattern in the unbalanced excitability between the two muscles, and the degree of unbalanced excitability was correlated with the sensation of aftereffect. The kinesthetic aftereffect seems to be elicited by a sensory process that is determined by the preceding illusory movements. Motor–cortical processing of the unbalanced sensory information from the stimulated and non-stimulated muscles may contribute to the elicitation of kinesthetic aftereffect.