Decreased motor cortex excitability mirrors own hand disembodiment during the rubber hand illusion

• Published in: eLife Vol. 5
• Main Authors: della Gatta, Francesco, Garbarini, Francesca, Puglisi, Guglielmo, Leonetti, Antonella, Berti, Annamaria, Borroni, Paola
• Format: Journal Article
• Language: English
• Published: England eLife Sciences Publications Ltd 20.10.2016; eLife Sciences Publications, Ltd
• Subjects: Action Potentials; Adult; body ownership; Cortex (motor); Cortical Excitability; Excitability; Experiments; Female; Hand - physiology; Humans; Illusions; magnetic transcranical stimulation; Male; Motor Cortex - physiology; Motor evoked potentials; Motor task performance; Neuroscience; Physiology; primary motor cortex; Proprioception; Questionnaires; Ratings & rankings; Short Report; Touch Perception; Visual Perception; Young Adult; magnetic transcranical stimulation; body ownership; human; neuroscience; primary motor cortex
• ISSN: 2050-084X; 2050-084X
• DOI: 10.7554/eLife.14972

During the rubber hand illusion (RHI), subjects experience an artificial hand as part of their own body, while the real hand is subject to a sort of 'disembodiment'. Can this altered belief about the body also affect physiological mechanisms involved in body-ownership, such as motor control? Here we ask whether the excitability of the motor pathways to the real (disembodied) hand are affected by the illusion. Our results show that the amplitude of the motor-evoked potentials recorded from the real hand is significantly reduced, with respect to baseline, when subjects in the synchronous (but not in the asynchronous) condition experience the fake hand as their own. This finding contributes to the theoretical understanding of the relationship between body-ownership and motor system, and provides the first physiological evidence that a significant drop in motor excitability in M1 hand circuits accompanies the disembodiment of the real hand during the RHI experience. The feeling of body ownership — that the various parts of your body are all part of you — is something that we typically take for granted. However, brain damage can disrupt this sensation and leave individuals convinced that an arm or a leg is no longer their own. Even in healthy people, the ‘rubber hand illusion’ can temporarily produce a similar phenomenon. Individuals watch a lifelike rubber hand being touched while their own hand – which is hidden from view – is touched at the same time. This creates the feeling that the artificial hand has become part of their body, while their real hand is left in a ‘disembodied’ state. How does the brain generate this illusory sense of ownership and accompanying sense of disembodiment? A person’s ability to move their body is thought to contribute to their feeling of body ownership. Therefore, della Gatta, Garbarini et al. asked whether the brain’s ability to move the real hand changes during the rubber hand illusion. In the experiments, the region of the brain that controls hand movement was artificially stimulated in a number of volunteers. When an individual had been primed by the rubber hand illusion to perceive a fake hand as part of their own body, their brain was temporarily less able to activate the muscles of their real hand. This is as if the brain no longer considered the real hand as part of the body. Thus, the altered sense of body ownership experienced during the rubber hand illusion is not a bizarre fantasy, but corresponds to a physiological reaction that accompanies changes in brain activity. The next step is to further define and quantify the relationship between the sense of body ownership and the control of body movement. Specifically, how does activity in the brain areas that control movement contribute to the sense of body ownership? And how do these brain regions communicate with one another to generate a sense of self?