New research published in eLife reveals how the brain can be tricked into feeling disembodied from a body part and a sense ownership over a fake one.
Experiments by researchers at the Universities of Turin and Milan have provided the first physiological evidence that changes to muscles and the brain help create this altered sense of self. They conducted the rubber hand illusion: subjects watch a lifelike rubber hand being touched while their own hand, hidden from view, is touched at the same time. This bizarrely leads to a sense of ownership over the rubber hand.
Using magnetic fields to stimulate nerve cells in the brain, the researchers discovered that the brain loses some sense of control over movement in the real hand. When it feels less ready to move the hand, the brain stops considering it as part of the body.
“Our discovery is a crucial step in understanding that movement plays a central role in creating our sense of self-awareness,” says neuropsychologist author Professor Francesca Garbarini from the University of Turin.
“It is also relevant to understanding how a paralyzed person can feel disembodied from their limbs, how brain damage can cause an altered sense of body ownership and it might also lead to new insights into body identity integrity disorder, where a person feels that only by amputating a limb can their body match their sense of self,” she says.
Subjects sat with their forearms resting on a table and their right hand hidden in box. An artificial hand was lined up with their right shoulder and participants were able to see its right index finger being stroked. Out of sight, their own right index finger was stroked simultaneously.
They were asked to pinpoint the perceived location of their unseen finger. During the experiment, the location drifted towards the rubber hand. Participants reported that it seemed their own hand had disappeared and that the fake hand had become their hand.
To understand what lies behind this sensation, the team used transcranial magnetic stimulation to stimulate the part of the brain involved in controlling movement, the motor cortex. Tiny electrical signals travel down the spinal cord and through the nervous system to muscles, generating muscle contractions that can be measured called “motor evoked potentials” (MEPs).
The size of MEPs reflects the ability of the connection between the motor cortex, the spine and the muscle to create movement. MEPs from the stimulated right hand were recorded.
During the experiment, the longer the participants experienced the illusion, the more the signals in the hidden hand weakened. This shows that the hand’s motor neurons were less excitable and therefore less ready to elicit movement. The pathway between the brain and the hand was altered, causing the sense of disembodiment with the real hand and allowing participants to experience the rubber hand as their own.
“There have been quite a few suggestions to explain the sensation caused by the rubber hand illusion but we wanted to find out whether it has a real physiological impact on the motor system that can be measured,” says Garbarini.
“For the first time, we showed that bodily disownership and bodily self-consciousness are strictly dependent on the possibility of movement: if I believe that the hand is mine, then I must be ready to use it; if not, then the activity of the motor system is down-regulated,” she says.
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eLife is a unique collaboration between the funders and practitioners of research to improve the way important research is selected, presented and shared. eLife publishes outstanding works across the life sciences and biomedicine — from basic biological research to applied, translational, and clinical studies. All papers are selected by active scientists in the research community. Decisions and responses are agreed by the reviewers and consolidated by the Reviewing Editor into a single, clear set of instructions for authors, removing the need for laborious cycles of revision and allowing authors to publish their findings quickly. eLife is supported by the Howard Hughes Medical Institute, the Max Planck Society and the Wellcome Trust. Learn more at elifesciences.org.
