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f you’re planning a Day of the Dead party, we heartily recommend the rubber hand illusion as the perfect combination of spookiness and science. While the gag itself is a great way to fill the time between trick-or-treaters, you’ll score extra points if you regale your guests with the neurobiological explanation of their bizarre experience.

Fortunately, a new paper in the journal eLife has arrived just in time to offer some new details about what’s going on in the brains of people during this trick — providing “a major advance in our understanding of our sense of body ownership,” according to one scientist not involved in the study.

You’d think people would always know what’s their body and what isn’t, but no.

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Patients with brain damage from stroke or trauma sometimes don’t recognize their hands, arms, and legs as their own. They scream to “get it out of my bed!” when they see their own leg and, when they see their hand approaching their face, think they’re being assaulted. In the worst cases, people with “body identity integrity disorder” feel so strongly that one of their limbs is not theirs they demand to have it amputated. Other forms of brain damage produce the opposite illusion: a patient feels that another person’s body part is hers, causing immense frustration when she can’t move it.

The rubber hand illusion creates that bizarre feeling in people with a healthy brain. It works like this: seat a volunteer with her forearms resting on a table and her right hand hidden in a box that’s open at both ends. Align a lifelike rubber hand with her right shoulder, where her real hand would be. Using a paintbrush, stroke the right index finger of the rubber hand and, simultaneously and in sync, her real right index finger. Tell her to carefully watch the rubber finger being stroked.

Hand experiment
Research analyst Lindsey McIntosh demonstrates the rubber hand illusion experiment in Professor Sohee Park’s lab at Vanderbilt University. John Russell/Vanderbilt University

Ask her where her right index finger is. (To reduce the risk of lying or cheating, have her close her eyes and use her left hand to point to it.) In all likelihood, within 15 seconds or so her answer will drift toward the rubber hand, as she feels that her own hand has melted away and the fake hand has become hers. Seeing only the rubber hand being stroked, she’ll feel as if her own is.

Even odder, the temperature of the real hand usually drops — as if the brain, which controls body temperature, no longer considers it part of the body.

Some basics of what’s going on in the brain have been sussed out. When visual information (seeing the rubber hand being stroked) conflicts with tactile information (feeling but not seeing one’s own hand being stroked), the brain trusts what it sees over what it feels. As the illusion takes hold, activity in the brain’s premotor cortex, which integrates information from the different senses, increases, as if the brain is struggling to resolve the visual/tactile conflict. The stronger the illusion, the greater this premotor activity, earlier research has found.

In a study published on Thursday in eLife, scientists led by neuropsychologist Francesca Garbarini of the University of Turin and Francesco della Gatta of the University of Milan found that something else goes on. As volunteers see the rubber hand and their own being stroked simultaneously, brain circuits that move their right hand become more and more difficult to kick into gear.

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“The brain is less ready to move the hand,” Garbarini said. That produces a loss of the sense of control over the hand. That lost sense, in turn, makes the brain stop considering the hand as part of the body. The longer someone experiences the illusion, the less excitable the circuits that move the hidden hand.

Their results “leave open the direction of causality,” said neuroscientist Luke Miller of INSERM, France’s leading biomedical research center, who coauthored a commentary on the paper. “Whether a reduction in motor cortex excitability causes or is a consequence of disembodiment is a fascinating question that should be addressed in future research.”

The reduced excitability of hand-moving brain circuits, nevertheless, fits with the idea that movement helps create the subjective experience that a body part really is yours. That sense is “strictly dependent on the possibility of movement,” said Garbarini. “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” — an insight that, she hopes, will eventually lead to new ways to help people whose brain has forgotten which body parts are theirs.

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