If your arm falls asleep for a while, it can sort of feel like it's made of rubber. But how do you make your arm feel like it's made of marble? According to Italian researchers, all you need is the sound of a hammer tapping stone. And some psychological trickery.

You may be familiar with the old rubber hand illusion: researchers hide a subject's arm from his or her view and have them look at a rubber hand. After rubbing a paint brush across both the real and the rubber hand simultaneously, the subject starts to think he or she is feeling sensation through the rubber hand. When the researcher brushes the rubber hand, but not the real hand, the patient reports that he or she can feel it.

Irene Senna, a psychologist at Milano-Bicocca University in Milan, Italy, decided to trade touch for sound to see if the illusion works across senses. She had subjects put their arms behind a small wall, where researchers swung a tiny hammer (lightly) at the hand. Subjects could see the hammer coming towards their hands, and when the hammer touched the subject's skin, it triggered a recording of a hammer tapping marble. Each time the subject felt the hammer, he or she heard the sound of a hammer on marble.


After about five minutes of this association, subjects reported that the arm being tapped felt heavy, stiff, and in many cases, numb—as if their limbs had turned to marble. A control group, where the sound effect was not in sync with the sensation of the hammer tapping the skin, reported no such sensation.

Research like this might sound like an elaborate prank, but it's helping scientists better understand the relationship between our senses and our bodies. This is a hugely important area of study, related to the treatment of neurological disorders and providing a better understanding of how to integrate prosthetics to feel and function like an amputee's natural limb.

Then again, it shows just how easily our brains can be fooled. Don't trust everything you see—or hear. [PLOS one via Discover Magazine]

Image: Shutterstock / Axel Lauer