Marble hand illusion. Deceptive body perception

Hand feels like marble by simple trick

03/16/2014

Neuroscientists have shown in a simple experiment that the human body can easily be deceived when it comes to feeling its own flesh. With the help of a hammer and noises, the illusion of a marble hand was evoked in test participants.

Deceptive body perception
In fact, everyone knows that the human body is made of flesh and bone. But researchers now showed that this body perception is not self-evident. For this they presented a body illusion that shows how people change their assumptions about what material their hand consists of. The neuroscientists around Irene Senna from the University of Milan-Bicocca, now working at the University of Bielefeld, asked volunteers for their experiment to place their hands on the table in front of them. The involved young adults were then knocked on the right hand with a small hammer. However, they did not hear the natural sound of the hammer hitting the skin, but the sound of a hammer bouncing on marble, which was played to them every time they knocked over headphones.

Hand of participants feels hard and cold within minutes
According to the participants, the right hand felt stiffer, heavier, harder, colder, less sensitive and unnatural within minutes. If the hammer sounds were out of sync with the touch, or if pure tones were heard instead, this effect would not work. This reported the researchers in the journal „PLOS ONE“. The scientists also investigated whether this subjective impression can be objectively measured by registering skin resistance with electrodes. This changed as expected depending on the illusion. „Our brain continually examines sensory perceptions about our environment and our body. As the experiment shows, it goes so far that the brain also permanently controls which material the body consists of - even if that seems unnecessary, because the material does not change normally“, so senna.

Contrary to all previous experience
According to the neuroscientist, the reason for this is that the brain, in parallel, gathers information from various sensory organs to assess the properties of its environment and body. When the tapping of the hammer (visual stimulus) is combined with the sound of a hammer beating on stone (acoustic stimulus), the brain adjusts the perception so that both pieces of information harmonize with each other. Even if this contradicts all previous experience, it gives the impression that the hand is made of stone. This fusion of information from different sense organs is called „multisensory integration.“ Senna explained: „Our newly discovered illusion of the body - the marble hand illusion - proves that the perceived material of our body can be changed through multisensory integration.“

People perceive prostheses as part of their body
It has also been shown that the brain apparently does not only test its own flesh but also attributes it to properties of non-biological material such as marble or metal. „This surprising flexibility of our perceptions may help to understand why people whose body parts have been replaced by prostheses perceive them as part of their bodies despite their artificial material“, so the neuroscientist. (Ad)