Marble hand illusion: Deceived body perception



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Hand feels like marble with a simple trick

In a simple experiment, neuroscientists have shown that the human body can be easily deceived when it comes to the feeling of one's own flesh. With the help of a hammer and noises, the participants were given the illusion of a marble hand.

Deceived body perception Actually everyone knows that the human body consists of flesh and bones. But researchers have now shown that this body awareness is not a matter of course. For this purpose, they presented a body illusion that shows how people change their assumptions about what material their hand is made of. The neuroscientists led by 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 young adults involved were then tapped 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 hitting marble, which was played to them every time they knocked on headphones.

Hand of the participant 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 not synchronized with the touch or if pure tones were heard instead, this effect did not occur. The researchers reported this in the journal "PLOS ONE". The scientists also examined whether this subjective impression can also be measured objectively by registering the skin resistance with electrodes. As expected, this changed depending on the illusion. “Our brain constantly tests senses about our environment and our body. As the experiment shows, this goes so far that the brain also permanently controls which material the body is made of - even if this seems unnecessary, because after all, the material normally does not change, "says Senna.

Contrary to all previous experiences According to the neuroscientist, the reason for this is that the brain brings together information from different sensory organs in parallel in order to assess the properties of its surroundings and its body. When the knocking of the hammer (visual stimulus) is combined with the sound of a hammer hitting a stone (acoustic stimulus), the brain adjusts the perception in such a way that the two pieces of information harmonize with one another. Even if this contradicts all previous experience, it creates the impression that the hand is made of stone. This amalgamation of information from different sensory organs is referred to as "multisensory integration." Senna explained: "Our newly discovered body illusion - 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 bodies. It has also been shown that the brain does not only test its own flesh, but that it can also attribute properties of non-biological material such as marble or metal to it. "This surprising flexibility in our perception may help us to understand why people whose body parts have been replaced by prostheses, despite their artificial material, perceive them as part of their bodies," said the neuroscientist. (ad)

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