Believe it! You can turn your hand into marble

Would you believe your hand could turn into marble? A study has demonstrated a surprising illusion about the material qualities of our bodies based on multi-sensory perceptual experience.

To induce an illusory perception of the material properties of the hand, a group of neuroscientists asked volunteers to sit with their hands lying on a table in front of them.

They repeatedly hit the participants' right hand gently with a small hammer while replacing the natural sound of the hammer against the skin with the sound of a hammer hitting a piece of marble.

Within minutes, hands started feeling stiffer, heavier, harder, less sensitive and unnatural.

Moreover, when approached by a threatening stimulus (a needle that the experimenter moved near their hands), participants showed an enhanced Galvanic skin response, thus demonstrating increased physiological arousal.

"This novel bodily illusion, the 'Marble-Hand Illusion', demonstrates that the perceived material of our body can quickly be updated through multisensory integration," explained Irene Senna from Bielefeld University in Germany.

What is more, it shows that even impact sounds of non-biological materials - such as marble and metal - can be attributed consistently to the body, as if its core material could indeed be modified, she added.

"This surprising perceptual plasticity might help to explain why tools and prostheses can merge so easily into our body schemas despite being made of non-biological materials," said Senna, who is associated with the Center of Excellence Cognitive Interaction Technology (CITEC).

To perceive our bodies and the world around us, our brains constantly combine information from different senses with prior knowledge retrieved from memory.

However, unlike most bodily properties that frequently change over time (such as posture and position), our body material never changes.

Hence, in principle, it would be unnecessary for the brain to constantly try to infer it, said the study published in the international scientific journal PLOS ONE.