You’ll probably answer with a resounding “no,” thinking that this is one of the craziest questions you’ve ever read. However, if you know how flexible our brains are, you’ll realize that the possibility of someone convincing you that your hand is made of marble or stone isn’t as unrealistic as it sounds.
In fact, our brains are quite easy to fool. In the past, experiments have been done to make people believe that one of their limbs has disappeared, turned into rubber, or become the size of a small doll.
All of these illusions are closely linked to vision and touch. That is, by playing with what we can see and touch, our brain becomes convinced that what it sees and feels is real, even if it is not and does not even make sense.
Given this surprising malleability, some neuroscientists have wondered whether sound can also trigger this type of illusion. If we hear the sound of a hammer on marble, could we think that our hand is made of stone? Researchers at the University of Milano-Bicocca were quick to test this idea.
Can our brain accept an external body part as its own?
The researchers recruited 23 people and asked them to sit with one of their arms on the table in front of them. They then placed a screen that blocked their view of their own arm. However, the people could see a small hammer hitting the arm repeatedly. A metal strip was attached to the arm and connected by a wire to the hammer and a computer so that each time the hammer touched it, the typical sound of hammering on marble was produced.
The blows were repeated for five minutes. Another group heard the same sound but out of sync with the hammer blows and a third group heard a completely different sound with each blow. What happened?
At the end of the experiment, only the people in the first group reported strange sensations in their arms. They said that they felt it was unnatural, much less sensitive, very heavy and more rigid (characteristics of marble).
This experiment shows us that the boundaries between our bodies and external objects may actually be more blurred than we originally thought, and opens up a new world of possibilities for people who have lost one of their limbs and want to use artificial limbs. Perhaps, with the right stimuli, the adaptation process could be accelerated.
Reference:
Senna, I. et. Al. (2014) The Marble-Hand illusion. PLoS ONE; 9 (3).
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