Scientists Convince People Their Hands Are Rocks

By Elizabeth Preston | March 28, 2014 8:58 am

marble hand

No matter how much of a critical thinker you consider yourself, your brain is pretty gullible. With a few minutes and a couple of props, your brain can be convinced that one of your limbs is made of rubber or invisible, or that your whole body is the size of a Barbie doll’s. All these illusions depend on your senses of vision and touch interacting. But a new illusion trades sight for sound. By hearing the sound of a hammer striking marble each time it tapped their hands, subjects came to feel that their limbs were made of stone.

In the original rubber hand illusion, first published in 1998 by psychologists Matthew Botvinick and Jonathan Cohen, subjects sit with one forearm resting on a table. A little wall blocks the subject’s own arm from his or her sight, while a rubber arm just inside the wall is clearly visible. A researcher sitting on the opposite side of the table simultaneously strokes the backs of the rubber hand and the real hand with paintbrushes. As subjects watch the paintbrush stroking the rubber hand and feel the action on their own skin at the same time, they come to feel a powerful illusion that the rubber hand is their hand.

Scientists have since performed many variations of this experiment, bending people’s perceptions of their own bodies in all kinds of freaky ways. (See below for links to stories about a few of those studies.)

“The focus of those experiments is embodiment,” says Irene Senna, a psychologist at Milano-Bicocca University in Milan, Italy. “Can our body incorporate an external body part?” The findings are relevant to amputees and people learning to use artificial limbs. But Senna and her coauthors were interested in a slightly different question: instead of convincing ourselves that an inanimate hand is our own, can we come to believe that one of our existing body parts is made of a non-human material?

This being Italy, the material the researchers chose wasn’t novelty-shop rubber, but stately marble. To create the illusion, they had 23 subjects sit with one forearm on a table. A wall blocked the subject’s hand from his or her view—but subjects were able to see the small hammer an experimenter swung toward the hand repeatedly. A piece of metal foil tape on the back of each subject’s hand was connected by a wire to the hammer, and to a computer. Whenever the hammer touched the foil, it triggered the desired sound effect: the chink! of hammer striking marble.

marble hand setup

This lasted for about five minutes. A control group of subjects heard the same sound effect, but out of sync with the hammer tapping their hands. Another control group heard a pure tone with each hammer strike, instead of the marble sound.

Before and after the experiment, subjects filled out a questionnaire rating the physical qualities of their hands. After scientists performed the illusion on them, people reported that their own hands felt more unnatural, stiffer, heavier, and less sensitive. Many of the subjects also told researchers, without being asked, that their hands felt stiff or numb. (In the control groups, the illusion didn’t work.)

Like previous rubber-hand-inspired studies, this one could be helpful to the science of prostheses—in this case, helping us understand why people can so easily adopt a limb that’s not made of skin and bones. It also shows that a combination of sound and touch cues is enough to trigger the illusion.

Yet it’s not really clear why our brains are susceptible to this kind of illusion in the first place, Senna says. Obviously it’s important for our brains to pay attention to where our limbs are in space, their temperature, and other details that constantly change. But the flesh of our body is a constant.

“It would be ideally unnecessary for our brain to infer the material of our body,” Senna says. “However, our illusion demonstrates that our brain does not take for granted such information.”


For more on the rubber-hand illusion:

Little People, Big World (on turning people into giants and Barbie dolls, plus my eighth-grade science-fair version of the rubber hand illusion)

Rubber Hand Experiment Shows Kids Have More Flexible Body Boundaries

Swapping Bodies with a Child Makes Everything Seem Bigger

Images: Ian Scott (via Flickr); Senna et al.

Senna, I., Maravita, A., Bolognini, N., & Parise, C. (2014). The Marble-Hand Illusion PLoS ONE, 9 (3) DOI: 10.1371/journal.pone.0091688

CATEGORIZED UNDER: brains, magic, sound, top posts, Uncategorized
MORE ABOUT: Psychology, Senses
  • Longmire

    “It would be ideally unnecessary for our brain to infer the material of our body,” Senna says. “However, our illusion demonstrates that our brain does not take for granted such information.” Perhaps our minds are not the same as our brains and merely inhabit our body and/or whatever else it can. “Reality” is all but figured out.

    • Ether_Man

      Our minds ARE not the same as our brain… Our mind is a product of our brain, not the same thing. If you’re braindead as an example, you still have a brain, but it is no longer maintaining your mind… As for reality being figured out… It all depends on which definition of reality you use… If you use a scientific meaning of the term, then reality itself, is defined by us and as such, entirely figured out… If you use a philosophical definition however, then it’s true that it’s not figured out and never can be because it’s an entirely hypothetical concept since you can never prove that we’re not the figment of someone elses imagination…

      • Longmire

        It is the current consensus that our minds are a product of our brain, if that is the case how can ones mind control the brain, which it can be proven with biofeedback, besides just ignoring the facts to allow the current theory to prevail. If you look at what you said about the scientific definition of reality then one could easily see just how hypothetical our knowledge reality is. Quantum mechanics is a joke read up on it.

        • Ether_Man

          1. Biofeedback isnt controlling the brain… Biofeedback is in a quite minor way AFFECT the brain… 2. Why wouldnt it be able to? I can construct a gun, which certainly can affect me. Just because A is a product of B, doesnt mean B cant be affected by A…

          As for how hypothetical our knowledge is… There’s a reason why… Because we dont know what we dont know… And without knowing what we dont know, makes it impossible to know anything with absolute certainty… ANYONE claiming to know ANYTHING general with absolute certainty… Is delusional… Science also doesnt try to hide this fact and that’s why the highest degree of certainty within science, is called a theory… You can only know things within specific contexts with absolute certainty, and most of the time, that’s only because WE define it ourself… Same thing as our existance, because WE define existance… We dont know what our existance IS, but we do know that we DO exist, simply because it’s self evident… And yes I’ve read quite a bit of QM… It’s nowhere near a joke… The joke is people thinking they understand it… But no scientist actually claims to understand it… Science is atleast working on it… What are you doing?

          • Longmire

            I was trying to point out that the mind may actually be independent of the brain/body yet entangled with it, and all the data is being presented in a way to preserve the current theory of mind. All I’m saying is keep an open mind and try to see alternatives to any consensus lest delusions of certainty cloud you as well.

          • Ether_Man

            That’s a statement that contradicts itself… If it’s entangled, it’s no independant… If it’s independant, it cant be entangled… So no, as far as we know, the mind is in no way independant of the brain… And sorry but your concept of having an open mind is kindof wrong… I suggest having a look at as an example…

          • Longmire

            That statement did not contradict itself, but yours did. Anyways there is a possibility that the mind is currently entangled with the brain/body and once the body dies the mind is set free. May sound religious but even within quantum mechanics its possible, I think the EPR paradox touches on it in an overly formal way.

          • Ether_Man

            Yes it did… Something CANNOT be entangled, and still be independant… They’re contradictory states and thus CANNOT be both…

            As for quantum mechanics allowing your mind to live on with the brain dead… There’s NOTHING in QM that would allow for something like that… There’s ZERO evidence for something like that in ANY field of science. There’s a possibility of it sure… Just like it’s a possibility that at the center of every black hole there’s a tiny old man searching for the circuit breaker… That something is, possible in theory, does NOT mean that something is in any way likely, or reasonable to believe… And how did you even imagine that the EPR paradox has ANYTHING to do with the mind living on after the brain dies? It has NOTHING to do with that area in any way…

          • Longmire

            What is entangled now doesn’t have to be entangled forever, hence independent. Our mind could be somewhere else entirely and we would never be able to tell because most of our ideas are based squarely on what we perceive yet EPR paradox would allow instant transmission of consciousness, I suppose it has nothing to do with telepathy either. Give it a few years and black holes wont exist.

          • Ether_Man

            That’s not what you said… Something that WAS entangled could BECOME independent, but then it is no longer entangled… And again… While that is theoretically possible, it’s about as likely as the tiny man looking for the circuit breaker… And about as much evidence for it… And no, the EPR paradox does not allow instant transmission of consciousness… You’re vastly missunderstanding what the EPR paradox actually says and means… And yes black holes will still exist in a few years… There’s no reason to believe they will ever cease to exist… Hell we dont even have any information to suggest that they CAN cease to exist… Black holes are not “black holes” because we cant see them… They’re black holes because the speed which is required to overcome the gravity, is faster than the speed of light. But since nothing can move faster than the speed of light, that means nothing can escape, not even light. Hence, they are completely dark, hence the name… Not as you seem to think that they’re black holes because of some lack of knowledge about them…

          • Longmire

            Black holes theoretically “exist” because they explain anomalies present in the current equations used in physics. Somehow we all believe that mathematics explains reality when it might not, imagine for a moment that Zeno’s paradox proved that. Again reality is all but figured out. If you examine the nature of the mind it is not at all far fetched to believe consciousness can indeed be transmitted instantaneous.

          • Ether_Man

            Your information is vastly out of date… It’s correct that black holes was simply a hypothesis to begin with. Something predicted to be there but couldnt be seen… You should read up on newer information however since we have directly observed black holes today… And noone said that reality was figured out… As I said before… Reality is quite figured out because WE DEFINE IT… Even if we are the figment of someone’s imagination, which is possible, however unlikely. Then reality is still defined as the existance we see and experience… As in, it doesnt matter if we are or not, it doesnt change anything about our reality… And in that definition, there are multiple realities. Existance is another cookie though and the only thing we really know about existance, is that we ourselves exist… You cant with absolute certainty know if someone else actually exist. And the only reason one can know that they themselves exist, is because they have to, to be able to even pose the question…. And you can believe what you wish… But there is absolutely ZERO evidence for that so it’s not a reasonable belief….

          • Restita DeJesus

            Rehearsing your own prejudices, perhaps?

          • Ether_Man

            Which prejudice would that be? Seems you have a warped idea of what prejudice even means…

          • NeoTechni

            Quantum entanglement cannot transmit data. The mind is in our brain and is a result of it.

          • Longmire

            I didn’t mention data I said consciousness could be transmitted instantaneous across space. If you actually want to know just stare at someone across a room, they can feel you which is currently ignored or outright denied by all the faithful members of the group for what reason. I believe that one example should sum up our current lot, do not see for yourself what can be taught to you by another.

          • NeoTechni

            Thatd count as data.

            Your example sums up that you’re just bullshitting as it’s not a real example :

          • Longmire

            That’s a pretty good example to any one bold enough to see.

          • NeoTechni

            No, its a pretty stupid example to any one smart enough to see

            1) wheres your scientific tests/data to show it happens at a reliable (read: not random) rate?

            2) its not related even in the slightest to what you propose, even if you have the data. They’re in no way similar. Even if you can prove it happens (go ahead, your boldness depends on it) it wouldnt prove anything about this discussion. You’re assuming the cause of X proves Y when y isnt x. That proves you’re insane if anything. Which is bold to advertise online cause it opens you up to criticism which i hope you take

          • Longmire

            The mind is more than the brain does. Why would the mind be so willing to see that a foreign object was connected to it if it was only a product of the body, its not, its way more than an over simplistic the “mind is what brain does” it may tide over the masses but not me. I gave an example that is there for all to see, if you deny that connection between minds exists then you’ll deny anything.

          • NeoTechni

            You grasping at straws isnt proof of what you claim. It just means you’re not educated. I don’t see how you were dumb enough to claim you accepting stupid theories is proof of anything. That doesnt prove the theories, it proves you’re gullible.

            If its way more than that, prove it. Dont tell me you acvepting that is proof of it, it’s not proof of anything.

            Your example was bullshit, i explained why. You still havent given the data to prove your example even exists or explained how its relevant after i told you its not.

            Your entire argument so far is you accepted a srtupid idea, and you accepting it is proof of the stupid idea. Therefor another completely different stupid idea is true. No, the only thing it proves is that you accept stupid ideas

            I demand proof.

          • Longmire

            Proof of what?

          • NeoTechni

            If you’re not going to read what you reply to, dont reply to it

            I demanded proof of your example, that it happens at a reliable rate (ie: not random) and then proof that the cause is actually related to your argument. Cause as I said, even if you have proof, it doesn’t prove your claim. It’s completely unrelated. Hence why I’m insulting you, your argument is batshit crazy

          • Longmire

            Your insulting me because you don’t know how to react to someone saying “look out your own eyes you might notice things that actually point to the unexplained”. Anyways most people are unaware because that is what is expected of them. I very well could be insane, as could we all be but I choose to trust my perception over anothers theory.

          • NeoTechni

            No, I’m insulting you cause you’re basing your entire argument on bullshit you refuse to prove that is both true, or related to your claim.

            You’re also using poor grammar, and logical fallacies. I’m insulting you cause you are proving to be an idiot. And I hope that me pointing that out to you will make you realize and correct the problem.

            “but I choose to trust my perception over anothers theory”

            My perception is that your theory is bullshit and unproven. Prove me wrong, then talk.

          • Longmire

            “Cause as I said, even if you have proof, it doesn’t prove your claim.” A closed mind sees only what it wants to. Quantum mechanics points more to the mystical than the concrete. It is also the science of “reality” which uses language in such a way as to make it inaccessible to 99%(my number of course) of people. Because why? perhaps the answer is not ready to be known, or perhaps it points to Something that doesn’t want to be known.

          • NeoTechni

            ” Quantum mechanics points more to the mystical than the concrete”

            Only if you’ve done absolutely no research. Quantum mechanics don’t back up your theory cause as I said, quantum entanglement can’t transmit data. Your consciousness counts as data. You keep using examples that don’t back up your claims.

            You also don’t get to bullshit your way out of this, back up your claims. Saying you don’t need evidence tells me that you don’t have any. Your mom told me so. Oh what’s that? You want proof she told me? Quit being so closed minded. (That’s your entire argument, you’re bullshitting me and then saying you don’t have to prove it. You do. )

            “Something that doesn’t want to be known.”

            a physical impossibility. knowledge doesn’t have desires

          • epreston8

            Hello, people of this thread! You’re welcome to have metaphysical debates here as long as you follow Discover’s comment policy, which calls for being cordial and not using foul language. Thanks very much.

          • NeoTechni

            my apologies, I find being presented with hilarious claims and a refusal to back up said claims just as offensive

          • RalphT

            Here is the example that exposes the thinking that HAS NOT been substantiated “If you actually want to know just stare at someone across a room, they can feel you..”
            What?? Lots of fluffy stuff followed but no attempt to provide evidence for this odd statement. Please provide that evidence so that this discussion can proceed.

          • NeoTechni

            Exactly. I’d like that evidence too

          • Longmire

            There a specific cell is involved. Since scientists can convince people there hands are made of stone then convincing the unaware to remain unaware would be near effortless. Science should be a platform not a crutch.

          • punkchobit

            You’re an absolute idiot! That article didn’t cite any supernatural forces as you suggest. It’s just the brain keeping track of where people within visual range, and where they look.

            As NT said, it’s not related to your claims.

          • Longmire

            Thanks for pointing out I’m an idiot. But he wanted evidence, I think, that people could actually sense other people staring at them which that article from a respected website acknowledges. Now I also didn’t claim any supernatural force was at play but a fundamental force of physics was. What is not yet understood doesn’t have to be supernatural or denied but it can be theorized about if you wish to be ridiculed.

          • NeoTechni

            And the evidence didn’t back your claim. There was no supernatural cause. The people SAW where the other people were looking. You misunderstood the data. As I said, its not related. Hence you deserve the insults

            The cause was understood. Its you who didn’t read it

          • Longmire

            “Studies that record the activity of single brain cells find that particular cells fire when someone is staring right at you, but—amazingly—not when the observer’s gaze is averted just a few degrees to the left or right of you (then different cells fire instead).” He says AMAZINGLY because if you consider the different appearance of eyes shifted a few degrees it shouldn’t be so absolute as to which cell fires. My uneducated/intuitive guess is that there is some energy stimulating that specific “cell”. The article also doesn’t mention whether the cells fired when the stare was from an undetectable area i.e. from behind the back or direct eye contact which should have been noted. If I were researching this there would be no doubt I would check both perspectives. Just a stupid question but why do you think we call it eye “contact”?

          • NeoTechni

            Yes I read it. It cited no supernatural causes related to your argument. It said the people saw the other. Your uneducated guess was flat out wrong.

            It not mentioning if it was from behind is a flaw in your argument though. Meaning its not proof of your claims

          • Longmire

            Its not a flaw in my argument its a flaw in the mentality of current researchers not being willing to publish information contrary to the consensus, why give em proof. But the specific cell being activated could point to my conclusion.

          • NeoTechni

            You can’t attack your own evidence as being flawed. That admits your argument is flawed. Try again

          • Longmire


          • Restita DeJesus

            I cant believe this etherman guy….throw a little science somewhere and the intellectuals jump at the chance to call people stupid….for what, to prove they’re oh-so-much-more endowed with superior intellect? Give me a break.

          • NeoTechni

            “throw a little science somewhere”

            He did no such thing.

            “and the intellectuals jump at the chance to call people stupid….”

            Because what he did had nothing to do with science.

  • jumbybird

    It’s a bug in the software.

  • gatorallin

    time to better understand phantom limb pain and the trick used to release that cramping pain with box and mirror trick. Maybe same thinking is going on in both examples… just another way to see/feel it. Just saying…

    more here

  • Oren Carstensen

    To create the illusion, they had 23 subjects sit with one forearm on a table. Films Streaming

  • Neil Crabtree

    According to Immanuel Kant: ‘Reality, in the pure conception of the
    understanding, is that which corresponds to a sensation in
    general; that, consequently, the conception of which indicates a being (in time).’

    As I wrote in my recent essay on Global Warming and the Nature of reality –

    A starting point is to realise that our reality is determined by our inherited disposition combined with the knowledge we glean
    from life’s experience; and unless we have the insight of a yogic guru, the reality of our own world is subtly and sometimes obviously, at variance with the reality of every other living thing! We all have different beverage preferences, our own understandings of death, our own views on refugees and our own political leanings – to the right or to the left?

    The fact is that the observer creates reality!



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


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