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Body Snatchers, Phantom Limbs, and Alien Hands

by Steven Novella, Feb 21 2011

One of the critical bits of wisdom central to a skeptical outlook is the realization that our brains are not objective perceivers of reality. Not even close. What we perceive as reality is constructed in an active process that is rife with assumptions and flaws. Everything you take for granted about what you experience as yourself and the outside world is actively constructed by specific brain processes.

There are some assumptions that are so fundamental to your construct of reality that you take them for granted – you are not even aware they are happening. We only know about them from cases where these mechanisms break down. For example, there is specific brain processing that makes you feel as if you are separate from the rest of the universe. If this processing breaks down, you will have the sense of feeling merged or one with the universe. Most people who experience this, usually as a consequence of psychoactive chemicals, feel that they have had a profound experience. In a way they have – they have had a “for the world is hollow and I have touched the sky” moment. They have peeked behind the curtain of their brain’s function and have experienced what it is like to have their brains construct reality in a different manner from what they are used to.

In essence they have had a profound internal experience. Many people however will interpret this as an external experience – that in some meaningful way they have touched the universe or experienced God or something similar. It is no surprise that many cultures have traditions that involve the use of psychoactive drugs in order to induce profound spiritual experiences.

We see this internal/external confusion in many other contexts as well. People who have a hypnagogic hallucination may feel they were abducted by aliens or visited by demons, rather than had a neurological episode of waking dreams. People frequently experience optical illusions (a break down of the visual construction of external reality) and react to what they constructed in their brains as if it were an accurate representation of external reality.

Another example of this overall phenomenon is the fact that our brains perform specific processing in order to create the sensation that we are inside our own bodies, that we own our body parts, and that we have agency or control over these body parts. These are all constructed experiences that can break down with damage to one or more brain structures. Further, as will optical illusions, these brain processes can be tricked into creating an alternate construction.

Henrik Ehrsson of the Swedish medical university Karolinska Institutet has published a series of studies in which he explores how the brain constructs the sense that we own our own bodies, and experimental conditions in which this process can be tricked to make us feel as if we are not inside our bodies, we do not own parts of our body, that we occupy another body (real or virtual), or that we own non-existent body parts. Not only is this absolutely cool, but what better demonstration of the fact that our realities are constructed by brain processes – and that you cannot absolutely trust what you experience.

In one experiment he was able to induce subjects to feel as if they had swapped bodies with another person. Subjects actually feel as if they occupy another body. This and other studies reveal some information about the process. The brain uses multiple sensory clues – primarily vision and tactile sense – compares these sensations and then uses the comparison to construct the sense of location and ownership. For example, you can see something touching your hand, you also feel that your hand is in the space that is being touched (proprioception) and you feel the touch (tactile sensation). Your brain compares these sensory streams and if they all match up that creates the ongoing sensation that what was touched is your hand.

This creates the opportunity to trick the brain. What Ehrsson has done is use virtual reality goggles or other setups to show a subject either another person’s arm or a virtual arm. That other arm is then touched and the subject’s real arm is touched in the same place at the same time. The brain then assumes that the arm it is seeing must be the subject’s arm, and creates the sensation that it is, in fact, the subject’s arm.

In a separate experiment he has made subjects feel as if they have an additional limb – not just swapped their body for another, but actually have a third limb. This occurs also in some stroke patients – the ownership module may be cut off from the person’s actual limb and as a result supernumerary phantom limbs will be created.

There is a distinct phenomenon of agency – the sense that we control our body parts. Alien hand syndrome is a category of syndromes in which one feels that they do not control their own hand. This too is caused by a disconnect among various sensory streams that do not match. In this case the brain compares the activity in those parts of the brain that plan and execute movement with the sensory information about what the limb actually does. If they match – then that creates the sensation of agency. If that process breaks down (again, usually from a stroke or other brain damage) then the limb feels like it is under alien control.

The theme among these various phenomena – body swapping, phantom limbs, and alien hands – is that the brain is comparing different sensory inputs to see how they match and then uses the comparison to create the distinct but related sensations of occupying a physical space and body, owning body parts, and controlling body parts.

The implications of this are extremely interesting. For the skeptic it is another example of the fact that our perceptions of reality are constructed. This means that seeing (and experiencing) is not believing. What we experience is largely a fiction. It works well enough most of the time – but not all the time. It can be tricked into incorrect perceptions that we cannot distinguish from accurate perceptions – it all seems really real to our brains.

For neuroscientists a key insight is the fact that our brains have specific processing that involves the comparison of different sensory streams. They are not all separate. Further, there is a separate line of research (which I can discuss in a separate post) that demonstrates that one sensation can affect the perceptions of another. What we see affects what we taste and hear, for example.

And for the futurephile, this research means that one day we can be made to feel as if we occupy that virtual reality character we are playing in the latest video game. Or (more practically) be made to feel as if we own a prosthetic (artificial limb), or that we occupy the robot we are remotely controlling.

Ehrsson is exploring the limits of our ability to body swap. For example, we cannot be made to feel as if we occupy a block of wood. But can we be made to feel as if we occupy a barbie doll? What are the limits of size, shape, number, etc?

If you are not blown away by this research, then read the post again.

23 Responses to “Body Snatchers, Phantom Limbs, and Alien Hands”

  1. Jacob says:

    Consider my mind blown. Now how do we get these types of rides into a theme park? If anyone interested could experience this firsthand we might have a fighting chance of dispelling the new age woowoo.

  2. Chris Howard says:

    I wonder if it could be used to treat people with psychological disorders, specifically personality disorders?

  3. Ed Seedhouse says:

    Of course, no matter how we feel about it the plain physical fact is that we are *not* separate from the physical universe, but connected with it. The “mystical” experience is closer to the reality than our normal perceptions.

    I wonder how our common perception that we are separate contributes to survival. It seems odd that, what basically amounts to a shared hallucination has survival value, but if it doesn’t then why would it persist in us?

    Do we know if this separation perception is inherent in our genetic structure or is it possibly a learned behaviour? Freud believed that newborn babies lack this sense of separation. Do we know if he was right or wrong and if he was right do we know anything about how we learn it as we grow?

    • Chris Howard says:

      Perhaps, “the shared hallucination” creates a closer connection between people, increasing social bonds, and kinship ties, thus increasing the chance of survival?

      • Max says:

        By “shared hallucination”, he means the sense of separation, I think.

        I don’t know, are people even functional without a sense of separation from the universe? Can they walk and hunt for food? Do they experience pain and hunger?

      • Chris Howard says:

        I think we’d suffer a psychotic break, if we experienced something like that, all the time. Yeah, sorry. What I meant was when people come “down” off the experience, and then share the event with others. How the “mystical” experience eventually turns into a religion, which inturn acts as a social suport network.
        The “no pain” thing is interesting, though. Maybe we developed this detachment thingy, as a way to die, somewhat peacefully, shutting down the bodies lesser functions, in a last ditch effort to survive. People might be triggering this response with out the mess of being near death. I dunno’?

    • Jacob says:

      I’m not sure why you think were not separate from the physical universe. There is a physical barrier between our bodies and the majority of nature. We can’t control anything outside our body, so why would we consider ourselves anything but our own bodies? If you had no sense of your body, your brain wouldn’t last long in being connected to the universe!

      It would seem to me that much of it is definitely learned. The brain structure is there, but it has no idea what its doing. Hence babies not being able to walk out of the womb, hands and arms going all over, etc. You can also see this in sports where some people know where their hands/feet are better than others.

      Some people are more clumsy probably because they think their feet extend shorter or longer than their feet actually are. Same thing with one’s shoulders (I tend to bump corners by taking them too sharply). I think these all result from the same phenomena. If you strip the brain of the knowledge about its body (temporarily), I could easily picture it grasping for straws… or extra bodies in the area. The interesting thing is that it can be so EASILY stripped away!

      • Rabbit says:

        “We can’t control anything outside our body, so why would we consider ourselves anything but our own bodies?”

        Why do you think that you’re in control of your own body and not in control of anything beyond the skin? When you “hear” music, are you in control of hearing it? If you chop down a tree, are you in control of chopping it?

        I rather like Gregory Bateson’s perspective on the issues of control and the mind-body separation. He argues that the question of control is misstated. Any action can be seen as part of a larger system (brain-hand-axe-tree-eye-brain, for instance), and no part of a system can ever have unilateral control over the whole. So you can never have unilateral control of your body…but what is this concept ‘you’?

        When he talks about ‘self’, Bateson uses the analogy of a blind man with a walking stick. If you wanted to delimit the blind man’s ‘self’, where would you draw the boundaries between his self and the external world – at the tip of the stick? At his hand? Half way down the stick? If you are in control of your eyes, and the man’s stick is a substitute for his eyes, the man is in control of the stick, isn’t he? Therefore, the stick is part of his self…

        But when he sits down to eat and puts down the stick, the stick is no longer a part of his self!

        Perhaps the concept of ‘self’ is a product of the entire system, the size of which depends on the phenomenon you want to understand – whether it be the blind man’s walking, or his eating. Maybe there’s no real reason to draw the boundaries of ‘self’ at the skin.

  4. Max says:

    I can create the illusion of having two noses, if anyone wants to fund my research.

    And totally off-topic, but have you ever googled “baffles scientists”? Fun stuff.

    • Max, what happens if you google “baffles scientists” + “santorum”?

      More seriously, it sounds like this is an extension of research on tricking individuals into believing that a manikin arm is actually their own.

  5. feralboy12 says:

    I remember that merging with the universe stuff from my own LSD days in the 70’s. Far from being profound, though, I found it oddly uncomfortable and disorienting. Imagine trying to talk on the phone when the damn thing merges with your head. Taking a leak can be an adventure as well–“I hope that wasn’t my soul I just flushed.”
    I don’t miss that stuff much.

    • Chris Howard says:

      I hear ya’. There are times when I get odd, pre-epileptic, auras. It feels like my body is merging with the bed, then the room, then the house, and eventually I start visualizing strange fractal-like, Mandelbrot imagery, which I, for lack of better description, disolve into.
      It only happens when I am near sleep.
      It feels like my sensation is extended well beyond my bodies physical limits, and then, when I reach the Mandelbrot stage, perception disolves, and not to be too “woospeak” but there is a feeling of being, and not being.
      Of course, as you all have probably already discerned from my posts, my brains not quite right. ;-)

  6. Kyran says:

    Ehrsson is not the only person out there who is researching these phenomena. I myself am actively using these constructs with patients with schizophrenia.
    What sets Ehrsson and his group apart is both the variety and quality of techniques they use.

    Another finding of Ehrsson that is relevant to skeptics is the induction of out of body experiences in healthy humans using a rather complicated setup of video googles, video cameras and mirrors.

    The distinction between yourself and the rest of the world is such a continuous sensation that those of us lucky enough to be healthy take it for granted. Even though there is a physical barrier between us, the brain still has to perceive that barrier and recognise it as being a barrier! This is demonstrated in various conditions where the distinction between yourself and the world breaks. Psychoactive drugs (LSD and ketamine are the two that most spring to mind) and schizophrenia are best-known for their ego destruction. If you don’t understand the importance, think of it this way: if you don’t where you end and the universe starts, how do you know what is part of your body and what isn’t? If you don’t know that, how do you know what part to move if you want to walk? And how do you know that it is you who is moving it? As I just said, there are conditions in which these processes break down and they can be very debilitating. There are whole neural networks of the brain dedicated to these processes.
    As for the sentence “We can’t control anything outside our body, so why would we consider ourselves anything but our own bodies?”… we control things outside our body all the time. You type, write, kick a ball, use a knife, turn a light on/off etc etc. We are forever influencing our environment. Even if there is a barrier between myself and a light switch, how do I know that the light turns on because it is a light and not a part of myself? These distinctions need to be made!

  7. Ed Seedhouse says:

    “I’m not sure why you think were not separate from the physical universe. There is a physical barrier between our bodies and the majority of nature.”

    Well no there isn’t. The skin is not only a barrier but also a connection. Air is perfectly physical and we all share the same atmosphere. Atoms that Abraham Lincoln once breathed out are in your lungs right now to a virtual statistical certainty.

    Take away the world and we die. We live only because we are physically connected to it. Do not confuse connection with identity. I am a distinct part of the universe while being physically connected to it, at least locally. I am not you and you are not me, we are distinct, but we are still connected by a perfectly real and actually physical connection.

  8. highnumber says:

    Where is Massimo Pigliucci when you need him?

  9. Brandon Z says:

    Can you imagine the ‘literally’ mind blowing recreational drugs one could engineer with these mechanisms fully understood. Puritanism sucks!

  10. Tommy says:

    Really interesting research!

    A couple of videos of the experiments:

  11. Paulina says:

    Vilayanur Ramachandran is another scientist who has done research into phantom limb pain, and has used simple mirror illusions to try to trick the brains of his patients into believing it has gotten feedback from its missing limb. It has varying effectiveness, in his studies, depending on how long the person has had the pain.

    • Paulina says:

      Sorry about the syntax. I really enjoy torturing English for fun in my spare time. :/

  12. Billy says:

    When you mention the “specific brain processing that makes you feel as if you are separate from the rest of the universe”- what is that called? I have been looking (quite passively, really) for this info for a while now. And has it been shown which psychoactive chemicals disrupt these processes and how??? I am very curious about this…