Sep 23 2013

Body Image and Giving the Finger

The most astounding sensory illusions occur by exploiting brain processes you are not aware even exist, or need to exist. Definitely in the running for the best such illusion are body ownership experiments. Your brain uses sensory information to decide which parts of your body you own, control, and where they are in three-dimensional space. This process can be easily fooled into creating an alternate image – making you feel as if you own and control fake body parts, and even virtual avatars.

Various researchers have consistently shown this basic effect, while they explore the details and limits of this phenomenon. One such experiment, published today in the Journal of Physiology, adds a few new details to the picture.

Researchers had subjects hold an artificial finger in their left hand. They then had them flex their right index finger, while the artificial finger also flexed itself. They were blocked from seeing their hands, and the skin of their right hand was numbed with medication. Subjects then reported that they were holding their right index finger.

Therefore – their brains constructed an internal model of their bodies based upon the fact that when they commanded their right index finger to move, the finger in their left hand moved, therefore that must be their finger. Because visual and surface sensory information was blocked, the researchers concluded that this body-image illusion was created with a single sensory modality – sensory feedback from muscles. That’s the first time one type of sensory input has created this illusion.

This does not necessarily mean that the brain generally only uses that one modality. Skin sensation and vision were blocked. If they weren’t, the brain would have used them also and compared all the sensory input to “decide” where its finger was (as indicated by prior research). But this study does show that the brain can assign body ownership based on one type of sensation alone.

This research also suggests what many other types of research have, that the brain is not a passive recipient of sensory information. The brain builds or constructs models of reality based upon sensory information. However, this is a “thoughtful” process (to use the researcher’s words). The brain decides on what the most likely scenario is, based upon what sensory information it has and on various assumptions which are likely based on passed experience and its overall model of how reality works.

Developmental research shows that over time babies learn how the world works – that objects do not disappear from existence when they are no longer visible, that objects that are not supported will fall down in a straight line, etc.

The type of such assumptions that are probably best known are those related to vision. Our brains assume that objects that are smaller are farther away, objects that obscure other objects are closer, and objects that are visually contiguous are physically contiguous. It then takes the visual information it has, it performs moment to moment a subconscious processing that determines what the most likely scenario is for the construction of the visual images it is receiving, and then that is what we “see.” If this construction does not fit reality, or contains internal contradictions, then the result is an optical illusion.

The finger experiment is exactly the same – creating a body ownership illusion by tricking the brain’s algorithm for constructing its ownership model.

Magicians exploit this all the time as well. They have learned through trial and error that people consistently make the same assumptions about how reality works and that they will tend to pay attention to and process certain kinds of information in front of them. Magicians trick their audience into constructing reality in a false but entertaining way. Magic tricks are literally illusions.

Research also reveals that the brain processes multiple streams of sensory information simultaneously and compares them to each other. So – it does not just use visual information to construct what you see; it also uses sound, tactile, and other information to construct one seamless narrative of reality.

The builders of amusement park rides know this. The latest rides and high-end parks, like Disney or Universal, use multiple sensory modalities together to create powerful illusions. On Universal’s Spiderman ride, for example, you sit in a car with about a dozen other people as you move slowly through a three-dimensional set with video screens to show the action. At various points you are made to feel as if your car is falling to the ground. This sensation is created by tilting the car forward so you feel gravity’s tug, while the video shows you falling through the air. At another point a super villain threatens you with a blow torch, and a blast of hot air is enough to create the illusion that the threat is real.

This latest research shows that the brain will get by with one type of sensation if that is all it has, but other research clearly indicates that our brains weave multiple sensations together when available.

The researchers also found that when the subjects grasped the artificial finger, some of them perceived their real index finger to be closer and level with the hand grasping the artificial finger. In other words – the brain’s construction of where the two fingers are in relation to each other was based not only on directly feeling where those fingers were, but also incorporating the sensation that one finger was grasping the other.

Professor Simon Gandevia, Deputy Director of Neuroscience Research Australia, who conducted the research said:

“Grasping the artificial finger induces a sensation in some subjects that their hands are level with one another, despite being 12 cm apart. This illusion demonstrates that our brain is a thoughtful (yet at times gullible!) decision maker: it uses available sensory information and memories of past experiences to decide what scenario is most likely (i.e. ‘my hands are level’).”

That is as good a summary as any – our brains are thoughtful but gullible.

8 responses so far

8 thoughts on “Body Image and Giving the Finger”

  1. locutusbrg says:

    This study reminds me of pediatric suturing. If you secure an toddler to repair a finger laceration and numb the extremity, the extremity disappears underneath the sterile covering and appears visually to the child in the middle of the sterile field. They will go from abject terror(while numbing) to interested observer. They lose the ability to understand that the hand is theirs since they cannot feel it plus the visual illusion. The lack of object permanence in the young child shows how their brain reconstructs a reality where someone else’s hand is getting sutured. The child calms down(most times) and intently watches the procedure. Interesting that you can pull the similar result in adults.

  2. tmac57 says:

    While doing some ‘herbal research’ back in the mid 70’s, I devised an experiment where I and a friend of mine who had similar a appearance,sat in identical chairs at right angles to each other with a barrier in between such that we could not see each other.Then I placed a tall door mirror in such a manner that as we sat in our chairs,we could see the other person,but it had the illusion that you were looking straight into a mirror and seeing yourself.Next we took turns initiating slow gestures that the other could mimic (ala the famous Lucy and Harpo Marx routine).It was an extremely weird and funny illusion that became very real for the person who was not the mimic.

    Ahhhh…good times 🙂

  3. superdave says:


  4. TheFlyingPig says:

    locutusbrg, So the toddlers generally weren’t concerned that their arm or hand had gone missing?

  5. ccbowers says:


    Most toddlers have a fairly good understanding of object permanence. I do not doubt locutusbrg’s experience as much as the explanation. I’m sure that not feeling the pain and altering of the typical visual perspective reduces the fear of the toddler, but I question the idea that the fear is reduced because they no longer think that they have the limb that is covered (which is what alluding to object permanence implies).

  6. Bruce Woodward says:

    I find this fascinating and is a great post.

    I do worry, however, that this research could be used by self help quacks and “prove” their point that you can create your own reality by thinking positive and pretending to pet puppies etc.

    I guess all research can be twisted if you try hard enough and what this demonstrates is how we really have to be careful in analysing even what our brain is telling us.

  7. locutusbrg says:

    @ theflyingpig.

    I am guessing but only the finger is numb, and they can still feel their hand. Since it is immobilized they become so engrossed with watching that the just kind of go with it.

    @ccbowers I would disagree. Small children (and many adults) flinch and struggle when you approach with a suture needle. It is instinctive. Demonstrating that the finger is numb(by poking with a needle), does not alleviate the distress in a little child. Even after multiple sutures that do not produce pain they respond strongly every time the needle approaches. Same as in suture sites much closer to the torso. You immobilize the child, numb them and then cover the extremity and they passively accept and watch the suturing. It is anecdote I admit but there is no way that small child will accept that the needle will not hurt when it is approaching their skin. It is reasonable to assume by the relaxed fascinated observation that they think they are watching something happening to someone else. Object permanence age is in itself a controversial subject. But the key factor is applying the sterile field, without it you get continued resistance. For those lay persons reading this, the hand comes out through a square hole in the center of a larger square of sterile disposable covering. The key factor is the visual disconnect of the covered arm and the hand through the center. A average size field will cover most of toddlers extremity. Showing them that they cannot feel the needle does not produce the same response. It may not be object permanence but more in line with the post above where the brain is drawing a picture from multiple sensory lines that make the child think it is not them getting the needle.

    Discussion of this anecdote that has actually spurred me on to consider researching this officially.

  8. ccbowers says:

    “It is reasonable to assume by the relaxed fascinated observation that they think they are watching something happening to someone else. Object permanence age is in itself a controversial subject.”

    A lack of object permanence in this senario would imply that the toddler does not realize that his or her limb exists anymore, and this is not what you are arguing to begin with. The age at which object permanence begins is somewhat controversial as you say, but the trend had been identifying the ability at younger and younger ages.

    “The key factor is the visual disconnect of the covered arm and the hand through the center. A average size field will cover most of toddlers extremity. Showing them that they cannot feel the needle does not produce the same response.”

    I agree with what you are saying, but my point it that it has little to do with the concept of object permanence. Even as an adult, I imagine that the visual disconnect of the covering would probably alleviate my own anxiety somewhat, and I believe my understanding of object permanence is fully developed. In the past, I have just closed my eyes and this had alleviated the fear response.

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