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Body Image and Giving the Finger

by Steven Novella, Sep 23 2013

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.

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One Response to “Body Image and Giving the Finger”

  1. Max says:

    What if instead of numbing the right hand, they just have an artificial hand hold the right index finger?

    I assume the illusion lessens when the hands are farther apart, like if the right arm is extended to the right, and the left arm to the left, because proprioception ruins it.

    The first of these illusions I learned was to cross your fingers and rub your nose inside the ‘V’. It’s supposed to feel like you have two noses. For me, the stronger illusion is to rub the other hand’s index and middle finger outside the ‘V.’ It feels like one finger.