What our changing body image teaches us about our brains.
Posted Feb 07, 2010
Body image - In one of life's funny coincidences, the first doctors to use this term were the neurologists, Henry Head and Lord Russell Brain. When doctors or scientists speak of body image, they are not referring to your opinion of your body (I am too fat! I hate my nose!), but rather the sense that you have a body and that you know where its various parts are in space. This information is crucial for movement. If you could not sense your legs or know their position in space, it would be extremely difficult to get up and walk.
Our sense of body image, of our ownership of our own bodies, is integral to our sense of identity. At first blush then, you might think that we maintain a stable, robust body image throughout life. But a few simple experiments might change your mind. I just picked up a pencil and, holding it vertically and with the eraser end down, pounded the pencil against my desk. I felt the desk push back, but this feeling was not centered at my finger tips but was transferred to the end of the pencil instead! Since my pencil had a good eraser at its end, the feeling seemed "rubbery," as if my finger tips had transformed into the eraser. Indeed, studies of brain activity in monkeys and people show that when we use a tool, our mental body image expands to encompass the tool and that this change happens remarkably quickly. Our body image is in constant flux.
I do a popular "body image" experiment with my students each year. This experiment was first described in a paper in the journal Nature and then popularized in the book, Phantoms in the Brain. I take a dishwashing glove and fill it with cotton so it resembles a solid right hand. Then I place the glove on the table just to the right of a student at a point where her own right hand could comfortably lie. The student places her real right hand to the right of the glove and behind a barrier to that she cannot see it. Then I stroke the glove and the student's hand in exactly the same way and at exactly the same time. If I touch the thumb of the glove, I touch the real thumb and so on. The student watches me stroking the glove and feels her own unseen hand being stroked. In less than a minute, many students develop the eerie sensation that their own hand is taking on the latex feel of the glove or that the glove is their own hand! It is easy to see when this illusion works because the student gets a look of horror and disgust on her face and instantly pulls her hand away. One student was so impressed by this phenomenon that she thought that every student in the college should participate in this experiment!
Why is your mind tricked into thinking that a dishwashing glove is the real hand? Because sensory information from several sources is telling you that this is the case. You see the dishwashing glove being stroked in the same way that you feel your unseen hand being touched. From this pooled information, your brain (actually you) make the best guess. The glove must be the real hand. Our sense of our own bodies, our body image, is constantly being updated by the input we receive.
Over and over again, timing has been shown to play a crucial role in how we perceive ourselves and how we learn. We must combine information coming from different sensory organs with our prior experiences and knowledge to produce a single, coherent perception of the world. If we see and feel two things happening simultaneously then we ascribe them to the same source, even to the point of believing a lifeless hand to be our own.
Timing was critical too in my acquisition of stereovision, the ability to see in 3D. Since I was cross-eyed since early infancy, my two eyes did not look at the same thing. In order to develop a single view of the world, I had to shut down or suppress the input from one eye. With optometric vision therapy, I learned to do what most infants can do within the first months of life. I learned to aim the two eyes at the same point at the same time. Now the input from the two eyes was correlated and, to my astonishment, I began to see in 3D. As with the hand experiment described above, temporal correlation, two inputs happening at the same time, was critical for the perceptual change. But something else was needed, a crucial step, which I'll explore in next week's post.