Science Update - Body Image

Body Image

Introduction
If you feel fat but don’t look it, there may be a good reason. Researchers in England have found that when it comes to body image, the brain has a mind of its own.


Podcast: Body Image


Transcript

How the brain sizes up the body. I'm Bob Hirshon and this is Science Update.

Whether you feel flabby or fit depends on your brain as well as your waistline. This according to neurologist Henrik Ehrsson and his colleagues at University College, London.

They stimulated the nerves in volunteers’ bodies in a way that tricked them into feeling like their waistlines were shrinking. The illusion activated a part of the subjects’ brains called the posterior parietal cortex, which integrates sensory signals from all over the body. The nerve stimulation for each person was the same, yet some experienced the shrinking sensation more strongly—and they had more activity in this part of the brain.

Ehrsson:
That suggests that two people who have identical bodies might experience their body image differently.

This may lead to a better understanding of anorexia and other body-image disorders. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

When you put your hand on a hot stove, it activates heat receptors: specialized cells on the surface of your hand custom-designed to detect heat. We have receptors for pain too, and other kinds of touch. There are even receptors throughout the body that tell us how our body is oriented in space (in other words, whether our arm is up or down).

But there aren’t receptors specifically designed for detecting our body size. Yet we’re constantly judging our body size, and not just to figure out whether we’ll fit into our favorite jeans. For example, in order to reach across a table to grab a can of soda, your brain has to factor in not only how far away the soda is, but also how long your arm is.

How does the brain do it? According to this research, it looks like body size is a higher-level process than simply perceiving heat or pain. It requires the brain to consider several different streams of information, and put them all together into a coherent picture. The posterior parietal cortex is a part of the brain that does just this sort of thing. It’s the part that was activated when the subjects were confused about their body size.

The illusion that Ehrsson’s team created is called the Pinocchio Illusion, and it’s been around since the 1980’s. Here’s how it works. You put your hand on your waist and then the researcher uses a small, vibrating device to stimulate the tendon on your wrist. This creates the sensation that your wrist is flexing, even though it isn’t actually moving. As a result, your brain gets conflicting information: “My wrist is flexing but I’m not moving it.” Brains don’t like these sorts of conflicts, and they sometimes solve them by creating illusions. In this case, your brain decides that your wrist is flexing because your waist is shrinking.

Not all of Ehrsson’s subjects experienced this illusion. Of those who did, some experienced it more strongly than others. Since the input to the body was exactly the same for each person, the researchers concluded that everybody’s brain calculates our body size a little bit differently. This conclusion is supported by the fact that the stronger the illusion, the more activity there is in the brain.

It’s a long way from this study to eating disorders like anorexia, but it may help to answer an important question: why do people with eating disorders think they’re so much heavier than they actually are? Could it be that their body-size-perception system has gone awry? And if so, can the problem be fixed? It may not be easy, but it’s possible that basic research like this could someday make a real difference in patients’ lives.

Now try and answer these questions:

What’s the difference between how we perceive body size and how we perceive pain, heat, and touch?

Describe the Pinocchio Illusion. Why was it chosen for this experiment?

Suppose all the subjects experienced the illusion in the exact same way, yet brain activity during the illusion was different from person to person. How would you interpret this result?

Suppose, instead, that everyone experienced the illusion differently—some strongly, some weakly, some not at all, just as in the real experiment—but regardless, everyone’s brain activity during the illusion was identical. What would your conclusion be in this case?