Tuesday, 15 June 2010

A distorted image of science

[trigger warning for eating disorders]

The BBC, in its usual uncritical way, reports on a paper claiming that people's sense of proprioception is not great. Proprioception, if you haven't heard the term before, is the internal sense of position - you know where any limbs you have are in relation to the rest of your body, at what angle your head is to your shoulders, and so on, without having to look.

The paper itself (payment/subscription needed for full text) is really quite interesting. They tested how exact this sense of proprioception is by asking people to point to the fingertips and knuckles of one hand with the other hand while not being able to see it, and found that people tended to miss by a fair bit, estimating their hands to be shorter and wider than they really were.

I tested this briefly in an uncontrolled fashion with my own hands, and yes, this does seem to happen to at least some people.

So, the next time someone tells you that they know X like the back of their hand? Worry.

This is not an area I have much knowledge about, so if there are giant flaws in the research methodology, I don't stand much chance of finding them. There are none stunningly obvious to me as a scientific generalist, though some things that do stand out are:

  • All of their experimental samples (for four variations of the study) have more women than men, some by a significant margin. If gender was significantly affecting the results they didn't mention it, but this suggests some other bias in how they recruited their samples which might be significant.
  • They don't mention, but it seems likely by implication, that all of the sample had relatively average vision. Whether this effect also occurs in people who aren't able to use vision to supplement proprioception in most cases seems an important question.

The flaws, as usual, come in the interpretation for the press release and news articles, where, according to the BBC:

The brain naturally distorts body image - a finding which could explain eating disorders like anorexia, say experts.


Distorted perception may dominate in some people, leading to body image problems, a US journal reports.

Lead researcher Dr Matthew Longo said: "These findings may well be relevant to psychiatric conditions involving body image such as anorexia nervosa, as there may be a general bias towards perceiving the body to be wider than it is.

"Our results show dramatic distortions of hand shape, which were highly consistent across participants."

...and in come the wild extrapolations.

It is, I suppose, possible that the body image distortions of anorexia nervosa have the same basic cause as this one, and it would be entirely reasonable to do some follow-up studies in this direction.

But there's no evidence yet to justify Dr Longo's speculation that there's even a tendency to misperceive your own body width in the general1 case, or that people with anorexia nervosa misguess where the edges of their body are more than people without.

The proprioception errors they report on are fairly clearly not "proprioception plus vision" errors, but anorexia nervosa is not generally a condition that only takes effect when not observing oneself visually, which is some evidence that they aren't the same effects.

The hand distortion seems to be fairly consistent across the sample and not related to any obvious social effects. Anorexia nervosa has been fairly strongly linked to social effects, and varies widely in severity across the population.

It's an interesting study with potentially interesting follow-ups, but both the news article and Dr Longo's quotes are assuming prior to doing the needed follow-up studies that the distorted body image of proprioception and the distorted body image of anorexia nervosa are distorted in the same way, or even involve the same meaning of "body image". As usual, the BBC seems to have largely uncritically reported and extended a press release.

Here's the original press release - there's one relevant quote at the end, in a press release that otherwise actually summarises the research very well:

“These findings may well be relevant to psychiatric conditions involving body image such as anorexia nervosa, as there may be a general bias towards perceiving the body to be wider than it is. Our healthy participants had a basically accurate visual image of their own body, but the brain’s model of the hand underlying position sense was highly distorted. This distorted perception could come to dominate in some people, leading to distortions of body image as well, such as in eating disorders,” said Dr Longo.

It's not as bad as some of the BBC's science reporting, but it's got all the same types of error - unjustified extrapolation, uncritical reprinting of press releases, not reading the actual paper, etc.

1 They might also need to sharpen up their recruitment strategies to get a control group from a society where "thin is good / fat is bad" isn't the overwhelming social message.