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Misperceiving the Affordances in Anorexia Nervosa

By Andrew D Wilson @PsychScientists

Misperceiving the affordances in anorexia nervosa

Alice had a few problems with doors too

It's always hard to evaluate the impact of your research, although funding bodies are increasingly asking that we do so. I always struggle a bit, because the connection between studying, say, the perception of affordances and impact on policy or clinical practice is rarely a straight forward story. My problem is that I think that, if you ask a question the right way the answer will always come in handy to someone at some point, and that this is a big part of how science has its impact. 

Case in point: in 1987 Bill Warren was investigating the perception of affordances for passing through an aperture, such as a door. Apertures can vary in width, but the question facing the observer isn't 'how wide is it?' but 'can I fit through it?'. Bill was describing affordances using pi numbers, ratios of widths in the world divided by widths of some relevant body scale. An aperture wider than your shoulders, say, would yield a ratio larger than 1. For a given person's shoulder width, different apertures would produce different ratios. Across people, apertures of different sizes will produce the same ratio as shoulder width changes. It's a way of factoring out the effect of the size of the observer, and it's based on the idea that we percieve the world in terms of our own ability to act on it.

Warren & Whang (1987) asked, at what value of the aperture/shoulder (A/S) ratio do people start turning in order to fit through the gap? Large and small people turned for different apertures but the A/S ratio that triggered the turn was fairly constant over the groups and came out at around 1.30. In effect, people give themselves a 30% window for error and turn whenever the aperture is smaller than 1.3 times their shoulder width.

A nice result; the ratio describes the relevant affordance property (a property of the world measured on an action relevant unit) and people's behavior is generally organised with respect to that affordance. The margin for error reflects how difficult it is to turn while walking (high inertia), the fact of postural sway during locomotion and the significant cost to locomotion of banging into the edge of the gap. 

Impact (and it only took 26 years!)
Keizer et al (2013) have now taken advantage of Warren's more detailed, basic science analysis and applied this task to people with anorexia nervosa (AN). People with AN are well known to have distorted ideas about their own appearance, but where does this distortion come from? Are they biased in what they pay attention to, or do they actually misperceive their body's size? 

Keizer et al had patients with AN and some healthy controls do a version of the aperture crossing task. Participants walked 6m to an aperture and beyond to a table where they interacted with some haptic stimuli. (The cover story for the study was testing whether different actions, such as walking, influenced haptic perception. People with AN are obsessed with issues of body size and they wanted to distract them from the issue of fitting through a gap.)

The control  participants turned on at least 2/3 trials at an A/S ratio of around 1.25, essentially replicating Warren & Whang (1987). The AN participants, however, turned at a ratio of 1.4, significantly higher.This clearly suggests that the AN patients literally perceive their bodies to be larger than they really are, and they are turning as if their bodies were bigger.

This opens up a whole new potential avenue for treatment. First, it explains why many AN patients persist in body image distortions even after treatment; the treatment has not changed their perception of themselves. Second, it may be possible to recalibrate these people's perception of themselves, Calibration is just the process of perceiving information that specifies the action-relevant unit; you use that information to scale the perception of, say, the size of an aperture. You can recalibrate people almost indefinitely (see some thoughts further down on this post) and perhaps, with perceptual training, rehabilitation for AN can be enhanced.

One caveat

One other interpretation is that the AN participants correctly perceive the affordance but are giving themselves a larger margin for error for some reason. The 30% margin, although pretty consistent, is not compulsory. The authors might think to check if the AN participants sway more from side to side as they walk to rule this out (I posted a comment to this effect on their paper, a nice feature of PLOS One!)

One Note

We often hear that our description of standard cognitive psychology is unfair; that no one really talks about representations the way we suggest, and that we're describing a straw man. This paper does it all, and is a nice demonstration of just how pervasive this thinking is. The problem people with AN have is they haven't updated their body schemas recently. Motor programmes to control locomotion are working fine, but the input they take from perception is flawed. This, in a paper about the perception of affordances, and that word is only mentioned once, in the title of Bill Warren's paper in the Reference section!
Any questions asked correctly produces an answer that will be of use to someone, some day. Warren's theoretically motivated exploration of affordance perception laid out some methods and analyses for measuring how people perceive their own bodies, and this question, it turns out, is very interesting with respect to anorexia. It turns out getting the right answers to the right questions is reason enough to study things like affordances.

Keizer, A., Smeets, M., Dijkerman, H., Uzunbajakau, S., van Elburg, A., & Postma, A. (2013). Too Fat to Fit through the Door: First Evidence for Disturbed Body-Scaled Action in Anorexia Nervosa during Locomotion PLoS ONE, 8 (5) DOI: 10.1371/journal.pone.0064602
Warren, W., & Whang, S. (1987). Visual guidance of walking through apertures: Body-scaled information for affordances. Journal of Experimental Psychology: Human Perception and Performance, 13 (3), 371-383 DOI: 10.1037//0096-1523.13.3.371 Download

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