"Magic shoes" illusion is different for people with eating disorders

The classic 'magic shoes' studies show that our footstep sounds influence our perceptions of our size and weight. When footstep sounds are filtered, so that a walker hears, through headphones, only the higher frequencies, they perceive their body to be thinner and lighter. When the sounds are filtered to leave only the lower frequencies — which would typically indicate a heavy walker — this makes people feel that their body has become bigger and heavier.

Ana Tajadara-Jiménez at Charles III University of Madrid, Spain, and colleagues now report in a paper in Scientific Reports that women diagnosed with the eating disorder anorexia and also healthy women with relatively high levels of symptoms of an eating disorder do not respond in the typical way to filtering of their footstep sounds. Their findings may improve understanding of the brain processes involved in anorexia, and perhaps lead to new types of treatments.

For the first study, the team recruited 58 healthy young women with a normal-range body mass index. Based on their scores on an eating disorder questionnaire, they were split into three groups: those with low, medium and high levels of symptoms. For the second study, the team recruited 15 women who were being treated for anorexia at a centre in Seville, Spain.

All the participants walked for a minute in the experimental sandals in each of three conditions: while listening to their natural walking sounds, while listening to the low frequency component, and while listening to the high frequency component. Once at the beginning, and then after each walk, they used software to indicate their perceived body shape and size.

The results showed that for those in the medium and low symptom groups, the 'footsteps illusion' worked as expected: in the low frequency condition, they perceived their body to be bigger than in the two other conditions, and in the high frequency condition, they perceived it to be smaller.

Earlier work has found that patients with anorexia are more susceptible to visual body-related illusions. The team predicted that it would be the same with this auditory body-related illusion: that these participants, and also those with high scores for eating disorder symptoms, would respond in the same way as the other participants, but even more strongly.

However, this is not what they found. The healthy women with high levels of eating disorder symptoms and the women with anorexia perceived their bodies to be widest and heaviest in the high frequency condition. The researchers also analysed the gait of the participants as they walked. They found that the healthy high-symptoms group walked as though they were heavier while listening to the high frequency sounds. Among the women with anorexia, those who had initially perceived their body to be wider/heavier than it actually was did the same thing.

What's going on?

The team suggests an intriguing explanation based on the popular predictive processing model of perception. This states that the brain uses sensory data and also expectation (based on past experience) to generate its 'best guess' of what's going on — and this best guess is what we perceive. A typical brain is pretty good at judging, moment to moment, how much to rely on sensory data vs expectation. Generally, when sensory data is judged to be high quality and reliable, it is given more weight, and allowed to update expectations. But when it's unclear or otherwise seems unreliable, expectation has more of an influence on perception.

So, the team reasons, what if people with anorexia and high levels of eating disorder symptoms have a very strong expectation that 'I am heavy' — and their brains have trouble integrating new sensory data to update this belief? In the low frequency condition, footstep sounds match this expectation of being heavy, so there is no conflict. But in the high frequency condition, they don't. This could lead their brains to decide that the sensory data must be unreliable, causing them to over-rely on the prior expectation — and to perceive their bodies as being heavier.

This is just a pilot study, and this is only a tentative explanation, the team stresses. But if further research supports these findings, they could inform treatments for eating disorders, which are currently often based on the visual experience of the body. Perhaps finding methods to help patients respond to manipulations of walking sounds in a more typical way might help — especially if a patient is blind, or suffers extreme anxiety while looking at their own body, the researchers suggest.