Diminished size-weight illusion in anorexia nervosa: evidence for visuo-proprioceptive integration deficit

Individuals with anorexia nervosa (AN) experience pronounced body image distortion in combination with a pernicious desire to maintain a dangerously low body weight. Relatively little is known, however, about the mechanism underlying body image distortion in AN. Despite having normal visual perception, individuals with AN both feel and see themselves as large-bodied and show deficits in interoception and haptic perception, suggesting a potential deficit in visual and tactile integration. The size-weight illusion (SWI) arises when two objects of equal weight but different sizes are held. Typical individuals experience a strong and robust illusion that the smaller object feels much heavier than the larger object because of an implicit assumption that weight scales with size. The current study compared the strength of the SWI in individuals with AN to healthy control participants. Individuals with AN exhibited a markedly reduced SWI relative to controls, even though their ability to discriminate weight was unaffected. Because the SWI is strongly modulated by visual appearance, we believe our finding reflects decreased integration of visual and proprioceptive information in anorexia. This finding may explain the puzzling observation that visual perception of the body in a mirror does not correct an AN patient's distorted body image. We speculate that methods to correct visuo-proprioceptive integration in constructing body image may help rehabilitate patients' judgments of size and weight regarding their own bodies. We also suggest that a dysfunction in interactions between inferior parietal lobule (concerned with body image), insula, and hypothalamus may underlie AN.