Neurocognitive evidence favors "top down" over "bottom up" mechanisms in the pathogenesis of body size distortions in anorexia nervosa.

Perceptually-based disturbances in body image, or body size distortions, have been posited to occur in anorexia nervosa (AN). Perception does not result from a simple flow of sensory information from periphery to cortex ("bottom-up" processing), but involves the selection of inputs most likely to be relevant in light of an individual's experience and expectations ("top-down" processing). Most investigations of body size distortion in AN have used procedures likely to engage top-down processing, raising the possibility that attitudinal disturbances may play a role. To our knowledge, there have been no studies that assess the presence, in AN, of neurocognitive deficits associated with neurologically based disturbances in body schema. Such deficits, if found, could provide evidence of body image distortion unlikely to result from top-down processing. We tested 20 inpatients with AN on measures of proprioception, finger identification, right/left orientation, general cognition and eating disorders symptomatology, both before and after treatment. Matched normal controls were tested on the same measures over the same time intervals. Significant differences between the two groups occurred only prior to treatment, and only on those measures which involved executive, in addition to more body-schema-specific functions. This suggests that patients with AN do not have enduring deficits in the domain of body-schema, but may have subtle cognitive dysfunction, in the acute state, which is not specific to, but can interact with processing of body-schema-related information. This, in turn, suggests that their body image distortion may not be secondary to bottom-up perceptual disturbances.