The Malleability of Body Representation in Anorexia Nervosa: the Sixth Finger Illusion

The ability to imagine our body in our mind is called body representation. This ability is grounded in sensory and perceptual process and it is crucial to plan action, to interact with the environment and so on. Body representation can be explored using behavioural tasks such as motor imagery tasks as well as body illusions.

Evidence from healthy individuals shows that we depend on physical constraints when we imagine actions we can do with our body: we are faster and more accurate in imagining an action that we can really perform. This phenomenon is called "biomechanical constraints effect" and it is an index that our brain is performing motor imagery. When a lesion occurs to the spinal cord, for example, this effect is not present anymore, supporting the idea that what occurs in our brain is strongly dependent on what we can do in the real word. This dependency from the real word, however, does not mean that the representation of the body in the mind is an exact copy of the physical body. For example, findings from experiments using body illusions show that we have a distorted representation of our body, even in absence of any lesion. Overall, evidence from body illusions in healthy individuals shows that we tend to overestimate features when we imagine our body, width in particular compared to length and height.

In both cases, evidence from studies on healthy individuals as well as patients with lesions to the central nervous system show that body representation is not set in stone: how we imagine our bodies is a dynamic and continuously updated process, to reflect changes in our own body as well as the environment.

Recent studies suggest that body representation is quite different in conditions that affect the body even in absence of central nervous system or brain impairments, such as anorexia nervosa. In patients affected by anorexia nervosa, motor imagery does not occur, and patients treat their body like it is an object. Similarly, when subjected to illusions of ownership, patients with anorexia nervosa experience the illusion more strongly than healthy individuals.

In anorexia nervosa the representation of the body is very different from that of healthy individuals: the representation is more malleable, and easier to manipulate, while at the same time being more detached from physical constraints. These features of body representation in anorexia nervosa might contribute to the persistence of symptoms and to relapses too. Body representation has clear implications for treatment of anorexia nervosa too.

The study aims at evaluating the presence of differences in the malleability of the body representation, explored through the illusion of the sixth finger, between a group of people with AN and a group of normal weight people, taking into account the biomechanical constraints that characterize the physical and mentally represented body.

Inclusion criteria

• female
• 18-55 years
• bmi ≤ 17.5 Kg/m2 for AN and BMI between 18.5 Kg/m2 e 25 Kg/m2 for healthy controls

Exclusion criteria

• psychopathologies related to psychosis or brain injury lesions