The Sixth Finger Illusion in Anorexia Nervosa

Area of Science:

• Neuroscience
• Psychology
• Body Perception

Background:

• Anorexia nervosa (AN) is associated with altered body representation, particularly in embodiment and motor imagery.
• Previous research in healthy controls (HC) shows illusions of supernumerary body parts are not limited by posture.
• The body representation of supernumerary body parts in AN remains largely unexplored.

Purpose of the Study:

• To investigate differences in the malleability of body representation for supernumerary body parts between individuals with AN and HC.
• To assess how AN affects the perception of an illusory sixth finger across different hand orientations.

Main Methods:

• Employed the sixth finger illusion, inducing an illusory extra finger using conflicting visual-tactile signals.
• Tested 30 participants with AN and 30 HC across four synchronous visual-tactile stroking orientations (0°, 90°, 135°, 180°) and a control.
• Utilized an ownership questionnaire to assess perceived finger orientation and the sixth finger's felt orientation.

Main Results:

• Both AN and HC groups experienced the sixth finger illusion across all tested orientations, irrespective of biomechanical constraints.
• Participants with AN perceived the illusory sixth finger as being closer to the hand's actual position compared to HC.
• AN participants demonstrated reduced flexibility in the perceived orientation of the extra finger.

Conclusions:

• The study reveals that while individuals with AN exhibit altered body representations, this heightened malleability is not universal across all bodily self domains.
• Reduced flexibility in perceiving the orientation of a supernumerary body part suggests a specific deficit in AN.
• Findings challenge the notion of generalized body representation flexibility in anorexia nervosa, highlighting domain-specific alterations.