The Sixth Finger Illusion in Anorexia Nervosa.

People with anorexia nervosa (AN) exhibit altered responses to embodiment and motor imagery tasks, indicating a more malleable mental representation of their body compared to healthy controls (HC). While this has been observed with the actual body, little is known about the mental representation of supernumerary body parts. Recently, it was demonstrated in HC that illusions of supernumerary fingers are not constrained by posture congruency. The current study aimed to evaluate the differences in the malleability of body representation for supernumerary body parts comparing 30 participants with AN and 30 HC using the sixth finger illusion which is a body illusion based on conflicting visual and tactile signals that lead to create the illusory perception of having an extra finger (i.e. a sixth finger). Synchronous visual-tactile stroking was delivered at four abduction orientations: 0°, 90°, 135° and 180°, plus a control condition. Participants were asked to complete an ownership questionnaire to judge the little finger's perceived orientation and the sixth finger's felt orientation when they agreed to feel six fingers. Both groups experienced the sixth finger in all orientations, despite biomechanical constraints. However, AN participants perceived the sixth finger closer to the hand's position. Previous studies in AN report higher flexibility of some body representation domains, such as embodiment and motor imagery. Interestingly, our study highlights that this higher flexibility does not seem to apply to all domains of the representation of the bodily self: AN participants showed reduced flexibility for the perceived orientation of the extra finger.