Exploring specific alterations at the explicit and perceptual levels in sense of ownership, agency, and body schema in Functional Motor Disorder: A pilot comparative study with Irritable Bowel Syndrome

Functional Motor Disorders (FMD) consists in symptoms of altered motor function not attributable to typical neurological and medical conditions. This study aimed to explore explicit and perceptual measures of Sense of Ownership, Agency, and Body Schema in FMD patients, and assess whether these alterations are specific to FMD or shared with other functional disturbances. Twelve FMD patients, ten with Irritable Bowel Syndrome (IBS, a functional gastrointestinal disorder) and fifteen healthy controls (HC) underwent: (i) the Mirror Box Illusion (MBI), requiring participants to perform tapping movements with their dominant hand concealed from sight, while visual feedback was provided by an alien hand under visuo-motor congruency or incongruency conditions; (ii) a Forearm Bisection Task before and after exposure to the MBI, and the Embodiment Questionnaire after the MBI, as perceptual and explicit indices of the embodiment illusion, respectively. At the Embodiment Questionnaire, all groups self-reported embodiment of the alien hand only under visuo-motor congruency; at the perceptual level, HC showed the expected distalized drift (an “elongated” arm in the Body Schema) under visuo-motor congruency, while FMD and IBS patients did not. FMD patients showed a proximalized drift when sensory feedback mismatched, possibly reflecting reliance on altered priors to avoid losing control over their movement. Results in IBS patients suggest Body Schema alterations differ across functional syndromes. In conclusion, we found that explicit Sense of Ownership and Agency are preserved in FMD and IBS patients, but dissociate from their implicit measures, differing in degree according to the specific disturbance.