The rubber hand illusion describes a sense of embodiment over a fake hand induced by synchronous visuo-tactile stimulation. In Tourette Syndrome, the expression of involuntary tics and preceding premonitory sensations is associated with the perturbation of subjective feelings of self-control and agency. We compared responses to induction of the Rubber Hand Illusion in 23 adults with TS and 22 matched controls. Both TS and control participants reported equivalent subjective embodiment of the artificial hand: feelings of ownership, location, and agency were greater during synchronous visuo-tactile stimulation, compared to asynchronous stimulation. However, individuals with TS did not manifest greater proprioceptive drift during synchronous relative to asynchronous stimulation, an objective marker of embodiment observed in controls. We computed an ‘embodiment prediction error’ index from the difference between subjective embodiment and objective proprioceptive drift. This embodiment prediction error correlated with severity of premonitory sensations according to the Premonitory Urge for Tics Scale (PUTS). Feelings of ownership over the artificial hand also correlated with premonitory sensation severity, and feelings of agency with tic severity (YGTSS). Together our findings suggest that the subjective strength of bodily ownership, as measured by the rubber hand illusion, contributes to susceptibility to the premonitory sensations that are a precipitating factor in tics. These results also suggest that somatosensory neural pathways underpinning visuo-tactile integration are likely altered in TS and may interact with other sensory and motor systems to engender premonitory sensations and tics.
Drifting perceptual patterns suggest prediction errors fusion rather than hypothesis selection: replicating the rubber-hand illusion on a robot
