Sustained Facilitation of Embodied Fake Hand Movement on Voluntary Movement Execution

In a rubber hand illusion, participants experience illusory ownership (embodiment) of a seen fake hand when it is stroked synchronously with their unseen real hand. A recent investigation demonstrated that participants exhibited significantly faster reaction times when instructed to lift their index finger immediately after observing the index finger movement of an embodied (i.e., rubber hand illusion) versus a nonembodied (i.e., nonrubber hand illusion) fake hand. The current study examined whether this facilitation in reaction times arises from enhanced visual processing of the observed movement or from motor facilitation driven by a visuo-proprioceptive conflict between the embodied fake hand and the participant's hand. Two experiments were conducted, in which participants were required to lift their index finger in response to a neutral auditory stimulus following illusion induction. To isolate the contribution of visual processing, the visual stimulus (i.e., fake finger movement) was presented before the auditory cue with different stimulus onset asynchronies (500, 1000, or 1500 ms). In Experiment 1, the fake finger remained elevated until the participant initiated their movement, whereas it lowered soon in Experiment 2. The results revealed that the reaction time advantage in the rubber hand illusion condition was independent of stimulus onset asynchronies and emerged exclusively in Experiment 1. No significant differences were observed in peak velocity and acceleration of finger movement. These findings suggest that the ownership-dependent facilitation of reaction times is not due to visual processing alone but rather to motor facilitation mechanisms driven by visuo-proprioceptive discrepancy at the termination of the fake finger movement.