No causal link between changes in hand position sense and feeling of limb ownership in the rubber hand illusion

The rubber hand illusion is a perceptual illusion in which participants experience an inanimate rubber hand as belonging to their own body. The illusion is elicited by synchronously stroking the rubber hand and the participant's real hand, which is hidden from sight. The feeling of owning the rubber hand is accompanied by changes in hand position sense (proprioception), so that when participants are asked to indicate the location of their (unseen) hand, they indicate that it is located closer to the rubber hand. This "proprioceptive drift" is the most widely used objective measure of the rubber hand illusion, and from a theoretical perspective, it suggests a close link between proprioception and the feeling of body ownership. However, the critical question of whether a causal relationship exists between changes in hand position sense and changes in limb ownership is unknown. Here we addressed this question by devising a novel setup that allowed us to mechanically manipulate the position of the participant's hand without the participant noticing, while the rubber hand illusion was being elicited. Our results showed that changing the sensed position closer to or farther away from the rubber hand did not change the strength of the rubber hand illusion. Thus, the illusion is not dependent on changes in hand position sense. This finding supports models of body ownership and central body representation that hold that proprioceptive drift and the subjective illusion are related to different central processes.