Visuo-Tactile Congruence Leads to Stronger Illusion Than Visuo-Proprioceptive Congruence: a Quantitative and Qualitative Approach to Explore the Rubber Hand Illusion.

The Rubber Hand Illusion (RHI) arises through multisensory congruence and informative cues from the most relevant sensory channels. Some studies have explored the RHI phenomenon on the fingers, but none of them modulated the congruence of visuo-tactile and visuo-proprioceptive information by changing the posture of the fingers. This study hypothesizes that RHI induction is possible despite a partial visuo-proprioceptive or visuo-tactile incongruence. With quantitative and qualitative measures, we observed that gradual induction of the sense of body ownership depends on the congruence of multisensory information, with an emphasis on visuo-tactile information rather than visuo-proprioceptive signals. Based on the overall measures, the RHI observed went from stronger to weaker with full congruence; visuo-proprioceptive incongruence and visuo-tactile congruence; visuo-proprioceptive congruence and visuo-tactile incongruence; full incongruence. Our results confirm that congruent visual and tactile mapping is important, though not mandatory, to induce a strong sense of ownership. By changing index finger and thumb postures rather than the rotation of the whole hand, our study investigates the contribution of visuo-proprioception and postural congruence in the field of RHI research. The results are in favor of a probabilistic multisensory integration theory and do not resonate with rules and constraints found in internal body models. The RHI could be illustrated as a continuum: the more multisensory information is congruent, the stronger the RHI.