When one part feels, the whole belongs: associations between local touch referral and illusory full-limb ownership in individuals with leg amputation

The rubber limb illusion (RLI) describes a perceptual phenomenon where synchronous stroking of a hidden real and a visible artificial limb induces ownership for the fake limb, i.e., the sensation as if it belongs to one's body. Previous research suggests that the induction of ownership in the RLI is closely related to the referral of touch, i.e., the perception that the observed touch on the artificial limb causes the tactile sensation. However, the ultimate importance of touch referral for eliciting limb ownership remains unclear. To help answer this question, the present study re-analyzed the data from a recently introduced RLI paradigm for individuals with lower limb amputation. In 34 participants, synchronized touches were used to induce ownership for an artificial leg, with both the real amputated and non-amputated leg as targets. Crucially, participants were also asked for ownership of a non-touched artificial foot that was either connected (intact condition) or disconnected (impaired condition) to the artificial leg. Touch referral reliably predicted the occurrence of ownership for the touched leg regardless of the artificial limb being intact or impaired. A series of mediation analyses revealed that leg ownership significantly mediated the relationship between touch referral and foot ownership, but only if an intact artificial counterpart was used. This indicates that touch applied to a given body part indirectly relates to the perception of adjacent but non-touched body parts, even when those areas are physically absent. These findings may motivate studies testing whether sensory feedback promoting touch referral can support prosthesis embodiment.