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Supernumerary phantom limb (SPL) is a rare neuropsychiatric syndrome, under which patients perceive more limbs than they anatomically possess (e.g., two instead of only one right arm). Remarkably, the concurrent limb percepts can thereby differ in respect to their primarily experienced modality, for instance, that one kinesthetic and one moving limb are experienced. Although SPLs are well-documented after various neuropsychiatric conditions, their neurocognitive pathomechanisms remain elusive. Therefore, an experimental paradigm by which such aberrant body misperceptions could be transiently induced and systematically investigated in healthy participants would be helpful. Inspired by the virtual hand illusion, we developed a virtual supernumerary limb illusion (SLI) version, in which two embodiable virtual hands are presented to the participant via a head-mounted display. One virtual hand is thereby presented medially and the other laterally aside the participant's hidden real hand. Using this general setup, we examined the inducibility of a SLI in three consecutive experiments: Experiment 1 explored by which induction type (visuotactile congruency, visuothermal congruency, and visuomotor congruency) SLI experiences can be induced most robustly, Experiment 2 explored whether SLIs can be induced by a combination of these induction types, and Experiment 3 explored how visuoproprioceptive congruency influences the inducibility of a SLI. Sense of ownership toward the virtual hands was systematically assessed by means of experience sampling and by an implicit electrodermal embodiment measure. Results reveal a robust effect of stronger sense of ownership toward the medially than laterally presented virtual hand, while only a subgroup of participants (on average, across experiments: ~ 25%) reported concomitant sense of ownership toward both virtual hands (i.e., a full-blown SLI experience). The highest SLI responder rate (~63%) was observed for the combined application of visuotactile and visuothermal congruency, whereas a combined application of visuothermal and visuoproprioceptive congruency induced the lowest SLI responder rate (~4%). In conclusion, our study demonstrates that although under specific experimental conditions, a majority of participants can be made to experience a SLI, under most conditions, sense of ownership is only experienced toward one virtual hand. This indicates that multiple type-identical limb percepts are avoided to be co-instantiated by our brain, wherever possible.

Experimental Inducibility of Supernumerary Phantom Limbs: A Series of Virtual Reality Experiments