Crossmodal congruency measures of lateral distance effects on the rubber hand illusion

Body ownership for an artificial hand and the perceived position of one's own hand can be manipulated in the so-called rubber hand illusion. To induce this illusion, typically an artificial hand is placed next to the participant's body and stroked in synchrony with the real hand, which is hidden from view. Our first aim was to test if the crossmodal congruency task could be used to obtain a measure for the strength of body ownership in the rubber hand illusion. In this speeded location discrimination task participants responded to tactile targets presented to their index or middle finger, while trying to ignore irrelevant visual distracters placed on the artificial hand either on the congruent finger or on the incongruent finger. The difference between performance on congruent and incongruent trials (crossmodal congruency effect, CCE) indicates the amount of multisensory interactions between tactile targets and visual distracters. In order to investigate if changes in body ownership influence the CCE, we manipulated ownership for an artificial hand by synchronous and asynchronous stroking before the crossmodal congruency task (blocked design) in Experiment 1 and during the crossmodal congruency task (interleaved trial-by-trial design) in Experiment 2. Modulations of the CCE by ownership for an artificial hand were apparent in the interleaved trial-by-trial design. These findings suggest that the CCE can be used as an objective measure for body ownership. Secondly, we tested the hypothesis that the lateral spatial distance between the real hand and artificial hand limits the rubber hand illusion. We found no lateral spatial limits for the rubber hand illusion created by synchronous stroking within reaching distances. In conclusion, the sense of ownership seems to be related to modulations of multisensory interactions possibly through peripersonal space mechanisms, and these modulations do not appear to be limited by an increase in distance between artificial hand and real hand.